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R-value for floor over uninsulated crawlspace?

There are standard recommendations for high-performance R-values in homes, from the ground up -- R-40 for walls, for example, but typically much less for slabs (not going to enter that controversy by naming a number).

For various reasons, I'm considering building over an uninsulated crawlspace. What would you recommend as a rule-of-thumb R-value for a high-performance home, for the floor system, in this case? Since it won't be thermally buffered the way a slab would be, I'm thinking it should be closer to R-40 than, say R-10. But doing something less than R-40 would be preferable due to the complications of a floor system with that much insulation. Thoughts?

Asked by Anonymous
Posted Dec 8, 2009 1:44 AM ET


5 Answers

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Of course, the bare minimum is code compliance. The 2006 IRC (in Table N1102.1) has a minimum requirement for floors to be insulated to R-30 in climate zones 5 though 8 (Ohio, Pennsylvania, Massachusetts, and farther north).

Rather than following a prescriptive code, it's possible to do a performance design using trade-offs. A wide number of software programs allow you to experiment with the effect of lowering the R-value of one component and balancing that heat loss by increasing the R-value of another component. Of course, trade-offs have their critics -- since such trade-offs can easily result in comfort problems -- but playing with these software programs gives you a feel for the energy effects of changing insulation levels.

If you are going for an above-code home, you will probably want to insulate your floor to a higher level than R-30. Remember, heat doesn't rise; heat moves from hot to cold in all directions. If you choose to use fiberglass batts to insulate your floor -- not my first choice -- be sure to install a layer of rigid foam on the underside of the joists, with sealed seams, to create an air barrier.

Answered by Martin Holladay
Posted Dec 8, 2009 5:15 AM ET


Martin, I was thinking I would run some calcs, but at this point I wanted to get a rough idea of all of the assemblies. Packing R-30 or more insulation into a floor system implies certain design considerations. I was thinking, per your note on thermodynamics, that if a high-performing wall is understood by some to be R-40, then the floor in this case is a wall and should be at least R-40.

I was looking for some details on GBA for this situation, and came up empty. Did I miss something? My preference would be to insulate with dense-pack cellulose, and if I needed to use a rigid board, then to use mineral wool rather than foam. I typically like to frame with dimensional lumber sawn onsite, but perhaps to get enough insulation value in a floor it would be wise to go with engineered joists?

Answered by Tristan Roberts
Posted Dec 8, 2009 8:59 AM ET


There's one critical piece of information missing from your question, Tristan: What climate zone are you in? If you're in Tuscon, Arizona, floor insulation won't be critical. If you're in Minneapolis, you definitely don't want to go lower than code via tradeoffs, as described by Martin. If you're in Macon, Georgia, R-value's not that important, but moisture certainly is.

In a hot-humid or mixed-humid climate, I'd strongly recommend an encapsulated crawl space, with insulation on the foundation walls rather than the floor. There's usually less area to insulate, and it doesn't fall off the way fiberglass batts do in floor systems. The main benefit in the humid climate, however, is moisture control.

I'd recommend an encapsulated crawl space for cold climates, too. It's just a better way to deal with the space. You didn't mention having considered encapsulation, so if you don't know about that option, go to Advanced Energy's page about their research and recommendations for crawl spaces at crawlspaces.org.

If you do end up insulating your floor, I agree with Martin that batts are a poor choice. You can install foamboard below the joists, as he described, but that can difficult to do with all the penetrations for electrical, HVAC, and plumbing. Batts with the steel pins holding them up do a terrible job. Spray foam is your best option.

Allison A. Bailes III, PhD
Energy Vanguard

Answered by Allison A. Bailes III
Posted Dec 8, 2009 9:04 AM ET


Perhaps the main reason that you didn't find a lot of discussion of how to insulate the framed floor over a cold-climate crawl spaces at the GBA site is that the best practice is to seal the crawl space and install insulation on crawl space walls and, if necessary, on the crawl space floor (rather than installing insulation in the framed floor above the crawl space).

If you decide to go ahead and insulate the framed floor, your plan to use a mineral wool board under the joists and dense-packed cellulose between the joists will work fine — as long as you are able to create an air barrier with the mineral wool board.

Answered by Martin Holladay
Posted Dec 8, 2009 10:05 AM ET



Assuming your crawlspace is closed to the outside (not a floor built up on piers or dry-laid stone), and not ventilated (which is the current recommendation for cold climates in the building science world), then a floor is NOT a wall. Even with maximum insulation values in the floor assembly, the crawlspace will still be semi-conditioned by both the earth and the heat loss downward.

If your preference is rough-sawn lumber for framing, then there's no reason to go to engineered lumber since a full-dimension 2x12 will offer R-39, even with the thermal bridging through the joists (24" oc). With semi-rigid mineral wool board under the joists, you would be beyond the R-40 "high-performance" standard.

What I would suggest however, is the installation of a taped-seam polymeric housewrap over the joists and under the subfloor, both to create an air barrier but more importantly to prevent spills or floor mopping from saturating the cellulose.

I designed just this kind of floor assembly last year for the Yestermorrow Natural Building Intensive project at Knoll Farm in Fayston VT. It was a drained rubble-trench foundation with an 8" reinforced concrete grade beam, rough-sawn cedar sills and a rough-sawn 2x12 floor assembly, dense-packed with cellulose.

I used 4" wide ledgers nailed under and parallel with the joists to support short lengths of 1" rough-sawn pine boards, on which was installed filter fabric to contain the cellulose and allow breathing downward. I installed filter fabric around the perimeter of the floor assembly, installed an upper 2x6 rim joist (to support the wall assembly above), and left the lower half of the rim joist off until cellulose was blown through slots in the filter fabric (after roof assembly). The joists were covered with Typar before the rough-sawn subfloor was installed.

I would be glad to consult on this if it would be helpful.

Answered by Robert Riversong
Posted Dec 8, 2009 2:36 PM ET

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