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Roxul comfort board in basement?

I spoke to someone at the Roxul company the other day who recommended the following basement insulation. First, tyvek or similar against the concrete basement wall, then 1 1/4 " roxul comfort board for a thermal break, then a stud wall filled with roxul batts--- sheetrock. Do you see a real problem with water condensing on the concrete with the comfort board and tyvek there? Still seems like too much work but I am curious performance wise.

I read the article about roxul board as a foam alternative under siding. It seems like such a solid long term product compared to foam.

Hopeful for cost effective foam alternatives for basement--- no luck yet.

Asked by Nyle Phillips
Posted Sep 27, 2012 11:50 AM ET


5 Answers

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I strongly advise you not to use this approach.

Your concrete basement wall is in contact with the soil and is usually cool or cold. When humid indoor air contacts this cold concrete, condensation will form.

If you install insulation on the interior side of the concrete, you will ensure that the concrete stays cold.

If you install an air-permeable insulation like Roxul, the humid indoor air will have easy access to the cold concrete (or the cold Tyvek). You don't want that.

Instead, you want a type of insulation that is air-impermeable. I suggest that you install rigid foam board or closed-cell spray polyurethane foam.

Answered by Martin Holladay, GBA Advisor
Posted Sep 27, 2012 12:55 PM ET


Hi Martin, what if you used a smart vapor barrier on the inside, under the drywall? Say MemBrain or the likes? I too am looking for rigid foam alternatives and even now, 2 years from the date of original post, there is not much to choose from. My walls are block, a few rows below and rest is above grade.Climate zone 6. Alternatively, what do we know about fire retardands from rigid foam making it to the indoor air, when covered with drywal/texture/paint?

Answered by Igor Orlovich
Posted Oct 30, 2014 12:05 PM ET


My advice remains unchanged. I don't think that there is any way to install MemBrain in such a way that you will avoid the problem I describe.

I have never heard of any human illness that was caused by flame retardant chemicals in rigid foam when the foam was installed behind gypsum drywall.

Answered by Martin Holladay, GBA Advisor
Posted Oct 30, 2014 12:37 PM ET


Martin, I had a thread going a while back on this same topic. At the time I was persuaded by your advice, the same offered here. I ended up using polyiso for the project, but in the meantime I have had an extended back-and-forth with the technical person at Tremco Barrier Solutions in regard to their Fiberglass Insulation Board. A company pamphlet I picked up at a green building expo endorses this product for basement interior walls.

His argument: "As with most moisture topics relating to the home envelope, a simple condensation model [i.e. any condensation = problems] is not sufficient to characterize what is really happening and whether or not it creates a problem. As discussed above, moisture storage, temperatures, and durations are also key variables."

My question to him was about a hypothetical basement in CZ4, southern Ohio. He went on to say:

"To initiate mold growth, you need some specific conditions; a food source, enough moisture to have "free water", temperature above 39 F, and about 30 days where all of the conditions exist simultaneously. Looking at the monthly average temperatures in the Cincinnati area, the temperatures winter season temperatures are generally warm enough: Nov=55, Dec=44, Jan=39,Feb=44, and Mar=55 (although Jan would cycle below 39 regularly).

However lets look at "free water" that is likely to exist on the concrete wall surface. Concrete will absorb moisture, so there must be sufficient condensation to outrun the normal capacity to harmlessly absorb water that condenses on the surface. If the basement is maintained at typical indoor conditions , say 70 F and 35% RH (though the RH is probably lower in the winter), condensation only occurs on the wall surface when the exterior temperature is about 42 F or less. Since vapor is free flowing though un-faced fiberglass, as the exterior temperature is above 42 there is net evaporation from the wall surface, below 42 there is net condensation. Since this is occurring daily and the average temperature is above 42 for Nov, Dec, Feb, and Mar, there will be no net condensation in those months. [If the product had a non-permeable facing (hindering free evaporation) moisture could accumulate].

The last month to consider in January. In Jan the temperature is typically cycling to levels below 39 F; which is not conducive to initiation of mold growth. There is also cycling of temperatures above and below 42 so both evaporation and condensation are occurring, though there is modest net condensation. However, the net condensation is not sufficient to overwhelm the storage capacity of the concrete, so there is insufficient "free water" to support initiation and continuance of mold growth."

He also described mold problems behind XPS on basement walls. Of course he works for a manufacturer, but he's paid to communicate the science behind their product and, presumably, to avoid lawsuits over product failure. Aren't there some definitive studies that can be brought to bear on this? Are there some test walls somewhere?

Answered by Andy Chappell-Dick
Posted Oct 30, 2014 8:08 PM ET


First of all, I'm happy to stipulate that it's sometimes possible to insulate the interior side of a basement wall with fiberglass batts without incurring mold or moisture problems. However, this approach is risky -- so risky that it's not worth using the method.

There are a lot of variables at play. If you're lucky, and your basement is dry and warm, conditions favoring mold development or extensive condensation may never happen. If you are unlucky, however, your wall will be a moldy mess. You are rolling the dice. So I advice builders to use rigid foam in this location.

Q. "Aren't there some definitive studies that can be brought to bear on this? Are there some test walls somewhere?"

A. Tests have been conducted and model homes have been built by Building Science Corporation, and studies have been conducted by Paul Ellringer (see Ellringer, P., May/June 2002, "Minnesota Mold Busting," Home Energy Magazine) and Don Fugler (see Fugler, D., March/April 2002, "Dry Notes from the Underground," Home Energy Magazine).

You may want to read a Building Science Corporation report titled Basement Insulation Systems. Here is a quote from that report:

“The approaches used early in the [Building America] program were interior stud wall framing insulated with fiberglass batts and ‘blanket’ insulation. These two approaches are the most common approaches to basement insulation used by the home building industry in general. The experience by the Building Science Consortium with these two approaches has been bad. The Building Science Consortium has concluded that these two approaches are unsuitable for use by the home building industry due to serious problems associated with mold, decay and odors. This is consistent with reports from Canada where basements are insulated in a similar manner (Fugler, 2002) and from other researchers in the United States, notably in Minnesota (Ellringer, 2002). Continued use of these approaches by the home building industry will likely lead to a disaster of unprecedented proportions and may result in the construction of energy efficient homes being set back a generation.”

Answered by Martin Holladay, GBA Advisor
Posted Oct 31, 2014 4:44 AM ET

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