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Seeking Advice on Insulation & Partial Conditioning of Crawlspace

Posted by mark_g (My Page) on
Thu, Sep 9, 10 at 8:33

We're completing our build in NY State: house sits on 2000sf crawlspace of which approx 2/3 nom 6' height, 1/3 4' height. We have taken good care to prevent moisture infiltration, but the area is still damp/clammy, esp in the shoulder months (house about to be insulated and hvac not yet operating). The crawlspace is only accessible from the outside via bulkhead door (no connection or stairs from inside the house).

We have decided to insulate and partially condition this space (meaning to maintain 55-60 year-round). The house hvac is installed, and has no air supplies or returns to the crawlspace (we maxed out the cfms of the unit with the house).

This raises some questions:

1) We're planning to use 2" closed cell spray foam on rim joist and all the way down the walls. The slab floor was not insulated, but it has an excellent vapor barrier.

I have read that the rim should not be completely covered by foam--to leave a small area for visual termite inspection *and* for any vapor drive issues that might come from outside. But I cannot find specific details abt where that gap shd be, and how to achieve it?

2) Since this is crawlspace (never planned for living), and only accessible from the outside, will firestop (intumescent paint) be necessary over the foam to meet code? Our AHJ does not have much experience with this type of insulation and we have a good relationship--so he requests I give him details as published and will accept that as the standard.

3) To partially condition the space, we want to install an energy efficient unit to keep temp at 55 and humidity at <40% year-round. Is there a combo unit that will do that? We're looking at better dehumidifiers made for lower temp environments, but they're expensive and use a lot of electricity. Our hvac sub proposed a minisplit heat pump for both heating and dehumidification, but it seems most are set up for cooling which is not what we need, and the heating we will need will likely occur in the coldest months of winter, when the heatpump will prob not be efficient. We have propane and can use that for heating, but that means two different systems to control the temp/humidity. So the question is, is there an hvac unit that can operate in these conditions with high efficiency?

4) Lastly, we are concerned abt cold floors on the first floor of the house. Wd it make sense to do some insulating under that floor? We can spec open cell, closed cell, or blown-in mineral wool in every joist bay that's not occupied by hvac supplies and returns. I am thinking that 1" of closed cell wd slow transfer of heat and provide some sound deadening at nominal upchg, b/c it can't be used everywhere. But I don't know how that will affect wood floors above, for example. The mineral wool and open cell will allow some perming--is that good or bad in this application?

In all the houses I've built over the years, never have I given so much consideration to the non-living, partially conditioned spaces. But it seems as everything else gets tightened up, the requirements in this area increase, too.

Thanks so much all for your feedback. Worthy, I hope you're on line for this one!

Best, Mark

Follow-Up Postings:

RE: Seeking Advice on Insulation & Partial Conditioning of Crawls

Below is the relevant NYS Building Code section. This is from the draft Code, but later bidding specs I have seen on-line use the same language.

2603.4.1.6 Attics and crawl spaces. Within an attic or crawl space where entry is made only for service of utilities, foam plastic insulation shall be protected against ignition by 1.5-inch-thick (38 mm) mineral fiber insulation; 0.25-inch-thick (6.4 mm) wood structural panel, particleboard or hardboard; 0.375-inch (9.5 mm) gypsum wallboard, corrosion-resistant steel having a base metal thickness of 0.016 inch (0.4 mm) or other approved material installed in such a manner that the foam plastic insulation is not exposed. The protective covering shall be consistent with the requirements for the type of construction. has a lot of detailed information on thermal and ignition barriers.

You might also simply post on their forum and get an answer from industry experts on spray foam and thermal barriers.

The termite inspection gap is mentioned at the US Dept. of Energy site. I think this is more important for exterior installation. Even then, there's at least one brand of foamboard that isn't susceptible to termites. So I'd spray foam the sills entirely. That's what I do with basements now. But on the exterior, leave a min. 6" gap between the ground and wood elements of the home and always check for shelter tubes. Depending on your climate, you might want or have to leave a larger distance.

Since it's unvented space, I'd also treat it as conditioned space using one of the three variations mentioned by BSC in RR 0401. That means no insulation on the ceiling of the crawlspace, only on the walls and floor.

Here is a link that might be useful: Crawl Space Venting and Insulation

RE: Seeking Advice on Insulation & Partial Conditioning of Crawls

Thank you, Worthy! All this is very helpful.

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