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Is it possible to eliminate condensation with insulation?

I was wondering if it was possible to insulate so much that the dew point actually occurs in the insulation? Also if this was to occur would it eliminate condensation or would it still occur within the insulation? I can see this as maybe being a possibility in a ceiling but would it be possible in a wall?

Asked by Anonymous
Posted Dec 23, 2009 11:18 AM ET
Edited Dec 23, 2009 11:57 AM ET


15 Answers

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Building scientists have learned that condensation almost never occurs inside insulation. As William Rose wrote in Water In Builidings, "The language reaching dew point seems to indicate that one could plot a temperature profile through a wall, find the point where that profile intersects a horizontal line indicating indoor dew point temperature, and expect burgeoning water at that location. This impression is decidedly incorrect. If water accumulates, it does so on the surfaces of materials, not within the thickness of materials."

It is, however, possible to design a wall with cold sheathing. If interior moisture is allowed to reach the back side of the cold sheathing, condensation can occur on the sheathing.

Answered by Martin Holladay
Posted Dec 23, 2009 11:56 AM ET


The best way to eliminate potential condensation problems within the thermal envelope is to prevent moisture from getting in, and allowing any moisture that does get in to dry out.

Since almost all moisture that gets into a thermal envelope from the inside in a cold climate is from air movement, what's required is an excellent interior air barrier and no vapor barrier to prevent drying.

Answered by Riversong
Posted Dec 23, 2009 1:58 PM ET


Martin, is it a proven fact that moisture doesn't condense within insulation materials like fiberglass or cellulose? If so, that's good and I would appreciate if you could recommend some reading on the subject. I just have a hard time wrapping my head around the concept. Fog for instance is water vapor condensed onto condensation nuclei, airborne particles with surfaces smaller even than a single strand of fiberglass.

Answered by Lucas Durand - 7A
Posted Dec 23, 2009 8:59 PM ET


The best resource to answer your questions is the one I cited, Water In Buildings by William Rose.

For a variety of reasons, some of which I understand and some of which I don't quite, condensation in wall and ceiling assemblies just doesn't occur within insulation. If the fibers get cold enough to permit condensation, then adjacent solid surfaces -- usually the sheathing -- are also cold enough to permit condensation, and the solid surfaces are the ones where the moisture condenses first.

Answered by Martin Holladay
Posted Dec 23, 2009 11:01 PM ET


Of course water vapor condenses on glass fibers (or anything else) when the local dew point is reached. What Rose was refering to in his book, Water in Buildings, was that Teasdale's 1937 treatise on "Condensation in Walls and Attics" oversimplified the phenomenon by using the temperature gradient within an insulated wall and the indoor air dew point rather than the local dew point which would be dependent upon the moisture gradient.

But condensation within fiberglass insulation would be very diffuse and, because it would not be absorbed, would not accumulate to the point of causing problems beyond the degradation of the insulating properties of the fiberglass.

Condensation tends to occur in noticeable quantities and cause problems at surfaces where there is a sudden change of permeance, which causes an increase in local relative humidity sufficient to create dew point conditions. Additionally, if the condensation occurs on a hygroscopic surface (such as wooden sheathing), then moisture is absorbed, lowering the vapor pressure and increasing the vapor pressure gradient, driving more moisture toward that surface. This becomes, effectively, a water vapor "pump" which continues to drive moisture toward the condensing surface.

Answered by Riversong
Posted Dec 23, 2009 11:23 PM ET


Ok, that makes sense. Thanks guys.

Answered by Lucas Durand - 7A
Posted Dec 24, 2009 9:57 PM ET


The issue is not whether condensation can form within insulation - what happens is that condensate will settle to the lowest point. Inside a roof space that usually means into the insulation. The insulation then acts as a storage for the condenate, which as the temperature increases will evaourate into the interstital space raining the humidity. Without air movement within the space the incresed levels of humidity will result in the satuarted air reaching the dew point at progressively higher temperatures. Air circulation is not enough to ensure that you avoid condensation - correct application (location, type ) of insulation and control of the entry of moisture is also important, particularly as the addition of more insulation changes the nature of the risk.

Answered by Peter Sarlos
Posted Jun 25, 2010 3:25 AM ET


Isn't there some statute of limitations on reviving old threads?

what happens is that condensate will settle to the lowest point

This, too, is a vast oversimplification. Moisture moves by several mechanisms, including bulk drainage (which, I think, is what you're talking about), diffusion (absorption), surface diffusion (adsorption), capillarity, osmosis, convection. Only when moisture accumulates to gross quantities does it become subject to gravitational forces and fall the the lowest point, and this is rarely the case in structures except with bulk leakage.

Most moisture storage in a wood-framed building is in the hygroscopic materials, such as framing and sheathing, and in the insulation only if it too is hygroscopic, such as cellulose but not fiberglass. Desorption, or moisture release, can occur through the hygroscopic materials if they are vapor open on the exterior and the moisture drives are in that direction. Otherwise, moisture is removed by air convection, which is why attics traditionally were (and still are) ventilated.

Answered by Riversong
Posted Jun 25, 2010 12:14 PM ET


Isn't there some statute of limitations on reviving old threads?

I like to see old threads dredged up sometimes
This one had some extra good comments by You and Martin.

Bill Rose and his book are excellent.

Answered by John Brooks
Posted Jun 25, 2010 12:23 PM ET


You may notice that the google robot is posting an add for Bill's Book

My favorite book!

Answered by John Brooks
Posted Jun 25, 2010 12:24 PM ET


Otherwise, moisture is removed by air convection, which is why attics traditionally were (and still are) ventilated.

Unless you live in a Humid Climate.

Answered by John Brooks
Posted Jun 25, 2010 12:29 PM ET


Humid climate roof ventilation is problematic only if the average outdoor RH is greater than the average attic RH, or at those times when outdoor RH is greater than attic RH.

Attic RH without ventilation can easily reach saturation, even with a good ceiling air barrier, by solar vapor drive after a rain. If the outdoor RH is less than 100%, ventilation will reduce attic humidity under such conditions.

Answered by Riversong
Posted Jun 25, 2010 12:34 PM ET


I actually have a similar question, I was thinking of using zip system sheathing since I heard that was very waterproof, would that work?

Answered by Kevin
Posted Jun 30, 2010 1:36 PM ET


Your question is unclear. Huber Zip System sheathing certainly "works." But I'm guessing that you are asking a different question.

Perhaps you are curious to know the permeance of Huber Zip System sheathing? According to a Huber rep, the permeance is between 2 and 3 perms.

Answered by Martin Holladay
Posted Jun 30, 2010 1:56 PM ET



The ZIP System will do nothing to change the probability of condensation.

And the 2-3 perms they claim is based on an assumption of 2-3 perms for the underlying 7/16" OSB, yet all reputable sources suggest a dry-cup perm for 7/16" OSB of 0.75 - 0.9. It has a wet-cup perm of about 2, but that is not the industry-accepted test standard.

Answered by Riversong
Posted Jun 30, 2010 9:16 PM ET

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