crawlspace/under deck insulation

[DanMc]

I am trying to address an insulation problem that the builders of my house created. I am attaching a sketch of the area in question to hopefully make it more clear. The house is two stories and it has an unfinished basement. My office is in a corner room of the first floor and there is a large bay window area which extends out beyond the corner of the foundation. The foundation has a 90 degree bend at the corner of the house. The exterior walls are mostly directly above the foundation but right at the corner, there is a bay window area which is like 5 of the 8 sides of an octagon. These windows extend out beyond the foundation. The support for this structure is cantilevered joists under the floor of the first floor. This bay window/octagon shape thing is continued on the 2nd floor. The floor in the room goes all the way to the edge (i.e. there are no ledges or seats by the windows).

What I notice is that the floor in that corner of the room is quite cold. For example, yesterday it was 67 degrees F in the house and 52 degrees outside. The floor temperature on that corner of the room was 57 degrees while the floor temperature on the opposite corner of the room was 65 degrees. On a "cold" day (I'm in Georgia so that means 25 at night) it is much worse in that corner. Still, I want to move my desk over there and it is cold on my feet.

The top of the foundation is about 2 feet above ground in that corner of the house.

When I crawl under there what I see is that the joists which hold up that portion of the wall are just totally bare and the only thing between my toes and the outside world are a hardwood floor and the subfloor. No insulation, no vapor barrier, no nothing. To make it worse there is a floor outlet over in that corner. I see the wires totally exposed to the outside world and so there is a cold breeze right around the outlet. The outlet plate measured only about 1 degree above outside temperature and 16 degrees below room temperature.

Referring to my sketch, the places marked as "Area A" are totally exposed joists and the bottom of the subfloor is totally exposed. The only thing between Area A and the basement is a short piece of 2x10 between joists on top of the foundation. There are gaps which let cold air into the basement.

The place marked as "Area B" has a plywood cover on the bottom of the floor joists to hide the insulated flexible duct which connects to the heating/ac vent in the floor. There is no other insulation or vapor barrier in that cavity. On the basement side, there is nothing else which blocks air or insulates in between those two joists. This means not only do I have poor insulation in my office but also there is poor insulation in the basement.

In the area under the floor outlet, again the bottom of the subfloor is totally exposed to the outside of the house as are the wires.


So... how do I properly insulate this so my feet are happier? Do I use paper backed fiberglass insulation down there? Or should I use that pink 2" thick foam stuff and glue strips up between the joists? What about in that cavity around the floor vent? Spray in some of that "big stuff" expanding foam insulation? How about vapor barriers? Do I need something? If so, what and should it go right against the subfloor or should I insulate, then staple plastic on the joists over insulation and then cover in plywood?

What I didn't show in the sketch is there is also a wrap around porch on the other side of the windows so I'll have to crawl on my back way up under the porch to do whatever insulating needs doing.

Grrrrr. House is only 30 years old... Let's not even talk about the mechanics of a cantilever like this for a 2 story structure. I'm leaving that repair job to a pro.

Thanks
-Dan

[mtcummins]

If you want to do a complete, nice job, and spend a little mula doing it, get a spray foam contractor to come out and fill the joist bays in the cantilever, and while they're there, also spray your entire basement rim joist, possibly your attic, etc if you see fit. Its going to be prohibitive to bring them out for a small job, but if you wanted to take care of a bunch of things at once, closed cell spray foam is the way to do it.

A DIY smaller scale solution would be the foam panels, cut almost to width. You're actually better off leaving about 1/4" on all sides, then use a can of Great Stuff to fill the remaining space. This will give you a better seal than you'll likely be able to do trying to do a tight fit with the panels. Cut out around the outlet box in the foam, then spray around it as well. For your duct, you might just want to push it up as high as you can, put some fiberglass or something up around the sides of it, and then install a couple panels of foam underneath it.

You can go too crazy here... the walls of your house are likely R19 at most, so putting 10 inches of foam in the floor (which loses much less heat than the walls/ceilings) would just be crazy.

If you want a bit more warmth and your basement is reasonably warm, you could take out the 2x10s closing off the openings between inside and out (make sure they're not in any way structural first), then install your first foam panel fitting tightly and leave a couple inches gap between it and the floor so that some warm air can get in there. But if your basement is fairly cool, this will not help at all.

