Chart describes moisture profiles of building walls (C) Daniel Friedman Vapor Barriers & Building Condensation - Part 2

  • VAPOR CONDENSATION & BUILDING SHEATHING - CONTENTS: Vapor barriers and condensation in buildings - solving tricky vapor barrier location questions
    • How various building wall sheathing materials affect building condensation and moisture
  • Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
  • POST a QUESTION or READ FAQs about the need for & role of vapor barriers beneath building wall siding

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This article discusses how to solve difficult vapor barrier location & indoor condensation problems. In a series of questions and answers about vapor barrier location problems we explain when and why condensation occurs inside buildings, explains the problems caused by excessive indoor condensation.

We discuss how moisture enters building wall and ceiling cavities, and we summarize the best approaches to prevention of indoor moisture and condensation problems. Sketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

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Vapor Barriers & Building Condensation - Solving Tricky Problems

Insulation refrofit in an attic (C) Daniel Friedman"Vapor Barriers, Part II - Vapor Barriers and Condensation, building researchers are helping out with the tricky questions" - links to the original article in PDF form immediately below are followed by an expanded/updated online version of this article. Our photograph (below) shows an insulation retrofit that jammed fiberglass between rafters over an attic, combined with a foil "radiant barrier" that in our view risked moisture traps or future roof leak traps (and building damage) hidden under the roof decking.

Along with tables summarizing building moisture research from the National Forest Products Laboratory, this article answers the following building condensation and materials questions:

At part I, VAPOR BARRIERS & CONDENSATION in buildings we looked at the fundamentals of moisture condensation in buildings: what causes condensation, how to control condensation, and whether we should worry about moisture condensation in buildings. We concluded that small amounts of moisture condensation can occur and do occur in wall cavities, but that structural damage rarely occurs because the walls dry out before temperatures are warm enough to support wood rotting fungi.

Still, risks of paint-peeing, corrosion of metals, hidden costly mold contamination, and degrading of insulation R-values do exist. A dry wall cavity is certainly preferable to a wet one.

And the most reliable way to achieve a dry wall is by installing a continuous vapor-retarding membrane such as 6-mil polyethylene plastic, paying a lot of attention to joints and penetrations. In fact, the penetrations are usually more important than the main surfaces, since air leaks generally transport a lot more moisture into a wall cavity than does vapor diffusion.

In this article we will examine questions frequently raised about how various materials and applications affect moisture condensation. Even if you have a good handle on the theory, applying it can be trick. Some building materials both insulate and block moisture vapor flow, confusing the issue. And in some applications, moisture vapor flows reverse seasonally, or spaces need both ventilation and sealing. It is enough to make a moisture vapor conscious contractor move to Phoenix (where presumably it is warm and dry enough that not much mold grows).

Question: How about insulating building exterior wall sheathings? Do they cause moisture problems?

Vapor and condensation table (C) Daniel Friedman

Answer: 1980's tests at the Forest Products Laboratory in Madison WI confirmed earlier reports that in a 2x4 wall in a moderately cold climate (7863 degree days), insulating sheathings caused no greater condensation hazard than ordinary sheathings. In fact, in the FPL tests, the insulating sheathings seemed to protect the siding from condensation, probably by slowing the flow of moisture to the siding.

For thicker insulated walls, which will have colder sheathing, or for buildings with more humid interiors (greater than 40 percent RH, which is most buildings with conditioned air in winter) these findings should be applied with caution. See Tables 1 (at the top of this page) and Table 2 (at left) that present some of the FPL findings.

Also see SIDING WOOD, FAILURES OVER FOAM BOARD where we describe wood siding failures when installed over foam insulating building sheathing, and see SHEATHING, FOIL FACED - VENTS - do we need to vent building walls with siding installed over foam board insulating sheathing?

Question: Don't insulating foam board wall sheathing products used on a building exterior put a vapor barrier on the wrong side of the wall?

