Crawlspace VentilationCrawlspace VentilationCurrent best practices compare crawl space venting versus crawl space dry-out & seal-up as a conditioned space.
Should you ventilate the crawl space, close off crawl space vents, install crawl space vent fans or dehumidifiers or what? This article describes best practices as well as common codes and standards for venting (or not venting) the space beneath buildings.
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Crawl space venting versus sealing: this question was debated for decades in several countries including the U.S.. Today we understand that in humid weather venting outside air into a cool crawl space is likely increase crawl space moisture.
That's because moist warm air entering a usually cooler crawl area will drop its moisture out in that space, allowing moisture to accumulate as condensate (water) on foundation walls, floors, and in insulation where in turn it can become a key factor in problematic crawl space mold growth even if there is not other water leaking into that area.
Therefore experts no longer recommend simple passive venting nor humidistat-controlled venting except in special circumstances.
When our onsite inspection indicates a long-standing moisture problem in such an area the best current advice is to stop venting the crawl space and to convert the crawl space to a dry, conditioned space.
That means we close off crawl space vents, dry out the area, and add some heat to it.
Temporarily in a wet crawl space and certainly in a moldy crawl space that is waiting for cleaning and repairs we also recommend an exhaust fan or two blowing out of the crawl space to the outdoors. This creates a slight negative air pressure within the crawl area with respect to the rest of the building and thus it reduces the chances of moving moisture, insect allergens, bacteria (say from a sewage leak), and mold upwards into the occupied building space.
At CRAWL SPACE DEHUMIDIFICATION we explain the use of heat, dehumidifiers, or crawl space exhaust vents to try to improve the humidity level in crawlspaces.
Crawl space venting practices and building codes specifying crawl space vent areas also discussed
at CRAWL SPACE GROUND COVERS.
Watch out : in some conditions, dust containment, negative air, and more protective gear or help from professionals may be needed.
The following is excerpted from CRAWL SPACE GROUND COVERS
In the original 1984 Solar Age article, the same experts were recommending what was conventional crawl space ventilation wisdom - specifications that were consistent with building codes.
1984 crawl space advice from Solar Age"
" A ground cover should be used in conjunction with ventilation. The HUD standard [1984] typical of others, recommended four crawl space vents with a total minimum free vent equal to 1/150 of the crawl space floor area if there is a ground cover, 1/1500 with the ground uncovered. For best results, place two vents each on opposing walls. "
Really? Experts continue to study crawl space ventilation, sealing, air movement, and the effects of those choices on building indoor air quality, energy costs, and moisture - we cite some of this research at the end of this article.
But by the very next year Nazaroff (1985) had shown that there was significant air movement up from a crawl space into the occupied space, enough that with crawl space vents open radon gas contamination levels doubled and 50% of the radon contamination entered the living space. And Dickson (2013) confirms and sums up a key point about venting crawl spaces in hot humid climates.
Research has shown that vented crawlspaces located in mixed or hot-humid climates tend to increase the moisture level within the space instead of keeping it drier.
Our photo (above-left) shows a severe and problematic mold contamination on the underside of the first floor of a building constructed over a wet crawl space. Ventilation with outdoor air had not helped one bit to avoid this problem.
Studies of air movement in buildings including from crawl spaces has continued to the present. In 2007 Kalamees demonstrated that air leakage upwards through a building was significantly driven by temperature (and thus pressure) differences and that normal openings around wiring and plumbing as well as windows and doors produced significant air leakage and air movement in buildings.
The typical air leakage places in the studied houses were: the junction of the ceiling/floor with the external wall, the junction of the separating walls with the external wall, penetrations of the electrical and plumbing installations through the air barrier systems, penetrations of the chimney and ventilation ducts through the air barrier systems, leakage around and through electrical sockets and switches, and leakage around and through windows and doors.
According to the questionnaire conducted, fluctuating room temperature, cold floors and draught from electric sockets were related to the houses with air leakage rate >3 m3/(h m2) at 50 Pa.
Conventional best practice crawl space moisture control has thus shifted from the 1984 view.
Experts observed that crawl space venting was not effective in many instances, for example depending on wind direction as well as the source and amount of crawl space water or moisture, crawl space vents were simply ineffective.
