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Community and Q&A

Trading Roxul for plastic foam in basement wall insulation.

user-1137156 | Posted in Energy Efficiency and Durability on

The usual advice for interior insulation of basement walls is use rigid foam because it is not air permeable so condensation will not occur. Cost of material for foam is near $0.12/ r sq ft. Cost of Roxul comfort bat about $0.045/r sq ft.

I will have an inner basement wall of 2×6 to support my first floor and the inner walls of the exterior double wall. My initial sketch shows 3″ foam and R-15 batts with no drywall. If I switch to all Roxul I would want airtight drywall. I already have more insulation than I can justify.

So if I change the plan and use R-10 Roxul for steel studs, installed through the stud spaces and stick with the 3 1/2″ in the stud cavities and add airtight drywall will the insulation cost saving pay for drywall? If I can add drywall for less than $1.35/sq ft. the answer is yes! If I also switch to R-23 between the studs adding about $0.32 /sq ft. I’ll actually increase the wall R and still more than pay for the drywall.

Since it is required that bottom plates sitting on concrete be treated no damage will occur even if there is some condensation on the concrete wall. I will need to add the step of caulking the bottom plates to the floor to assure air tightness along with all the other ADA steps. Have I missed something?

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  1. GBA Editor
    Martin Holladay | | #1

    I don't recommend using Roxul mineral wool to insulate the interior of a basement wall. The insulation is air permeable, and you will get condensation on the cold concrete.

    The Airtight Drywall Approach won't help you. For one thing, it can never be truly airtight. For another thing, where do you think the air that is behind the drywall came from? And the air between the fibers of the Roxul? That's interior air -- moisture-laden air.

    You can't keep interior air away from the cold concrete with a drywall barrier. What you need is a layer of air-impermeable insulation installed directly against the concrete. That means rigid foam or closed-cell spray polyurethane foam.

  2. user-1137156 | | #2

    I understand there will be condensation. The question is how much or maybe at what rate. To answer I gathered some facts by looking them up on the WWW to learn that if SATURATED air at 68 f is cooled to -20f about 1/1000# of water is removed from each cubic foot of air cooled. Knowing water weighs about 8.3 #/ gallon means 1/83000 gallons is removed from a cubic foot. of air. Lets assume my walls are 8 foot tall and 9" deep and that I have 220 feet of wall, about my actual size, They will hold 1320 cu ft of air and if all the moisture in saturated 68 f air condensed on the walls 10 liquid ounces of water.would be spread over 1760 sq ft of cold wall. That's a mighty thin layer of frost painted on the wall. Clearly the initial air volume's moisture is not a concern even if the air was saturated. With more typical indoor wintertime air conditions less than 1/2 the water is present.. Blower door tests measure air infiltration under an artificially high pressure differential but airtight drywall routinely shows 0.6 ACH@ 50 pa. It has been estimated blower door tests show 17 times the air infiltration that occurs under actual use conditions. Or about one air change per day is likely. Will 5 liquid ounces of water evaporate in a day? Will 5 ounces per day cause a problem? That is the size of the problem. Since my slab will be well above the dew point I seriously doubt any water will ever make it to the floor drain.

  3. user-1137156 | | #3

    Not that I'm at all worried about using Roxul & ADA inside but I'll abandon it in favor of my newest "hair brained scheme" . The new scheme eliminates a bunch of foam in the 'rim joist' area, yet improves it's thermal performance, places equal amounts of wood, bot cross grain and total, under my inner and outer walls assuring equal shrinkage and assures that the main floor walls share support of the roof. The new plan may be a bit less " buildable" and require an unusual order of events, it also will cost about the same with savings in foam costs spent on pressure treated " foundation grade" lumber.. Basically the new plan is to retain a " double wall" basement with one wood and one poured concrete, but swap the wood to the outside. Which is load bearing would also be swapped With the concrete basement wall being the primary, almost sole support.
    I'll start a new thread with more detail.

  4. Expert Member
    Dana Dorsett | | #4

    And of course moisure would NEVER be drawn through the concrete into the stud cavity either, right? ;-)

    Any stackup/design that relies on true perfection in it's implementation is doomed to fail. Even a half-inch of foam gives it considerable resilience, but design foam/fiber R ratios up to a reasonable dew-point is better, and nearly guarantees success.

    Using reclaimed foam from commercial roofing demolition runs about 3-4 cents/R-foot (yes, cheaper than Roxul.)

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