Answer: A widely accepted rule of thumb holds that the building's exterior wall surface should be 5 to 10 times as moisture-permeable as the interior vapor retarder installed on the inside surface of the building's exterior walls. ("Retarder" not "barrier" is ASHRAE's preferred term). However, since insulating wall sheathings on a building exterior (under the siding) keep the wall cavity warmer and present a warmer face to the wall cavity, higher levels of vapor in the wall can be tolerated before condensation occurs. Hence the ratio of inside to outside permeability may be lower.

Question: How much lower can the ratio of inside to outside wall surface moisture permeability be without a problem on walls with exterior insulating sheathing?

Answer: You can play with the numbers if you're inclined, or hedge your bets by using a lapped and caulked poly vapor barrier with all wall penetrations sealed (our recommendation). This approach also controls air infiltration. Hence the awkward but useful phrase air/vapor barrier.

Question: Is polystyrene better than foil-faced foam insulating sheathing boards in preventing moisture condensation problems in building walls?

Answer: Theoretically, yes, because it is more permeable to water vapor; but no, because it has a lower R-value per inch. In the FPL tests,the foil-faced sheathing did slightly better, probably because the wall cavities were slightly warmer. Placing the rigid foam insulating board on the interior of the building wall side-steps the whole problem.

Question: How about using vent strips on exterior walls where foil faced building sheathing is to be installed?

Home made wall vent (C) Daniel FriedmanAnswer: These are probably not a good idea. Wall vent strips were tried on one wall in the FPL tests, and they actually increased the amount of wall cavity condensation. One possible reason is that the air drawn out of the wall through the vents was replaced with moist indoor air.

Vent strips were used only at the top of the walls. [That wall venting design is similar to the problem of installing a ridge vent on a home with no soffit intake venting. The presence of the high vent and no source of outdoor air leads to the ridge vent acting as a "pump" to draw indoor air out of the building, increasing home heating costs, or in the case of the wall top exit vent, also increasing the movement of indoor moisture into the wall cavity - DJF].

If wall vents are placed at both the top and bottom of the wall to solve this problem, the air movement through the wall cavity may degrade the R-value of the wall.

Our wall vent photo (above) shows a home-made wall vent installed by a building owner who hoped to avoid a moisture problem in the wall and in a raised wood floor over a concrete slab. At this building the ventilation system served only as an entry path for carpenter ants and water.

See SIDING WOOD, FAILURES OVER FOAM BOARD where we describe wood siding failures when installed over foam insulating building sheathing, and see SHEATHING, FOIL FACED - VENTS - do we need to vent building walls with siding installed over foam board insulating sheathing?

Question: What about stress-skin insulated building panels and moisture problems?

Answer: Many stress skin building panels have no vapor retarder on the inside, just drywall, and low permeance sheathing, such as OSB or waferboard on the panel exterior surface. Theoretically, water could condense within the panel, most likely at the foam/sheathing board interface.

However, since on a 0 degF winter day, less than quarter of an ounce of water will diffuse through an entire 4x8 foot stress skin panel of foam (3 1/2 inches thick) over 24 hours, I wouldn't lose sleep over this. I have asked around, and have heard of only one problem with moisture (frost under the plywood facing of the insulated stress-skin building panel) and that was under near-arctic conditions.

I would be more concerned about caulking the stress skin building panel joints well so that moist air would not leak out and contact cold surfaces - not to mention lose heat. Nonetheless, a coat of vapor barrier paint wouldn't hurt.

Question: How do you determine the amount of vapor transmission through foam insulating board?

Answer: Moisture permeability ratings are like U-values. So if you can calculate heat transmission you can calculate vapor transmission. Perms measure the grains of water transported per hour per square foot per inch of mercury vapor pressure (the difference between the inside and outside moisture vapor pressures on the surface or material).

So, multiply the perm rating times the number of square feet of the wall, times the vapor pressure differences on the two sides of the wall, and you can count the grains of water.

