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  1. #1
    Join Date
    Jul 2010
    Posts
    1

    Default Foam insulation under shingles

    I need to replace the roof on my house. It is a story and a half style built in the 1920's. There is no insulation in the roof, the attic is finished, and there is no venting. The current roof is Asphalt and I have not had any condensation issues.

    When I tear off the old shingles I am thinking of putting 4" of foam insulation (R-20), a new layer of OSB or plywood, and then the shingles. This seems to be a good way to add insulation with out tearing out most of my ceilings.

    Is there anything I am not seeing?

    Would condensation on the 1st layer of OSB be an issue or will the faom keep that layer warm like the rest of the living space?

  2. #2
    Join Date
    Jul 2010
    Location
    Alexandria, VA
    Posts
    4

    Default Re: Foam insulation under shingles

    Dan,
    Most shingle manufacturers won't give you a warranty if the material is installed to an insulated roof deck. They all require an air space. A 30 year shingle on an insulated/ un-vented deck will be trash in ten years.

    I have accomplished this on two projects two different ways:

    1)Lay out another roof; install 2x6 "rafters" in the same pattern as existing below your roof deck, then lay in new 4" rigid foam insulation board between your new rafters. Install new plywood roof deck. Now you will have a continuous 1.5" air space from eave to ridge; add soffit ventilation and a ridge vent and you are good to go. It's just basically building another roof on top of your existing roof.

    2)It wasn't cheap but it was much easier; http://www.hpanels.com/2009/pages/pd...ol-Vent-II.pdf These are rigid insulation with an airspace and then a nailable deck all in one. There was a little bit of lead time but they were a breeze to install. Again, allow for soffit/eave venting and a ridge vent and you will be good to go.

    Read this section on ventilation:http://lyonscontracting.com/resource...-new-roof.html

    Good luck with the job;
    Tom

  3. #3
    Join Date
    Aug 2007
    Location
    The Great White North
    Posts
    4,045

    Default Re: Foam insulation under shingles

    Quote Originally Posted by DIYDAN View Post
    I need to replace the roof on my house. It is a story and a half style built in the 1920's. There is no insulation in the roof, the attic is finished, and there is no venting. The current roof is Asphalt and I have not had any condensation issues.

    When I tear off the old shingles I am thinking of putting 4" of foam insulation (R-20), a new layer of OSB or plywood, and then the shingles. This seems to be a good way to add insulation with out tearing out most of my ceilings.

    Is there anything I am not seeing?

    Would condensation on the 1st layer of OSB be an issue or will the faom keep that layer warm like the rest of the living space?
    There's still some debate when it comes to shingles on unvented roof assemblies. There are some manufacturers that will allow their product to be used on unvented roofs. Usually those that will allow it will likely insist their premium shingles are used ( upsell ) and written confirmation from the maunfacturer for the warranty.
    Research has been done showing the shingles may only be exposed to anywhere from 2 - to 10 degree temperature increase to the roof sheathing. The estimated impact on the shingles might equate to only a 10% decrease of lifespan --- so a 25 year asphalt would have a predicted life of 22.5 years. Despite the research some manufacturers simply ignor changing their minds.
    Btw -- SIPS used for roofs aren't vented assemblies and asphalt shingles are sometimes used --- albeit with written confirmation from the manufacturer for warranty.

    Since you will be tearing off the shingles you have a couple of options..........

    #1 --- you could remove the existing sheathing --- sister a larger 2 by 6 or 2 by 8 to the existing rafters --- fill the rafter bay with whichever insulation you choose --- secure a 2 by 2 cleat ( or nailer ) along the new rafter --- secure to the under side of the cleat a vent channel ( could be plywood or rigid foam or .. ) all the way from the eve line to the ridge.
    Then re-sheath the roof deck and leave the appropriate opening at the ridge for a ridge vent.

    #2 --- based on your original tought. This system has merit and does work.
    However, to appease the shingle manufacturer gods a modification may be used.

    Lay the rigid foam down on the roof deck ( ensure a water barrier is on the wood decking before the foam ) completely covering the entire deck --- ensure to seal all the seams ( both horizontal and verticle ) to ensure an air tight seal --- on top of the foam secure 2 by 4's on the flat ( sleepers ) through the foam and into the rafters or plywood sheathing --- fasten new decking onto the sleepers.
    This method provides a continious thermal break for the entire roof surface providing better performace than the interuppted method of insulating between the sleepers. Also, this will allow the venting under the roof sheathing to appease the shingle concern.

    The R value needed is dependant on climate conditions where you live --- R 24 may not be enough but it will help with the continious thermal break.

    Condensation considerations will be dependant again on the climate wher you live and the indoor relative humidity both inside and outside. For example --- with R 24 and if you experience winter temps below 45 F for extended periods and your indoor RH is high ( 60 + % ) you could end up with vapour drive from the interior out and condensing on the cold side of the sheathing. This would require the interior RH would have to be maintained at a lower level --- 40 - 50 % .

    Just some thoughts.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  4. #4
    Join Date
    Nov 2009
    Location
    Galena IL
    Posts
    3

    Default Re: Foam insulation under shingles

    We have just completed our roof project on the 1890 Victorian in Galena IL: Same issue as you describe-no insulation, and as a lifelong log and timber home builder, both earlier methods described work fine. We chose the ventless roof type as we are trying to reduce roof profile to maintain historic consistency. After scraping the old roof deck clear, we repaired some soft areas and covered over the old chimmney hole. Just to make sure we did not have structure issues, we opened the cavities at various locations- then used CLOSED CELL foam slow rise foam (open cell acts as a sponge if no vapor barrier inside) We then used 2x6s on edge, applied dupont brand premium housewrap (blue)to the roof deck,and used 2 2" layers of polyiso thermax, and a 1/2" layer of foil faced thermax.cracks were foamed, and seams were taped. Experience and history has shown that multiple layers of vapor barrier trap moisture, thus, we wanted the origional roof deck to breathe. 1/2" osb was fastened with 8" polebarn spikes 16" oc, and occasionaly we used log boss screws to hit the rafters where we could. Water and ice shield was applied over the entire roof assembly. to maintain consitency with the addition and garage, rolled roof venting was applied, but serves no purpose. The Owen Corning shingles we used were lifetime (4 bundles per square), not the cheap ones. Talk to the company representative sales manager, not the warranty department for questions...
    After seeing roofs we did from 1993 on, including 6 on in the same lake subdivision, neither the vented or unvented showed issues of failure. The south facing roofs of both methods appear almost identical! Microclimate and yearly maintenance were the big factors- Given the ability (historic board here has rules) and if it would work in your budget and style, metal roofs are the better option in either method. If you look to save money, try craiglst for materials. Budget numbers:handi foam 600 kits $1100, insulation and other materials $800 (crglst), lumber/bldg materials/spikes $1350, roofing $27 bundle on sale, plus $400 for misc items (wip, edge, vent etc.) Hired out, labor cost would run $150-200 square for all labor to tear off, repair, insulate, sub facia boxes, finish facia and trim, foam, sheathe and lay shingles.

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