In some instances, such as blowing warm high-moisture laden air into a cool crawl space in summer months in some climates greatly increased the level of crawl space moisture and condensation, making crawl space moisture worse rather than better. See Walker (1998) for a scholarly study of the strength of the "stack effect" in moving crawl space air upwards in buildings.
Our crawl space photo (left) shows that poly was placed on the dirt floor of the crawl area and a heat source was provided, salvaging an old radiator. We'd have preferred to see the poly extending up the crawl space walls a foot or so. But we notice that this crawl space looks dry: there are not mold nor moisture stains on the floor framing overhead, and no leak stains on the crawl space foundation wall.
Below we summarize the best way to avoid wet or damp crawl space problems under buildings. If your crawl area is already wet or damp, also
- close off those crawl space vents - and convert the crawl space to conditioned space, providing a small amount of heat where climate dictates, to help keep the area dry and above freezing.
By closing off openings around pipes, wiring or framing between the crawl space and adjacent or overhead building areas we stop un-wanted air movement from the (usually) cooler crawl area into occupied building spaces.
This step prevents movement of moisture, mold, allergens or anything else into the building from the crawl area. Further by stopping possible convection currents between the crawl space and upper building areas we not only will stop natural air movement up into the building from the crawlspace, we'll also stop the simultaneous drawing of seasonally warm humid outdoor air into the crawl space.
The best way to prevent un-wanted air movement from a crawlspace into a building is to seal off all air leaks between the crawlspace and the house.
such as roof drainage spilling around the foundation. In roughly 90 % of inspections performed by experts, we find that wet or moldy crawl spaces or basements that had been blamed on "high water table", "rising damp", or "built over a spring" were actually being caused by gutter and downspout defects, perhaps combined with in-slope grade that concentrated roof drainage right against the building foundation.
and lower crawl space walls, up to grade level, sealed as we described above. In new construction the poly may be installed under a crawl space slab or gravel. In crawl spaces that are rarely entered, placing sand or gravel over the poly is probably not necessary, and its use can hide depressions in the poly that may actually be holding ponding water on top of the poly in some cases.
at least once a year, to be sure that the poly moisture barrier is working as intended. As we just suggested, an outside water leak, such as roof spillage entering through a foundation wall, or an inside water source such as from a leaky plumbing supply or drain pipe, can place water on top of your crawl space poly moisture barrier, leading to a costly building moisture and mold problem.
Remember these are minimum values for average conditions. Your building may need special measures. If, after identifying and fixing outside sources of a wet or damp crawl space, you still find high water levels right under the crawl space floor, you may want to install a sump pump as well.
Our photo at above left shows accumulated water above an improperly-located plastic vapor barrier that was stapled to the bottom of the floor joists over a crawl space. Moist air leaking into the floor space carried water that condensed enough to accumulate as puddles on the upper side of the poly. The wet insulation became a mold reservoir as did the wood framing and plywood subfloor above.
Below is a properly cleaned and sealed crawl space that had been mold contaminated. The white device atop my black clipboard is an air sampler.
The following is excerpted from Kurnitski (2000) but readers should note that the air change ventilation specifications those authors cite is for the occupied space, not the crawl area.
All HUD-code homes are required to have a ventilation system installed. Title 24 CFR Part 3280.103b ‘Light and ventilation’ states that…
“Each manufactured home shall be capable of providing a minimum of 0.35 air changes per hour continuously or at an equivalent hourly average rate. The following criteria shall be adhered to.
Natural infiltration and exfiltration shall be considered as providing 0.25 air changes per hour.
The remaining ventilation capacity of 0.10-air change per hour or its hourly average equivalent shall be calculated using 0.035 cubic feet per minute per square foot of interior floor space. This ventilation capacity shall be in addition to any openable window area.
The remaining ventilation capacity may be provided by: a mechanical system, or a passive system, or a combination passive and mechanical system….”
Currently, there are two main types of ventilation systems that are employed by the manufacturing housing industry to meet the 0.10 air change per hour requirement. Both types are used in the hot and humid climate; an exhaust only system that is located in a hallway or utility room and an outside air supply system that is ducted from the roof to the return airside of the air handler fan. The exhaust ventilation system is manually controlled with a simple on-off switch. The outside air supply system is linked to the operation of the air handler and controlled with an automatic damper (if applicable).
