Closed cell spray foam insulation to underside of flat roof
BrooklynFlatRoof
| Posted in General Questions on
I am closed cell spray foaming the underside of my FLAT roof as part of a gut renovation to the top floor apartment of a 3-family building in NYC. The building is an attached style building. Meaning there’s a building to the left and to the right.
The sprayable roof underside is 960 sq ft.
I’ve had a few questions and concerns:
- R-value target: Am I safe to target an R-value of R-28? I’ve read the benefit of insulation beyond 2-3″ greatly falls off in actual cost savings in heating and cooling a space.
- Rim joist spraying: How far down the rim joist should the contractor spray the home on the shared party walls?
- Should i spray the front and back of building? Seems like if the contractor is there, might as well get him to spray. The additional cost isn’t that great. All the studding and electrical wiring will be done.
- Pricing : Seems to be all over the place. One reputable guy quoted me $6500, another $5400. And two other contractors quoted me $3400.
- What brands of CC Spray foam are considered good? One of the contractors who quoted $3400 uses Thermoseal. Is that a reliable brand?
- How can I keep residual spray off of stuff? The floor is being gutted. I have metal stud wall and ceiling framing up. i also have my new subfloor down. I want these surfaces to be nice and flat. I don’t want to be scrapping off foam when Im sheet rocking or laying down a new floor. I will cover my tools and supplies with plastic sheeting the best I can. I also plan to install my new windows before I spray. So the windows will be covered with plastic sheeting as well.
- If my flat roof does develop a leak, the moisture will become trapped b/w the roof sheathing (white pine boards maybe??) and the close cell spray foam insulation. This could create rot on my roof sheathing. I cannot think of a way the moisture could escape. Sounds scary. Are my fears unfounded?
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Replies
In order...
1>Code minimum in US climate zone 4 (NYC) would be R49, not R28. As long as at least R15 of that R49 is closed cell foam applied to the underside of the roof deck the other R34 could be fiber insulation, as long as the fiber is in contact with the foam. How much rafter depth do you have to work with? At what rafter spacing?
2> For party walls you don't need R-value, only air tightness, but the whole wall needs to be air tight.
3> I'm not quite sure what "... spray the front and back of building..." means here. All exterior walls need to be insulated, but it's not clear than closed cell foam is going to be the best choice. More details on the wall assemblies and joist attachment might be in order...
4>Pricing can vary for a number of reasons. In my area closed cell foam runs about $1 per square foot per inch of depth (R6-ish) if blown with climate damaging HFCs, or about $1.30 per square foot per inch of depth (R7-ish) if blown with far more benign HFOs, but the size of the job matters too, as is how hot the market is. For R28 it would take ~4.5" of HFC blown foam, so for 960 square feet that would run ~"4300 +/-. With HFO blown foam it would take 4" for about $5000.
But 3" of foam (either type) + R30 fluff is what it takes to hit code min most economically, if it fits.
5>The competence of the installer makes more of a difference than the brand of closed cell foam. Thermoseal makes a half-dozen or more different types & densities of closed cell foam. They probably quoted 4" of Thermoseal 2000, if R28 was the target:
https://thermosealusa.com/assets/docs/technical-data-sheets/insulation/ThermoSeal%202000-2000W%20TDS.pdf
At 4" x 960' the $3400 works out to about 88cents per square foot per inch of depth, which is "in the range" of a hungrier installer and HFC blown foam. I'm not quite buying the stated R6.9/inch for an HFC blown foam though, but there are incremental year on year product improvments, so maybe.
6> The stuff sticks to the first surface it hits, so you have to mask off or cover anything that would be damaged by the foam with drop cloths, etc. In a full-gut application there isn't usually too much to worry about other than any windows & doors you're keeping.
7> Closed cell foam is fairly waterproof, but at 4" or more would also be fairly structural. But the drying of the roof deck is one reason why installing the insulation above the roof deck is generally better. To hit code-minimum performance on a U-factor basis (rather than R-value) would take 6-7" of 2lb density roofing polyiso under a new roof, and even with the cost of the new roof included may be comparable in price to what it takes to meet code insulating only from the interior.
In order...
1>How much rafter depth do you have to work with? At what rafter spacing?
>>Rafter depth is 8”, Spacing is 16” OC.
2> For party walls you don't need R-value, only air tightness, but the whole wall needs to be air tight.
>>The building is attached on both side. Like a row house. So I believe I dont need to insulate the party walls.
3> I'm not quite sure what "... spray the front and back of building..." means here. All exterior walls need to be insulated, but it's not clear than closed cell foam is going to be the best choice. More details on the wall assemblies and joist attachment might be in order…
>>The front and back walls are the exterior walls. Walls are framed with steel studs and are currently exposed. Studs are 3.5” deep, behind the stud framing is brick fill or brick noggin.
4>Pricing can vary for a number of reasons. In my area closed cell foam runs about $1 per square foot per inch of depth (R6-ish) if blown with climate damaging HFCs, or about $1.30 per square foot per inch of depth (R7-ish) if blown with far more benign HFOs, but the size of the job matters too, as is how hot the market is. For R28 it would take ~4.5" of HFC blown foam, so for 960 square feet that would run ~"4300 +/-. With HFO blown foam it would take 4" for about $5000.
But 3" of foam (either type) + R30 fluff is what it takes to hit code min most economically, if it fits.
>>Best priced vendor is quoting me $0.92 per board foot CC which he claims is R-7 per inch.
6> The stuff sticks to the first surface it hits, so you have to mask off or cover anything that would be damaged by the foam with drop cloths, etc. In a full-gut application there isn't usually too much to worry about other than any windows & doors you're keeping.
>>Yeah Im getting lots of plastic.
I will second Dana’s 5th point: it is VERY important to use a competent installer. Getting a good spray foam installation takes experience and you don’t want to be the job site where a brand new crew is figuring out their equipment. I believe most of the problems people have reported with spray foam are due to inexperienced installers.
Your interior walls would not generally need to be insulated since the adjacent tenant space is conditioned space. That assumes there is no outside air gap between the buildings. Even so, you will probably want to insulate those walls for sound to keep noise down. There are thinner mineral wool batts made for this purpose that work well for this, fiberglass is a close second. For the ultimate sound proofing, consider hanging 5/8” drywall (two layers with green glue between is best) on resilient channel. Try to minimize service penetrations in this wall, and any that you have to have, at least keep them in separate stud cavities from any in the “other” side of the wall. You will have much happier tenants this way since they won’t hear their neighbors.
Bill