[DanMc]

A DIY smaller scale solution would be the foam panels, cut almost to width. You're actually better off leaving about 1/4" on all sides, then use a can of Great Stuff to fill the remaining space. This will give you a better seal than you'll likely be able to do trying to do a tight fit with the panels. Cut out around the outlet box in the foam, then spray around it as well. For your duct, you might just want to push it up as high as you can, put some fiberglass or something up around the sides of it, and then install a couple panels of foam underneath it.

Do I just use some sort of construction adhesive to hold the foam in place?

Do I need some sort of vapor barrier in there somewhere?

And is it ok to leave the foam exposed or does it need to be covered? I'm wondering both in terms of any sort of fire code and in terms of protecting the foam.

Thanks for the pointers.

-Dan

[hj]

Use vapor barrier fiberglass with the barrier on top against the floor. Then put plywood on the bottom of the joists to seal it in.

[mtcummins]

if you want to do fiberglass, do as hj said. for foam, you won't need a vapor barrier, its part of the foam.

and yes, you should sheath the bottom to protect it from critters making a nice warm home in there, etc, either way. I'd run a bead of caulking or liquid nails around the perimeter first, to give it nice tight air seal between the joists/plywood. Don't use OSB here, too exposed to moisture - use an exterior grade plywood.

[cacher_chick]

If you want to do a good job air sealing and insulating, wait until it gets warm out, buy a froth-pak and spray it yourself. If you have extra you can do the rim joist/band too.

[Dana]

In any GA climate you're better off doing it without a vapor barrier. A full cavity fill of open cell foam would allow sufficient drying toward the exterior in winter, but wouldn't be so vapor-open as to cause rapid moisture loading of sub-floor during the cooling season (unless you really frosted yourself with 65F interior temps in summer.)

OSB would be fine as a bottom sheathing- it's not as if it is getting rained/ splashed on but plywood becomes more vapor-open than OSB if it actually DID get hit with a load of wind-driven rain during the hurricane of the century. If OSB works as wall sheathing (and it mostly does, if you do the stackup right), it's more than fine as the under-cantilever sheathing.

If you cut'n'cobbled rigid foam in there, you can use foam-board construction adhesive to tack it in, and FrothPak or Great-Stuff to seal it all in place. But virgin-stock EPS isn't really any cheaper than open cell spray foam, and required a lot more attention to detail to get the proper air seal. With open cell foam it should be a 2-pass job,~5"-6" per shot. If they try to do a single shot at 9" (a 2x10 joist) there's a high likelihood of shrinkage and separation from the joists, and a low but real risk of starting a fire (due to the exothermic reaction taking place as it goes up.)

If you can find a source for reclaimed rigid roofing foam (call commercial roofing contractors- they sometimes will scavenge goods from re-roofing and demolitions) the cut'n'cobble approach becomes much cheaper, but it's still a lot of neck-twisting labor to get it into a cantilevered floor like that.

It's not insane to do full cavity fill with open cell foam, and doing a full fill lowers (but does not eliminate) the possibility of insect or rodent infestation. If you only did 6"/R20 in a 2x10 joist you'd still be able to feel the difference in floor temp during the mid winter lows, and since you are required by code to put half-inch OSB or ply as an ignition barrier, and empty warm dry space like that becomes a very handy critter condo for rodents (been there, done that, and have the squirrel-tail nailed the the garage wall to prove it. :-) )

Plan-B would be to use dense-packed cellulose, which is more critter-repellent, but probably more expensive than open cell foam in a small space like that.

Fiberglass batts==mouse nesting material and it's not very air retardent. Don't go there, even as a last resort.

Insulating the interior of the foundation is cost effective on heating/cooling energy use in GA and provides some mid-winter comfort while lowering the summertime mold-risk. A couple inches of reclaimed roofing foam (any type) is the cheapest way to get there. To do it with a studwall and fiber insuation requires at least an inch of foam between the foundation and stud edges, but with rigid foam you can use furring through-screwed to the foundation on which you can mount the (code-required) half-inch gypsum as an ignition barrier for the foam. For more money you could use fire-rated poly iso foam board, but even if you never plan to use the basement as a fully-finished space, the cheap-foam + gypsum is well worth it- far more worthwhile and effective than insulating between the joists. Air-sealing and insulating the band joist and foundation sill with cut'n'cobble 2" EPS is also part of doing it "right", and worth it even if you never insulated the foundation walls themselves.