Abstract excerpt:
Some Swedish school buildings built in the 1960’s and 1970’s have indoor air quality problems. Many of these buildings have a crawl space from which contaminants are suspected to originate. The poor indoor air quality cause discomfort among pupils and teachers and a solution to the problem is not always found.
This study evaluates the current building ventilation code requirements for at-grade and below grade crawl space using computational fluid dynamic (CFD) software with experiment inputs.
The research first tested the soil moisture evaporation rate from two monitored crawl spaces in Colorado, US, which produces an average moisture load of 13.75 grains/(ft2·h) (9.6g/(m2·h)) and a maximum load of 42.7 grains/(ft2·h) (29.8g/(m2·h)). The soil moisture evaporation rates identified align well in magnitude with those recorded in the literature, supporting the estimation method used.
The experiment reveals that plastic ground cover can effectively reduce the moisture load from the soil by an average of 93%. The study then developed a CFD model of the monitored crawl space to assess the necessity and effectiveness of various ventilation code requirements.
The space effective leakage area to the exterior was determined through field pressurization testing and CFD analysis to be approximately 0.26in.2/ft2 of floor area. The CFD predictions, validated with the measured data, verify that the building code requirements for at-grade crawl spaces appear sufficient, but have limitations for below grade crawl spaces.
While there is no convincing technical basis for current code requirements for crawl space ventilation, most codes still require operable vents and the practice is well established among builders and architects. While the evidence against venting is compelling to many if not most in the research community, builders and code officials apparently want to see hard evidence. In this research project the author measured weekly energy consumption for space heating for a 1150 square foot home in a climate with 5900 heating degrees days.
The house was built to meet the 1992 Model Energy Code and the perimeter of the crawl space was insulated with R-10 foamboard. The supply ducts of the gas-fired, forced air system are located in the crawl space and were carefully sealed and pressure tested. Utilizing a flip flop research design, with the crawl space vents open one year and closed the next, the author measured the energy penalty resulting from leaving crawl space vents open during the vents was 21% (12 MMBtu/yr) in year two of the research.
Abstract:
Air change rate, humidity and temperature were monitored in a naturally and a mechanically ventilated crawl space of the test building, in order to determine the ground moisture evaporation rate and the mass transfer coefficient.
The average value of the moisture evaporation with uncovered ground was 3.6–5.7 g/h m2, corresponding to the mass transfer coefficient 0.0012–0.0018 m/s.
The mass transfer coefficient determined from evaporation showed sufficient agreement with the coefficient calculated from temperature differences. Normally it would have been sufficient to consider natural convection but with high air change, forced convection also had to be taken into account.
The higher air change rates increased moisture evaporation, but still brought about lower relative humidity.
The objectives of the study were to find out how the thermal capacity and resistance and the placement of insulation layers affect relative humidity (RH) in the crawl space.
The possibilities to reduce the air change rate to very low levels corresponding to natural ventilation, and to control RH by minimising the thermal capacity present in the crawl space were simulated.
Mold growth analyses were used as performance criteria to predict the acceptability of the hygrothermal conditions in the studied crawl spaces. Most of the simulations were carried out with a low U-value for the base floor which normally results in a low temperature and high RH in the crawl space during the summer.
The results show that there are two alternative ways to use ground covers in combination with air change in order to achieve acceptable conditions in crawl spaces.
First, a traditional thin layer of lightweight expanded clay aggregate (LWA) or expanded polystyrene (EPS) may be used, but in this case, the air change rate has to be increased in the summer at least to 1 ach.
An alternative solution is to use thicker ground covers, such as at least 30 cm lightweight aggregate or 10 cm polystyrene, and a low air change rate of approximately 0.5 ach all the year round.
Abstract excerpts:
The effect of ventilation on moisture behaviour in the traditional outdoor-air ventilated crawl space of blocks of flats with uncovered and moist ground surface is discussed in this paper.
The objectives were to compare in real conditions the mechanical supply and mechanical extract ventilation to natural ventilation, to determine the rate of ground moisture evaporation, and to test the reduction of humidity with plastic sheet cover. .
