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New construction sealed crawlspace R-value and air exchange question

In climate zone 2 what is the required R value if doing sealed crawlspace and insulating foundation walls instead of floor? Table N1102.1 lists crawlspace walls. Is this for sealed and when not insulating the floor? That value is 0 for my climate zone. Is that accurate or is that for vented?

2nd question:
Should an air exchange (in addition to air supply to crawlspace) be installed in the above scenario? I think the code asks for it but I read somewhere (possibly on here) that the positives are outweighed by the negatives. Is that true? Is it preferable to not install that?

Thanks for any help.

Asked by C Smith
Posted Mon, 04/07/2014 - 19:30
Edited Tue, 04/08/2014 - 05:32

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1 Answer

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C. Smith,
You are reading Table N1102.1 correctly. In Climate Zone 2, there is no requirement for any crawl space wall insulation. This applies to unvented as well as vented crawl spaces.

Of course, you are free to insulate your crawl space walls if you want. It is always legal to include more insulation than minimum code requirements.

I explain code requirements for sealed (unvented) crawl spaces in my article, Building an Unvented Crawl Space. The code requires unvented crawl spaces to include a little bit of air exchange or space conditioning; these requirements apply in all climate zones.

Here's what I wrote in that article:

"The code lists two options for conditioning unvented crawl spaces; both options require the installation of a duct or transfer grille connecting the crawl space with the conditioned space upstairs. Option 1 requires ‘continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area.’ In other words, install an exhaust fan in the crawl space that blows through a hole in the rim joist or an exterior wall; make sure that the fan isn't too powerful. (The makeup air entering the crawl space is conditioned air from the house upstairs; since this conditioned air is drier than outdoor air, it doesn't lead to condensation problems.)

Option 2 requires that the crawl space have a forced-air register delivering 1 cfm of supply air from the furnace or air handler for each 50 square feet of crawl space area. (Assuming the house has air conditioning, this introduction of cool, dry air into the crawl space during the summer keeps the crawl space dry.)"

These measures will help keep your crawl space air dry. However, the requirement for conditioning does entail a small energy penalty. If you choose to install an exhaust fan in your rim joist, there is another issue: you have to remember to check the exhaust fan every few months to be sure that it is still working.

Some crawl spaces are dry enough that these conditioning measures aren't necessary. However, I wouldn't disable a crawl space exhaust fan or close an HVAC register serving a crawl space unless (a) I knew what I was doing, and (b) I was monitoring the relative humidity of the crawl space to make sure that the humidity stayed in a safe range.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 04/08/2014 - 05:31

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