I was thinking of putting a concrete cap over my two story icf house.  The ceiling joists would be open web steel joists followed by steel plate and a 4 inch cap of concrete for strength.  The main reason was for safety, I live in the tornado belt.  I was thinking of the best way to insulate the ceiling.  Since the concrete acts as a air barrier, could I blow in a cheap cellulose since the attic will not be used for any storage?  Would I need a moisture barrier?  A 6mil plastic sheet on top of the concrete before insulation perhaps?  Thanks for your thoughts!

Why not look into an insulated concrete roof and not worry about blowing in anything?

I haven't put it to paper, but I assume the cap even with blow in insulation would be significantly cheaper.  I could be wrong though.  I'll look into it.

Don't look into it, blown in insulation is cheaper.

Don't concentrate on price all the time, look at the system as a whole, how it performs and how it protects.

I don't concentrate on price.  Do you think I'd be building an all ICF house from the footing to the roof joists if I was just concentrating on price?  My original question was not answered.  Does anyone know the best way to insulate the underside of a raised concrete slab?  I'm thinking this is probably fairly common in commercial buildings, but I don't know if I would need a plastic moisture barrier on the warm side or not.  Any ideas?

If you use spray foam insulation you do not need a vapor barrier

Again, consider Insuldeck or similar for your application. If you take all things into consideration, and the list is long, I'm not trying to be smart with you, this includes construction times, financing costs, everything, you may find more bang for your buck where you least expect it.

Commercial buildings when utilities are payed by the tenant are generally built by cost conscious developers, price is usually the issue with them...I.E. it get's minimal insulation batts or the styrofoam below the roofing materials

Great idea, one I have toyed with myself for a while. Can you give a bit more thoughts on how the cap would be supported? You mentioned open web steel joists- like Hambro? I just did a Hambro floor on my house. Span is important. Hambro is expensive. I was thinking of a hybrid concrete, like gypcrete or foamcrete. Ever heard of that? Might be easier to support. I think this idea needs exploring and some creative thinking, but would be of great benefit to homes, as the entire interior would be safe.

BTW, fastening 4x8 poly insulation to underside of deck would not be hard

I wouldn't use Hambro, probably Vulcraft.  I wasn't planning on it being supported because the live loads would be very small since the space wouldn't be used for storage.  I think I could get away with code minimum L/240, something to check with the engineer.  EPS can be used easily on the underside, but I'm not sure if I need a vapor barrier on the warm side of the roof.  Blow in insulation on the topside would be even easier to install than EPS on the underside and cheaper. 

Just curious - what goes over the concrete (on the outside)?

That is my main question.  If I put a concrete cap on my icf house and use roof truss's on top of the cap, how would I insulate the cap?  I was thinking of a 6mil plastic vapor barrier and then blow in insulation to get to R40.  Would I need a vapor barrier?  Could I just blow in insulation on top of the cap?

Most cold climate homes do not use a vapor barrier below the attic. So OK, just blow in cellulose on top of the concrete. But personally, I prefer the idea of a below grade concrete room for tornadoes.

If you use ICF, you have your insulation. With a low slope concrete roof there are numerous self adhered roofing products out there. I use polyglass roofing. Depending on the type of edge treatment, parapet etc.. it has an insane wind uplift resistance.

I am currently building a house and using ICF for my the 6/12 roof. All of the hips, ridges and valleys will be formed in place concrete beams and the roof overhang will be formed and poured. The ICF panels need to be shored. It is a bit labor intensive and more costly than a conventional trussed roof system. The walls and floors are all ICF as well.  The benefit is a home that can withstand 200 +mph winds not to mention all of the other benefits of ICF.

Jim
J C Marshall Construction Inc
[email protected]