The air change rate in the crawl space was monitored continuously, as were temperature and humidity. This made it possible to assess moisture evaporation rate from the ground soil. .
The reported results account for the behaviour of air change and moisture balance, and give certain validity to arguments for optimum ventilation and the reduction of ground moisture evaporation. It was demonstrated that air change is only one important parameter affecting humidity in crawl spaces.
Ground moisture evaporation was related to air change rate and pressure conditions: a higher air change led to higher moisture evaporation.
Pressure conditions in the crawl space affected humidity notably; these were varied by using supply and extract fans and were monitored continuously during the measurements. Supply ventilation led to the lowest relative humidity, and extract ventilation brought about even higher humidity than did natural ventilation.
No high relative humidity values were measured in summertime, and only brief condensation peaks over a few days were detected.
During the summer, the relative humidity level with natural ventilation and uncovered ground was less than 85%, and with ground cover or with balanced ventilation it did not exceed 80%.
This study shows that acceptable moisture conditions in cold climate outdoor-air-ventilated crawl spaces may be achieved by the optimal selection of ground covers and ventilation.
Abstract Excrpts:
. That project, funded under award number DE-FC26-00NT40995 and titled 'A Field Study Comparison of the Energy and Moisture Performance Characteristics of Ventilated Versus Sealed Crawl Spaces in the South' demonstrated the substantial energy efficiency and moisture management benefits that result from using properly closed crawl space foundations for residential construction instead of traditional wall vented crawl space foundations.
Two activities of this first project included
(1) an assessment of ten existing homes to document commonly observed energy and moisture failures associated with wall-vented crawl space foundations and
(2) a detailed literature review that documented both the history of closed crawl space research and the historical lack of scientific justification for building code requirements for crawl space ventilation.
The most valuable activity of the 2005 project proved to be the field demonstration of various closed crawl space techniques, which were implemented in a set of twelve small (1040 square feet), simply designed homes in eastern North Carolina.
. The key findings from the field demonstration were that
(1) closed crawl spaces stay substantially drier than traditional wall-vented crawl spaces during humid climate conditions, and
Abstract
This study simulated the effects of thermal insulations on the ground, in the foundations and in the base floor in order to achieve as dry conditions as possible in outdoor air ventilated crawl spaces in a cold climate.
The objectives of the study were to find out how the thermal capacity and resistance and the placement of insulation layers affect relative humidity (RH) in the crawl space. The possibilities to reduce the air change rate to very low levels corresponding to natural ventilation, and to control RH by minimising the thermal capacity present in the crawl space were simulated.
Mold growth analyses were used as performance criteria to predict the acceptability of the hygrothermal conditions in the studied crawl spaces. Most of the simulations were carried out with a low U-value for the base floor which normally results in a low temperature and high RH in the crawl space during the summer.
The results show that there are two alternative ways to use ground covers in combination with air change in order to achieve acceptable conditions in crawl spaces. First, a traditional thin layer of lightweight expanded clay aggregate (LWA) or expanded polystyrene (EPS) may be used, but in this case, the air change rate has to be increased in the summer at least to 1 ach.
A significant number of new manufactured houses built to HUD code and located in the hot, humid Southeast are experiencing moisture problems. Soft wallboards, buckled floors, damaged wood molding and extensive mold growth are the most common symptoms. These problems do not respond to the standard service and repair strategies for water intrusion.
At the request of four manufacturers, over twenty-five such moisture damaged homes were investigated in 1999 and 2000 to determine likely causes. One time blower door, duct tightness and pressure differential measurements were performed on all homes. Field data on ambient, crawlspace, belly and house temperatures and RH were collected on a few of the homes.
Recommendations and reports were prepared for the manufacturers service, production and design staff. Field repairs were performed in most of these homes. A generalized theme existed in the houses investigated.
A retrofit study was conducted in an unoccupied manufactured house to investigate the impacts of airtightening on ventilation rates and energy consumption. This paper describes the retrofits and the results of the pre- and post-retrofit assessment of building airtightness, ventilation, and energy use.
Building envelope and air distribution systems airtightness were measured using fan pressurization. Air change rates were measured continuously using the tracer gas decay technique. Energy consumption associated with heating and cooling was monitored through measurement of gas consumption by the forced-air furnace for heating and electricity use by the air-conditioning system for cooling. The results of the study show that the retrofits reduced building envelope leakage by about 18% and duct leakage by about 80%.
