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Spray foam insulation

danny77 | Posted in Energy Efficiency and Durability on

I have a 100 year old house in Zone 5. There is a sunroom (approx. 5’X20′) on the south side of the house which is all windows. It is built on piles (about 4′ to the bottom of the joists) and open underneath with lattice. The room is open to the living room of my home and is heated by 2 forced hot water radiators. When we bought the home the floor was insulated from underneath by batt insulation and rigid insulation between the joist cavities. We had an energy audit performed when we moved in and were told this was all good. Still, the floor has always been cold. This past year, some squierrels decided to make their home in the batt insulation. As a result, the insulation has been pulled down in some places. I am thinking of having the batt and rigid removed and spray foam installed in the joist cavities. Are there better options?

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  1. Expert Member
    Dana Dorsett | | #1

    If the rigid foam between the joists, or a sheathing attached to the underside of the joist?

    Does the foam have a facer, and are there any vapor barriers (polyethylene sheeting or foil)?

    Is there any structural sheathing to the underside of the joists?

    How deep are the joists?

    If there are any true vapor barriers, pull them out (kraft facers are OK.) Seal any seams or gaps in the band joist with duct-mastic, and caulk or foam seal the band joist to the sub-floor. This may require some dissassembly of what's there or not, depending on what you have.

    If it doesn't already exist, install plywood or OSB to the bottom sides of the joists (use ring-shank nails, or screws), then you can dense-pack cellulose in there leaving the existing insulation in places, or pull it all out and dense-pack. Seal the OSB to the framing at every edge with can-foam, construction adhesive or caulk, and seal any seams that aren't over framing with fiber-reinforced duct mastic. Seal any holed drilled for dense-packing with can-foam (or drill those holes with a hole-saw and re-install the plugs with construction adhesive.)

    If the joists are 2x6 the thermal bridging of the joists will be substantial, but it can be thermally broken putting 2" of EPS (about R8) which is sufficient for dew-point control at the sheathing. If the joist depth is are thicker, you would need to go thicker with the foam-R.

    While squirrels can and will chew through OSB or plywood, they usually don't. The fire retardents commonly used in cellulose are eye irritants. I've seen where a squirrel chewed a hole into a bag of celluose out in my garage, but it didn't nest there, nor did it use the material as bedding. The amount of material removed before it gave up was about the size of a tennis ball. (It took months for me to finally evict the sucker, but he did not continue his cellulose exploration, despite ample opportunity.

    "Borate only, sulfate-free" cellulose is recommended, since the sulfated fire retardents become corrosive when wet, but not the borates. Any product from National Fiber or NuWool are borate-only,- pull the MSDS sheets for products from other companies to verify the type of fire retardent. Most box-store cellulose contains sulfates, but most of the manufacturers will have a "stabilized formula" that is sulfate-free, which may be ordered through a box-store distributor.

    Any room with a lot of windows will have convection currents causing cool air to fall down the windows to the floor, lowering the temperature of the floor. It may be "worth it" to extend the existing radiator loop to include some amount of radiant floor for that room, which would counteract the convection-loop cooling of the floor nearest the windows, but that's a whole other project. A cheaper (but not as cushy) heating-system countermeasure would be to run a loop of fin-tube baseboard under the windows to interrupt the convection flow that may be super- cooling the floor near the windows.

  2. wjrobinson | | #2

    Add a radiant heat floor

    It'll be your favorite winter space

  3. danny77 | | #3

    Thanks for the response.
    The rigid is attached to the underside of the joists with the batt in the cavities. There's no structural sheathing and no vapor barrier. The lip of the lattice at the exterior is not deep, so I was hoping to limit the thickness of materials attached to the underside of the joists. The joist spacing is also uneven and not neccessarily parallel along the length of the room. That's why I thought spray foam would be a good idea to achieve both insulation and air sealing.

  4. Expert Member
    Dana Dorsett | | #4

    You can just dense-pack over the batts in that situation. You may need to install 1x4 furring 16" o.c. perpendicular to the foam through-screwed to the joists with pancake head timber screws to keep the rigid foam from popping off.

    Removing the rigid foam will result in cold-striping along the floor at the joist edges. How thick is it, and what type? Any facers? How deep are the joists?

    Dense-packing does a pretty good job of tightening it up- almost as good as an inch of closed cell foam, but not as good as a full cavity fill of open cell foam. If there is 2" of space between the batts & subfloor or between the batts & rigid foam it doesn't take dense-packing genius to make it work. If it's just as easy to yank the batts out even a DIYer with a rental blower can get there, gut DO keep the rigid foam- it's your thermal break over the ~R1.2/inch joist framing.

  5. danny77 | | #5

    The joists are 8" and the rigid insulation is 1" thick. The batt insulation has a lot of animal nests in it. What about 2" of closed cell spray foam to the underside of the floor, Roxul to fill the rest of the cavity and then reattaching the rigid insulation to the underside of the joists?
    Also, the rim joist in the basement is currently uninsulated so I am thinking 2" closed cell spray foam to fill the joist cavities there?
    I had a insulation contractor out to look at it and it is very little money to add the rim joist on if we do the cantilevered floor as the base.

  6. Expert Member
    Dana Dorsett | | #6

    What's the point of the 2" ccSPF in that floor stackup?

