Roof
I am looking at insulating an approx 6k square foot flat roof in Jersey City, NJ.  The roof has a worn torch down layer over an original tar coating.  The underside has uninsulated rough cut 2x8s.  I will be finishing the interior by sheetrocking to the  rafters -- there will be no attic.   Building is gutted and vacant.

My quesiton is what would be the best insulating strategy for the money.  I am presently planning 1.5" of foam on top the roof deck with a silicon coating, plus 5-6" icynene between the rafters, followed by 5/8 sheetrock.  This is obviously pretty costly. 

Is there a more economical diy solution that would give me a similar result??  (I do have excess of 1.5" rigid foam on hand). 


For 2x4 walls and 2x8 basement ceiling ... ?? -- will the icynene be a much better performer than well installed cotton or JM bats.  Is there another solution? 


Any suggestions...appreciated.

That much spray foam should be about R-20. If you have more 1.5" sheets, use them, if it is EPS it would be about R4 per inch. You should be able to use a few inches on a roof deck.

Are you removing the existing roof(s) or are you putting the new insulation and coating over the existing roof(s)? I have no experience with silicon roofing products. What product are you thinking of?

Bruce

It does seem like a little pricey ...

(I may be a little late on this but...)

Bumping the layer of EPS above the roof deck to 4" and DIY dry-blowing cellulose into the rafter joists would be cheaper & much higher-R than 5" of half-pound Icynene fill. Even 6" of EPS and no cellulose you'd be slightly ahead of 5" of icynene + 1.5" of EPS. (Icynene is only ~ R3.5/inch- some other half-pound foams are closer to R4/inch. Cellulose at 1.5-1.8lb density runs ~ R3.6/inch, but hits ~ R3.8/inch at 3lb density- twice the material. A low density fill of cellulose would be cost-effective, and would sag from ~8" to ~7" over a few decades, with a long-term value of R25. But you can run into condensation & rot issues with low density cellulose-only under in torch-down flat roof- but in Jersey City's climate you can safely go that route with at least 30-35% of the R in foam above the roof. 4" of EPS would be R16, which would be about 39-40% of the total R(=R41, a significant R-boost over 5" of Icynene + 1.5" of EPS.)

Dry blowing cellulose into the wall cavities (possibly DIY), or wet-spraying cellulose (not DIY) would be higher-R than Icynene, if not as absolutely air-tight (but only 10% or less of the air infiltration & convection potential of batts.) Bang for buck it's usually good option. If JM Spider (wet sprayed ultra-fine fiberglass) can be sprayed to 1.8lbs density for similar money, it'll be a boost in R with air infiltration rates similar to cellulose.

If you are planning in this way as you described it will be little costly but i dont have the exact idea about it.