Abstract: A system for forming an insulating vapor barrier in a building is especially suited for forming an insulating vapor barrier in a crawl space beneath a building. The system includes a series of separate vapor barrier panels that can be attached around a wall. A ground level vapor barrier can be sealed to the insulating vapor barrier panels, which can be sealed to each other and along a top edge to the wall.
This standard is being considered for adoption by various jurisdictions within the U.S. as well as by various voluntary programs. The adoption of 62.2 would require mechanical ventilation systems to be installed in virtually all new homes, but it allows for a wide variety of design solutions.
These solutions, however, may have different energy costs and non-energy benefits. The authors have used a detailed simulation model to evaluate the energy impacts of common and proposed mechanical ventilation approaches for a variety of climates. These results separate the energy needed to ventilate into the energy needed to condition the ventilation air and the energy needed to distribute and/or temper the ventilation air.
Executive Summary Excerpts:
One method of crawl space code compliance is to seal and insulate the crawl space, rather than venting to the outdoors.
However, codes require mechanical ventilation—either via conditioned supply air from the heating, ventilating, and air-conditioning system or via a continuous exhaust ventilation strategy. Recently, the U.S. Department of Energy’s Building America team Consortium for Advanced Residential Buildings (CARB) partnered with Ithaca Neighborhood Housing Services.
This partner intended to use unvented crawl spaces in a recent development. CARB was interested in investigating a hybrid ventilation method that includes the exhaust air from the crawl space as a part of an ASHRAE Standard 62.2-compliant whole-house ventilation strategy.
Data were collected to investigate the effects of this ventilation on the temperature, relative humidity, and condensation potential within the crawl spaces of two subject homes (one occupied and one unoccupied) in New York State.
These initial findings suggest that Section R408.2 of the International Residential Code may warrant a revision, though further validation is needed. A code revision could incorporate the following guidance:
An unvented crawl space that meets code compliance via an exhaust fan in the crawl space and a passive vent from the living space should either have air conditioning in the living space or be insulated (along with the continuous vapor barrier) to minimize the potential for summertime condensation in the sealed crawl space.
This article series describes the steps needed to get into, inspect, clean, and then dry out a building crawl space. We give a step by step crawl space entry, inspection, cleanout, dryout and keep dry guide explains how to get into or inspect a crawl space even if there is no ready access, how to assess crawl space conditions, how to stop water that is entering the crawl area, how to dry out the space, how to clean up and if necessary disinfect or sanitize the crawl space, and how to keep out crawl space water and moisture in the future.
Below you will find questions and answers previously posted on this page at its page bottom reader comment box.
On 2020-01-15 by (mod) - Mold in the crawl space
Mold in the crawl space ought to be removed and the crawl space converted to a dry, warmed, conditioned space; that, combined with keeping out any leaks from the crawl area will prevent mold recurrence and thus reduce the chances of sending mold up into the occupied space.
(There's probably no mildew in your crawl space - mildew only grows on living plants)
Your inspector is absolutely right. The mold genera/species won't change the cleanup and prevention measures one iota.
Whoever got people scared about "toxic black mold" has done a dis-service. Of the millions of mold genera species, there are hundreds of them commonly found growing on materials or surfaces in buildings and on building contents. Some of these are "black" or "dark" in color; those dark molds may be toxic or pathogenic, or merely allergenic, and in rare cases just cosmetic;
But those exact words, "toxic or pathogenic, or merely allergenic" apply equally to molds that are white, green, red, yellow, brown, etc.
And in fact Aspergillus sp. is one of the most-ubiquitious mold families in buidings, is often very small (far smaller than the famous black mold Stachybotrys chartarum), is therefore far more airborne, and is breathed deeply into the lung where it can give you a very serious, even fatal illness, Asprgillosis.
So forget about the mold color. (Mostly).
On occasion a doc may want to know a persons exposure to specific molds as part of medical history and diagnosis, though docs tell me they can often tell the difference between big sticky S. chartarum exposure (sticks in your nose) and Aspergillus or Penicillium exposure (gets deep into your lung) by where mold related illnesses appear.