Question: Is it safe to add retrofit insulation without also adding a vapor barrier?

Answer: If you add fibrous insulation to a cavity wall, it will increase the risk of a wall condensation problem and may exacerbate existing problems such as peeling paint. [Both Bliss and Friedman report having inspected buildings whose exterior paint was intact and sound until soon after insulation (without vapor barriers) was blown into previously empty wall cavities of homes in northern climates.-- DJF]

Nonetheless, various field studies in both moderate and cold climates have failed to find serious problems in the walls of retrofitted homes with or without vapor barriers. There are mitigating factors in older homes. Of the ones that were monitored for relative humidity, few were much over 40 percent. Plus, most had highly permeable wood plank exterior wall sheathing, which tends to store and release any moisture condensate.

A reasonable approach would be to seal around moldings, electrical outlets, and other wall penetrations and keep building interior in the 40 percent range. When you redecorate, consider vapor barrier paint on the interior surface of exterior walls.

[In other words, we probably agree that where a newly-insulated older home has had a serious paint failure, there was most likely also a pre-exiting indoor high moisture level and indoor leaks or moisture problems, such as a wet basement or crawl area -- DJF.]

Two detailed articles discussing insulation retrofits, air leaks, moisture problems, and insulation effectiveness are at ENERGY SAVINGS RETROFIT CASE STUDY and ENERGY SAVINGS RETROFIT LEAK SEALING GUIDE.

Question: What conditions create high indoor humidity?

Answer: In a very tight house, the normal moisture generated by human respiration and perspiration, along with cooking, bathing, and cleaning, can cause a moisture buildup. With additional moisture sources (building leaks, wet basements), high moisture levels can build up even in a not-so-tight building.

A frequent cause of high indoor moisture is the presence of a dirt floor crawl space, even if there is no obvious crawl space flooding. A water table three feet below the soil surface of a dirt floor basement or crawl space can release 12 gallons of water vapor per 1000 square feet in one day.

Covering the soil with a heavy polyethylene plastic cover should reduce this moisture movement into the home by about 80 percent and reduce crawlspace ventilation requirements by a factor of 10.

See these crawl space ventilation and dry-out articles:

Other building moisture sources are un-vented clothes dryers and combustion appliances, drying firewood indoors, and house plants.

See MOISTURE CONTROL in buildings for an extensive list of diagnostic and "how-to" articles on controlling moisture in buildings. Also see HUMIDITY LEVEL TARGET.

Question: How can I both insulate and ventilate a crawl space?

Crawl space floor poly (C) Daniel FriedmanAnswer: one option is to insulate the floor above the crawl area with a vapor barrier on the warm side of the insulation (over the joists) and to leave the crawl space vented in all but the coldest weather, perhaps using thermally operated foundation vents (1980's convention). Low permeance rigid foam board insulation is the best product to use here because it will also resist forming a problem reservoir of toxic but hidden mold. (See Mold in Fiberglass Insulation).

Current (2009) best construction practices no longer ventilate crawl spaces; rather we convert the crawl space to an insulated, "conditioned" space, making sure that we keep out rot and mold causing water. That's because experience and field studies indicate that it is just about impossible to control crawl space ventilation to work optimally for all weather and building conditions. -- DJF

Our photo (above left) shows a poly moisture barrier placed over dirt in a crawl space - also notice that radiator in the right of the photo - the owner converted this crawl to a dry, heated space - what may be missing is foundation perimeter insulation, perhaps using foam board, unless that step was already taken outside.

See these crawl space mold, ventilation and dry-out articles:

Question: Why do I have to ventilate an attic or cathedral ceiling if I don't have to ventilate the building wall?

Answer: No vapor-retarding system is perfect. And due to the stack effect (air movement upwards in buildings as warm air rises), a disproportionate amount of moist air will find its way into the ceiling cavity or attic space. Also, attic and roof ventilation help for summer cooling, ice dam prevention, and a cooler attic means a cooler roof deck which means longer roof life. See ROOF VENTILATION NEEDED? and also see ROOF ICE DAM LEAKS for more details.

