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  1. #1
    Join Date
    Aug 2008
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    Default Insulating from inside - flat roof dilemma

    I'm renovating a back room in my 1920's semi in the Northeast. The walls were in horrible shape, so I tore them down. Plus I wanted to insulate and rewire, and it was just going to be easier. The original walls and ceiling were wood lathe, plaster, drywall, dryall (yes, two drywall). The ceiling had blown in insulation (not sure what kind). No vapour barrier anywhere.

    The roof is where I'm confused. It is a flat roof, with true 2x10's on 16" centers. On the roof side, it looks like a mix of 8", 10", and 12" wide boards with a 0.5" gap between each running perpendicular to the joists. I've read so much about insulating, but not much pertains to a flat roof. There are no soffits or anything on the side of the house, just fascia and eavestrough.

    So, the dilemma is this: How should I insulate the roof, and where do I (if any) put the vapour barrier?

    Is a 2x10 cavity too much for spray foam? Should I just double a 6" and a 4" batt insulation? Blown in? Vapour barrier before the drywall (I know I don't need one for spray foam, what about batts?) So many options.

    Any ideas?
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  2. #2
    Join Date
    Aug 2008
    Posts
    54

    Default Re: Insulating from inside - flat roof dilemma

    Quote Originally Posted by in too deep View Post
    I'm renovating a back room in my 1920's semi in the Northeast. The walls were in horrible shape, so I tore them down. Plus I wanted to insulate and rewire, and it was just going to be easier. The original walls and ceiling were wood lathe, plaster, drywall, dryall (yes, two drywall). The ceiling had blown in insulation (not sure what kind). No vapour barrier anywhere.

    The roof is where I'm confused. It is a flat roof, with true 2x10's on 16" centers. On the roof side, it looks like a mix of 8", 10", and 12" wide boards with a 0.5" gap between each running perpendicular to the joists. I've read so much about insulating, but not much pertains to a flat roof. There are no soffits or anything on the side of the house, just fascia and eavestrough.

    So, the dilemma is this: How should I insulate the roof, and where do I (if any) put the vapour barrier?

    Is a 2x10 cavity too much for spray foam? Should I just double a 6" and a 4" batt insulation? Blown in? Vapour barrier before the drywall (I know I don't need one for spray foam, what about batts?) So many options.

    Any ideas?
    I would blow in cellulose or blown fiberglass, dense packing either, but fiberglass with the joist depth would be costly, roughly 4 -5 times that of cellulose. More importantly, what is the roofing material of your flat roof? Torch down, rolled, rubber? If it's rolled roofing, I would install a rubber diaphram first, since life expectancy of rolled roofing is pretty short, and the last thing you want is all the insulation getting wet, heavy, and the whole ceiling crashing to the floor.
    I have no knowledge of or experience with the P2000 one guy is pushing here, but that may be a viable option in your situation.
    Foam is light weight, but the foams I have encountered are very porous and offer absolutely zero value for air infiltration reduction. There may be other types of blown foam that will, but I've never seen them personally.

  3. #3
    Join Date
    Aug 2008
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    BISBEE JUNCTION, AZ
    Posts
    2

    Default Re: Insulating from inside - flat roof dilemma

    With a flat roof, I’d suggest staying away from cellulous insulation. You have to expect water to get in especially with a flat roof and that stuff is very bad, messy & heavy when wet. It also won’t “bounce back” when dry like fiberglass. Becomes a nasty paper blob after wet. I’d go with fiberglass with very good vapor barrier and find a way to vent it, if possible. You may also want to look at Styrofoam insulation boards stacked up to fill all the voids. Not sure what the R factor is for 10” of dense Styrofoam is, but ˝” to 1” keeps beer cold all day, so…

  4. #4
    Join Date
    Aug 2008
    Posts
    54

    Default Re: Insulating from inside - flat roof dilemma

    Quote Originally Posted by DR&F View Post
    With a flat roof, I’d suggest staying away from cellulous insulation. You have to expect water to get in especially with a flat roof and that stuff is very bad, messy & heavy when wet. It also won’t “bounce back” when dry like fiberglass. Becomes a nasty paper blob after wet. I’d go with fiberglass with very good vapor barrier and find a way to vent it, if possible. You may also want to look at Styrofoam insulation boards stacked up to fill all the voids. Not sure what the R factor is for 10” of dense Styrofoam is, but ˝” to 1” keeps beer cold all day, so…
    I've often questioned how R values are determined for the very reason you mention. Styrofoam typically has a fairly low R value per inch, but, as you mentioned, a small amount keeps beer cold all day. You cannot feel a significant thermal transfer on the outside of a styrofoam cooler filled with ice water. Styrofoam is quite flamable however. You are correct on the cellulose, which is why I recommended a rubber diaphram roof to protect it for a long time. Blown insulation, be it cellulose or fiberglass will not require venting in a flat roof if it is dense packed. Venting these types of roofs would be next to impossible, unless you put soffit vents at each end of each cavity and could be installed to eliminate the possibility of water seeping in.

  5. #5
    Join Date
    Aug 2007
    Location
    The Great White North
    Posts
    4,045

    Default Re: Insulating from inside - flat roof dilemma

    In my understanding the reason that a Poly based foam has a superior insulating property is the poly has very good resistance to thermal transfer. In the case of closed cell foam ..... air pockets are encased with the poly.

    Air in a stagnant environment has good insulating properties and combined with the poly provides a superior insulation material.

    Unlike fiberglass batts where moving air diminishes the insulation performance as much as 40%. However .... in a static air environment fiberglass can be adequate.

    In this case if there is a budget allowed .... you might have closed cell foam sprayed along the joist ends and about 2 inches thick along the underside of the roof deck to seal air infiltration and provide a very good insulation barrier. Then conventional fiberglass or rock wool batts can be installed for added R value.

    If the budget doesn't allow spray foam .... then using 2 inch thick rigid foam can be used instead along the same areas. With this it's important to provide a tight air seal. One thing that can be done is use spray foam in a can to seal along all the edges of the foam sheets or a good caulking.

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