    When you bridge R6/inch foam with R1.2/inch timber at a framing fraction of even 25% it totally robs the performance of the foam. The difference in "whole wall R" between a full-fill of R30 rock wool or fiberglass and a 2" /R12 foam + ~R22 (compressed) R23 batt is a whopping R1.5, a very EXPENSIVE R1.5(!). Adding 3/8" of additional foam thickness on the underside would achieve the same thermal performance for less money. Air seal it with caulk- it's really cheap stuff relative to ccSPF.

    If you're concerned about the vapor redardency from interior moisture drives, paint the underside of the subfloor with vapor barrier latex- it's a lot cheaper, and has about the same vapor retardency of 2" foam.

    On the band joist & foundation sill it's easier / better to go with 2" ccSPF + batts though, since it is protecting the cold band joists from interior side moisture drives.

  7. danny77 | | #7

    Thanks for the help. I really appreciate. Would you think the labor required to caulk between the joists for the whole floor would be more than ccSPF? What do you think about insulating the rim joists around the basement with 2" ccSPF from the interior, except at the cantilever where it would be insulated from the exterior? Is that a worthwhile investment?

  8. Expert Member
    Dana Dorsett | | #9

    Caulking a 100 square feet of floor joist would take less than a half hour. Don't know what your time is worth, but you'd be saving on the order $200 in material cost difference.

    It may still be worth using the 1-2" of ccSPF + R15 rock wool to insulate & air seal the foundation sill & band joist though.

  9. Expert Member
    Dana Dorsett | | #10

    Caulking a 100 square feet of floor joist would take less than a half hour. Don't know what your time is worth, but you'd be saving on the order of $100- $200 in material cost difference.

    It may still be worth using the 1-2" of ccSPF + R15 rock wool to insulate & air seal the foundation sill & band joist though.

  10. wjrobinson | | #11

    The areas are easy to spray full with open cell Icynene. Do that and you shall be most happy. It is one good place to use water blown open cell Icynene and I would do so. The joists should be encapsulated which should not be a problem. Cross the joists with 2x3s on edge, then sheath with PT ply.

    One airline ticket to there then on to Tahiti and I'd do the project.

  11. Expert Member
    Dana Dorsett | | #12

    Depends on how much of the project he wants to do DIY, but an all half-pound foam solution wouldn't be terrible.

    It would be quite a bit more expensive than a DIY R30 rock wool + caulk solution, re-using the rigid foam on the bottom for sheathing, which would cost maybe $200-250 total for the whole shebang at box-store pricing for R30 Roxul. I don't think you can get a foam insulation contractor to even show up for less than a grand's worth of work, but maybe they're hungrier in his neighborhood than mind. But if they WOULD show up and not gouge you deeply, an all open cell solution to the floor would be around $300-350.

    One fly in that ointment is that to do 7-8" right with open cell foam they need to do it in two lifts of no more than 5.5", with a cooling period in-between. I suspect most would opt to just blast away, which has higher potential for gaps, poor adhesion (and a small chance of catching fire while curing.)

  12. danny77 | | #13

    I was quoted 1182 to remove/dispose the existing batt and rigid, 2" ccSPF on the floor and band joist, rock wool and hang back up the rigid. Plus do the band joist around by basement at the interior (except where the cantilever is since it is being done at the exterior).
    I got another quote to do the cantilever floor for about the same 1000 plus another 1000 to do the band joist...

  13. Expert Member
    Dana Dorsett | | #14

    The job is basically too small to make money at using "normal" per board-foot foam rates. The 2" foam is a cost-adder for the cantilever, and completely unnecessary- it buys you nothing.

    As a DIY doing the cantilever without spray foam is really, really cheap, but contractors have labor & overhead to pay. A typical cc foam-only job runs about $1-1.20 per board-foot (one square foot at 1" depth), open cell foam is typically in the 30-40 cents per board-foot range, but with 7.25" of oc foam it takes 2-passes with a cooling off period between passes. The volume of foam is really low, and you're tying up the rig and 1-2 guys for the better part of a day, so the $/board-foot cost is going to be pretty high compare to a "real" job of decent size. Closed cell foam at 2" is a single pass, but they'd be spending more time setting up and breaking down than they would spraying foam for less than 200 board-feet of foam.

    If they'll do the band joists for $1000 it is probably "worth it", since sealing & insulating the foundation sill & band joist using other means is very labor intensive.

  14. Richard Beyer | | #15

    AJ for $500.00 the crew did not make any money. A 200 board foot DIY low-pressure system cost $340.00 not including delivery, sales tax, safety equipment and labor to spray. Your subcontractor may be taking care of you, but he's not going to be around long with business practices like that.

  15. wjrobinson | | #16

    Dana, My spray crew will spray a rim joist at the end of a short day in the past for $500 likes to take care of his contractors. They would Spray outside, then the rim and be right back in action to finish the outside. There is no down time.

    Richard, when I met my spray company they had two spray rigs going fulltime. Last I talked to them they had five I think. Since you posted that they are going broke they added another full time rig. They also are not the low price guys. They do like to do what I posted at least they do for me. Friends with benefits maybe, but who knows.. I do think you may be off base though. They finish out a barrel maybe... finish out a day... maybe... but... they do it. And it benefits them, me and the person who got their rim joist done, adds customer base and word of mouth for a very low cost...

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