Hi All

Just looking for some suggestions on insulating under the floor of our cabin. Well, first it's really a bungalow built in the early 1930s. It's built on about two foot high concrete forum with a dirt floor crawlspace. Is it possible to use spray foam insulation under the floor or is it better to just use fiberglass? Will insulating interfere with the ventilation of the house? Thoughts?
Linda
Seattle, Wa

The only concerns I could see about spray foam is the vapors in such a confined space overpowering the sprayer(depending on which foam), and the difficulty of actually maneuvering the spraying wand and getting it in the cavities right. It probably could be done, but it might be overkill, depending on how the rest of the house is insulated. Unless you have got some really great wall/attic insulation I would recommend fiberglass. You also might consider converting the crawl space from a vented to an unvented. You can find out more about that by just doing a google search.

Spray foam insulation will work just fine in crawlspace applications.
However, I might consider an unvented option. In this situation, the insulation is applied to interior of the foundation walls instead of between the floor joists. This and two layers of 6 mil poly groundcover and you have a sealed, conditioned crawlspace. Very good for plumbing pipes and any HVAC ducts that might be located in the crawl.

Wes;

We have done the unvented/insulated wall crawl spaces on a couple of jobs and it works great

Spray foam works just fine, but at a premium in cost. It's worth paying the premium in many instances to eliminate infiltration between the first floor and the crawl space. (It'll interfere with the ventilation of the house alright, but in a GOOD way, by creating a reliable air-barrier. Fiberglass is the antithesis of air-barrier.)

Whether you can get away with insulating the crawlspace walls, sill & band-joist an use a vapor barrier on the floor depends on a number of factors (not the least of which is selling it to the local building inspector.). Half-pound foam (Icynene, et al) isn't always happy in ground-contact situations (leaks, gets water-saturated, etc.) and isn't a very good vapor-barrier in thinner applications. Two-pound foam (Corbond, et al) can work, but is typically ~1.5x the cost per equivalent R-value of half-pound foam. But insulating the walls of the crawl space won't insulate the living space from the ground, so it won't be as-effective as insulating the floor, in heating dominated climates. Earth-coupling with insulated crawlspace walls can be a netenergy savings in cooling dominated climates, but even there it requires mechanical ventilation to limit crawlspace humidity when you go that route. In heating-dominated Seattle, insulating the inner 2' perimeter of the dirt floor with 1.5-2" XPS would be a good idea if you went the insulated-wall route.

There's also the fire-proofing issue- many/most building codes require a fire barrier over foamed insulation, even the non-flame spreading types, due to a toxic-smoke potential, even when it's in a crawlspace, not living space. In most places that means a minimum of 1/2" of sheet rock (use a mold-resistant variety in a crawl space), which adds some complexity & expense to the install. Any XPS floor insulation would need thin slab of concrete over it to meet fire code in a lot of places.

If you end up going with lowly fiberglass, note that the higher density "cathedral ceiling" grade is measurably more infiltration resistant than the cheap stuff. Seal the floor from below the best you can with caulking/expanding foam sealant etc.), and be sure that the fiberglass is installed in full contact with the sub-flooring to avoid creating thermal-bypass tunnels along the joist bay. Sealing the just the band-joist & sill with foam may not require a fire-barrier (check local code), and will certainly slow down the air infiltration. (Sill & band-joists are often 30-40% of the total air-leakage in homes- greater than the windows & doors!)

No matter what you do, covering the dirt with 6mil polyethylene, sealed to the walls with duct-mastic will reduce the mold-potential inside the crawl space, with or without mechanical ventilation. Where multiple sheets are required, lapping the seams by 12" with a stripe of mastic at the 6" point, the taping them with FSK tape (not duct tape) will form a reliable near-perfect vapor seal. Sealing the foundation with concrete sealer isn't a bad idea either, but air-sealing the sill & band joist usually is a bigger factor in moisture transport than wicking of the concrete.

http://www.seattle.gov/DPD/Publications/CAM/CAM322.pdf

In Seattle a  1/2" thermal barrier required between the insulation and the habitable spade, no exceptions for crawlspaces:

 "A thermal barrier, equivalent to a minimum ½-inch gypsum board shall separate the insulation from the interior of the building "

No specific exceptions are made for attics & crawlspaces. If you had a concrete slab first-floor or installed 1/2" of gypsum on the ceiling of a crawlspace insulated with spray foam you'd be legal.  Seems like they'd disallow application directly to the underside of the sub-floor here too, unless the finish floor was an adequate thermal barrier (I suspect ceramic tiles over portland cement floor leveler slab could be argued, but not 3/4" of birch over 3/4" of ship-lap or t & g  fir.)

In Seattle soil contact is strictly disallowed:

"the insulation shall not be installed on the exterior of foundation walls or below floor slabs on grade or in
contact with the soil”

There are several places we might take exception to the code in Seattle (like the prohibition against application directly to the underside of roof decking) but it is what it is... (It's always good to check- but most installers are aware of the particulars in their area.)

Dan1

  thanks for proving the point that it is a thermal barrier and not Fire as you originally posted

There is no exception to the code because common sense prevails. In Seattle and elsewhere most people to not live in their crawlspaces and attics, therefore that is not habitable space.

the 1/2" thermal barrier for the attic space is the ceiling drywall the 1/2" thermal barrier for the crawl space is the floor subfloor

:-) :-) :-) :-) :-)

....as if....


 Local building inspectors aren't always up to the "common sense" standards.  Hopefully they would be in Seattle (although 30 years ago when I last lived ther that wasn't always the case.)

Dana1;

Do you really consider a crawl space and attic habitable space? or think an inspector does?

Habitable, no.  A potential flame source, depends.

But it's not what you or I think, it's what the letter of the code is, and how far the inspector wants to push it.  Local installers will have a better idea about the codes & personalites involved.  (I have plastic insulation without thermal barrier in both my attic and crawlspace, but please don't call my local building dept. on it, 'cuz I KNOW they'd make an issue of at least some of it. :-)  Reasonableness is in the eye of the beholder, and some o' them beholders are downright crosseyed. ) 

Where the local building codes use "interior of the building" language instead of "living space" or "habitable space" you're stuck running to the law library to look up how local case law has defined the term. It varies more than I like (and probably more than you think.)

But nowhere have I seen 0.75-1.5" of horizontal sawn wood planking/decking deemed an acceptable thermal barrier without being treated with fire-retardents. In Seattle the typical ca 1930 sub floor is ship-lapped 8" douglas fir- makes nice kindling, it does.  If it were 3/4" tongue & groove Georgia yellow pine it might pass an ASTM 15 or 30 minute fire test though.

Codes in the southern US are somewhat more relaxed than in New England or the Pacific Northwest but not uniformly.  Often reduced thermal barrier minimums (like 3/8" gypsum instead of 1/2") is explicitly called out in codes for plastic insulation in attics & crawl spaces.  In Seattle this doesn't appear to be the case- they don't spell it out for you.  Where it's not clear, ask the local installer what to expect, and pull a building permit so they can't make you hack it out after the fact.

Dana1 ; 

My front porch is a potential flame source.

please cite the section of the code that designates attics and crawlspaces as habitable?

1930's 3/4" douglas fir does meet the 15 min thermal barrier by definition of the code

Most codes are based on the IRC

Relaxed in the south?

If you ever built here you would know that we build under the most stringent codes in the ALL the US

I came from the northeast US where there were no enforced codes in NY & PA I never had to call for inspection until I moved to FL in 1984

You need to get the facts straight before you make assumptions