Remove the mold and fix its cause.
On 2020-01-15 by Marci
I recently hired a "home health" inspector due to concerns about air quality and other issues in my recently purchased home. I bought the house 4 months ago and found out after purchase that the longtime residents had been heavy smokers for years.
The main odor I noted was more musty than tobacco-like. I had the duct runs cleaned, then realized how degraded they were. I had them replaced nearly 3 months ago with a noticeable reduction in the musty smell.
The inspector noted mold and mildew in the crawl space, along with some in the wood-paneled sun room. The inspector made some recommendations regarding moisture control, etc. but in one area where I had concerns about what appeared to be a blackish mold spot, he stated that the type of mold was irrelevent as the treatment was the same. This seems contrary to everything I know about black mold.
While I can't say that's what it is, I also cannot say it isn't. I've already spent a large amount of money (for me) in only a few months and feel as if I'm not much further along than I was. I have ongoing health issues and am looking for answers. Any insight would be greatly appreciated.
On 2018-10-19 by (mod) - can you run cable TV wire (presumably co-axial cable) through a foundation vent into a building
The final legal authority to answer your question of "can you run cable TV wire (presumably co-axial cable) through a foundation cent into a building" is your local electrical code inspector
That said, Most likely, yes you shouldn't run cable through a foundation vent into a building (though that's probably quite common).
In the U.S. (if you're in the U.S.) the pertinent section of the US NEC is Article 820
820.1 Scope. This article covers coaxial cable distributionof radio frequency signals typically employed in communityantenna television (CATV) systems.
There are specifications for the type of cable, its grounding, and its protection.
In particular if you are running cable through or into a fire-rated building floor, wall, ceiling, there are extra requirements and certainly running through a foundation vent opening would not be among them.
You can read versions of the NEC including the full (and lengthy) txt of NEC Article 820 and other countries' codes at the live link given in the ARTICLE INDEX for
BUILDING CODE DOWNLOADS - free downloadable PDF files of building codes & standards
On 2018-10-18 by Billy Luck
Is it a code violation to run cable tv wire through a foundation vent?
On 2018-09-29 by (mod) - yes we do find moisture and mold contamination problems in crawl spaces in the Northeast
It's absolutely the case that we find moisture and mold contamination problems in crawl spaces in the Northeast.
A conditioned crawl space is a good design but of course needs to be executed properly. And you still want to do what you can to prevent order entry or high humidity.
This article series gives a lot of detail about how to do that. It sounds as if your contractor is of the old school. It seems that building experts currently explain that conventional crawl space venting does not work to prevent moisture problems and in fact it can actually cause them.
As temperatures change and indoor outdoor pressures or winds directions change moist air enters the crawl space, moisture condenses, creating a moisture and mold problem.
On 2018-09-29 by Karina24
Dan, do you find issues with mold in crawl spaces here in the Northeast? It's generally presented as a problem in the Southeast, but it sounds like you are seeing mold in crawl spaces in the New York area.
The reason I'm asking is I have specified a conditioned crawl space in a multi-family project that I'm working on in upstate New York, but the general contractor wants it to be vented, I think at the recommendation of his mechanical engineer. So I'm trying to get a feel for whether or not I should insist on having it conditioned.
On 2017-11-03 by (mod) - does the ground vapor barrier change the venting area requirements?
On 2017-11-03 by Joddie
Is the total venting area required for a permanent foundation under HUD guidelines reduced with the installation of the ground vapor barrier.
On 2017-01-21 - by (mod) - response to reader prefers to vent crawl spaces
Thank you for questioning this matter and article. Notwithstanding an occasional surge in dislike for science, facts, objective data, when people face the high costs of mold contamination, rot, insect damage, and structural damage, the work by building scientists becomes more interesting,
I don't think it's a matter of simply "in some cases ventilation didn't help" as much as a matter of improvement in building science and the benefit, now of half a century of observation of crawl spaces, vented, un-vented, well-vented, inadequately-vented, and probably better equipment to detect and measure problems.
In 1910 buildings oil was cheap, buildings were barely insulated or not insulated at all, and windows, doors, walls, and crawl areas were as drafty as all get-out.