Question: How about insulation and vapor barriers for a full basement: where does the vapor barrier go?

Answer: My opinion is that the basement wall should be treated much like the rest of the building shell - waterproofed on the outside (or more important, keep surface runoff and roof spillage away from the building foundation), and vapor-proofed on the inside (if you are finishing the basement interior walls).

Exterior foundation insulation will help keep the foundation wall warmer and less likely to condense water in winter and summer. By the way, if you've got standing water or even occasional wet floors in the basement, vapor barrier placement is a moot point - you need to solve the water problem first.

See WATER ENTRY in BUILDINGS, and the detailed articles that appear under that category.

Question: Where do we place the vapor barrier in raised floors such as buildings constructed over open crawl spaces or on pilings?

Construction on piers over water - pilings, moisture, movement, vapor barriers (C) D FriedmanI am designing a pile supported wood building building and would like to know where to place the vapor barrier. Was thinking about placing is below the wood sheathing above the TJI , however this would not allow for the wood sheathing could not be glued to the TJI.

The under side of the TJI will be sheathed and could used bagged insulation however am concerned about the vapor getting stuck between the floor sheathing and sheathing attached to the underside of the TJI. The building is to be constructed in dead horse alaska and temperatures out door could be -50 degrees +-.

Do you have any recommendations on placement of vapor barrier in the floor system. Is it needed? Thanks for your time Chad


Our photo (left) shows an older building in northern Maine. The structure, built on pilings, is exposed to both cold weather and high humidity from the Maine coast and waters from the Bay of Fundi, though it is not exposed to temperatures as cold as those in the question above.- Ed.

1) Exterior plywood sheathing is a moderately effective vapor retarder with a perm rating of less than 1.0 as long is it is kept dry and the relative humidity is low. When plywood is wet or relative humidity is high, its permeance will range from above 1 to as high as 10.

This is because wood is a “hygroscopic” material that readily absorbs and releases humidity. If you are using T&G plywood subflooring, you will have an effective vapor retarder (and an air barrier, as well, if you caulk the joints with a long-lasting sealant such as urethane or silicone) without the use of plastic sheeting. If you are required by code to use a separate vapor retarder, you could place it in strips between the TJIs under the floor sheathing to allow proper gluing of the subfloor.

To avoid trapping water against the plastic sheeting from spills, plumbing leaks, or other sources, I’d suggest using felt paper rather than plastic. Like plywood, felt paper can absorb and release moisture and its permeance rises when wet, allowing drying to take place.

2) Most moisture movement is from air leakage, not vapor transmission through materials, so vapor retarders are less of an issue than once thought. In a cold climate, air and moisture are generally moving upward so movement of moisture down into the floor structure should be minimal.

3) To be on the safe side, I’d recommend using a vapor permeable material on the underside of your floor. Possibilities are fiberboard products or expanded polystyrene (or housewrap if you do not need a solid material).

4) Local requirements: We in the lower 48 are not used to -50 degree temps, so I suggest talking to local building experts about special details and code requirements for your area.


Steven Bliss, Building Consultant, Burlington, VT

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

This article series about building vapor barriers and condensation in buildings series begins at part I, VAPOR BARRIERS & CONDENSATION in buildings, (when and why condensation occurs inside buildings), explains the problems caused by excessive indoor condensation, explains how moisture enters building wall and ceiling cavities, and summarizes the best approaches to prevention of indoor moisture and condensation problems).

Part II at VAPOR CONDENSATION & BUILDING SHEATHING (detailed questions and answers about various building wall sheathing and insulating materials and their impact on building condensation problems) is followed by VAPOR BARRIERS & AIR SEALING at BAND JOISTS. Readers should also see VAPOR BARRIERS & HOUSEWRAP.

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