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  1. #1
    Join Date
    Jul 2012
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    2

    Default spray foam insulation

    We have a 75 year old cape, stone with slate roof. Replacing the roof with new slates. We want to insulate the roof. We are getting different opinons from contractors. Questions: What is the minimum foam depth that will suffice for thermal and vapor barrier? Can the masonry gables be foamed as well. How thick? Is it best to spray the entire roof deck or spray the back of the knee wall, attic deck and ventilate the soffit to a ridge vent? The roof currently has only gable vents. I assume the ridge vent option would create a good deal more cost. Any input is welcome.

  2. #2
    Join Date
    Dec 2007
    Location
    Tennessee
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    1,748

    Default Re: spray foam insulation

    R-38 is the recommended insulation level for roofs/ceilings in most parts of the country. Different foams have different insulation properties so you have to look at the particular foam in question to determine the depth required to achieve the R-38.

    Not all foams are vapor barriers. Some foams are open cell which are not, others are closed cell which maybe vapor barriers. You have two choices for how you insulate.

    One choice would be to make the attic and kneewall space conditioned spaces. You would spray the entire roof deck and gable walls and have no ventilation. You need the closed cell foam for this. Generally you finish the attic to use as additional living space or storage space. If you chose living space, you will probably have to add windows. You will have to tear out the walls on the angled part of your upstairs rooms to spray the rafters in this area and that of course involves additional cost to replace and finish them. Some contractors can use a minimally or slow expanding foam so that the walls don't have to be removed, check with each contractor on that.

    The other choice is to insulate the backside of the kneewalls, the floor of the kneewall space, the rafters in the angled wall and the floor of the attic. Again you have the issue with the angled walls, but in addition, a spacer has to be installed that will keep the insulation an inch away from the roof deck so that air can rise up under the deck. For this, you need soffit vents and at least the gable vents. A ridge vent might have some benefit here. For this choice, batt insulation for the walls and angled walls is usually used with blown in or batts for the floors behind the kneewall and in the attic. Foam is much more expensive.

    You cannot mix the choices above, it is one or the other. Regardless of which one you chose, I would suggest that you tear out the sheetrock or plaster on the angled walls to insure the insulation is installed correctly, then put 1" foam board insulation between the rafters and the new sheetrock. It will greatly reduce the conduction of heat through the rafters themselves. The rafters make up about 10% of the total roof area and the R value through them is only about R-1.25 per inch as opposed to the R-3 to R-6 of the batt or the spray foam insulation.

  3. #3
    Join Date
    Jul 2012
    Posts
    2

    Default Re: spray foam insulation

    Thanks for your reply Keith. Would you agree that if the roofer removes enough of the roof decking (tongue and groove) to access the rafters that tearing into the walls to insulate that space is not required?

  4. #4
    Join Date
    Dec 2007
    Location
    Tennessee
    Posts
    1,748

    Default Re: spray foam insulation

    Yes I would agree, but if you go this route you need iron clad contracts for specific dates for this work to be done with severe penalties for failure to show on assigned dates. Thats if you have two separate contractors. If you don't, the insulation crew might show up before the roof is torn off or the roof is torn off and the insulation contractor doesn't show up so the roofing contractor is on hold waiting.

    In either case, the contractor on hold will be entitled to extra compensation, that is why the severe penalties. And do not pay up front, otherwise you cannot enforce the penalties. Check out your contractors closely on this. When you need only one contractor, you can afford to be more tolerant of timing, but when you have two contractors whose jobs interlace, the schedule is very important.

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