I've inspected old buildings from the 1700s and 1800s that had water regularly running in and through their basement or crawl space. When the building was drafty and barely heated and not insulated, the moisture mostly got out without making much trouble.
Then things changed.
By the oil crises of the 1970's energy costs had risen so dramatically that we made buildings tighter, better-insulated.
Moisture or water in a crawl area is a problem for most buildings though it's not always noticed promptly.
Small crawl space vents that admit warm moist air in summer, allowing that water into a cool crawl space, cause trouble for the building.
Sure a crawl area in a very dry area where no moisture enters in air the matter is not such a worry.
Contemporary expert opinion is supported by observation and research is that
- older, small conventional foundation vents admitted moist air enough to cause trouble but not enough air to keep a crawl space dry by natural circulation (as might occur under a building built on piers without a skirt)
- blowing or circulating air even by force in to the crawl space - something we tried for a time - through the crawl area made matters worse - that forms a one-way moisture pump IN.
- blowing air out - negative air in the crawl space, has some temporary applications (mold remediation)
- closing off crawl vents, getting water away from the building and out of the crawl pace, and putting down a good 6 mil poly barrier over dirt in a crawl space stops what otherwise functions as a natural moisture pump - sending moisture from the soil up into the structure.
Where a crawl area has been so closed off it might be useful to run fans and a dehumidifier during the dryout process.
We'd prefer not to rely on fans for a building to survive the elements, and we'd prefer to lower our energy cost for the structure. So once we've fixed gutters and leaders, outside soil grading, other leaks into the crawl area, such active drying will very often not be further needed.
I've cut open structures and cavities that were "vented" by a one-inch hole, even two-inch holes to see how effectively those areas had been vented and dried. They were not effectively dried at all, even when the remediator using that approach added a blower fan.
Both insulated wall cavities and uninsulated interior partitions and other areas so-vented were wet and moldy in problem buildings.
On 2017-01-21 by JJ
I can't really understand the logic behind all this. Just because in some cases, ventilation did not help, that does not mean the absence of ventilation WOULD help. Turn an empty tote upside down over a pile of mud and leave it there for months.
Check it from time to time and you will see that even in the hot summer, it will stay moist. But if you put even a 1 inch hole on either side, the mud would have dried completely in a short time. The time it takes would be dependent upon the flow of air combined with the temperatures involved.
And I say that it is important WHERE you place crawlspace vents. The wind comes in differently at a given location. So one needs to design a system to effectively function.
I have seen my fair share of crawlspaces that stayed dry thanks to vents. And I will be using them on a new construction of my own. The absence of air movement hinders evaporation which of course promotes all kinds of nasty problems.
The idea of using dehumidifiers and/or heat to dry out a crawlspace is a crutch due to the lack of understanding or ignorance of how air flow is used functionally. Think about the gray, cloudy winter time when everything is blanketed by the sky. The promotion of mold is amazing. Everything stays wet and gross. Take away the blanket and that all changes. The warm air of the summer works if you know how to use it.
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Moisture evaporation was included in the moisture balance, but moisture storage in the constructions and in the ground was not. The results show that evaporation and thermal behaviour are the key elements determining the resulting relative humidity in crawl spaces. If moisture evaporation is entirely prevented, the crawl space may be left unventilated. In other cases, ventilation is always required to remove the moisture. The higher air change rates increased moisture evaporation from uncovered ground, but still brought about lower relative humidity. The rise from 0.5 to 3 ach increased evaporation from 2.4 to 4.9 g/m2 h and decreased the highest monthly average of relative humidity from 81 to 74%. For the uncovered ground the lowest relative humidity was achieved at 2–3 ach air change in winter, but in the summer, the higher the air change the lower the relative humidity.
The possibilities to reduce the air change rate to very low levels corresponding to natural ventilation, and to control RH by minimising the thermal capacity present in the crawl space were simulated. Mold growth analyses were used as performance criteria to predict the acceptability of the hygrothermal conditions in the studied crawl spaces. Most of the simulations were carried out with a low U-value for the base floor which normally results in a low temperature and high RH in the crawl space during the summer.
Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The HOME REFERENCE BOOK - the Encyclopedia of Homes and to use illustrations from The ILLUSTRATED HOME .
