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Old 08-05-2013, 09:17 AM   #1
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roof insulation


Hello,

I have read some articles about contractors using rigid foam under the roof decking but my question is if you do that then aren't you assisting in keeping the underside of the sheathing warmer? By doing this you would be melting snow on the roof and creating possible ice damns on the eves?

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Old 08-05-2013, 09:23 AM   #2
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Is this going to be a finished attic that's heated and cooled, cathedral ceilings, open attic with no heating and cooling? What type roof is there now?
With questions like this it's best to go back and add your location to your profile. Just go to quick links to edit.

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Old 08-05-2013, 11:46 AM   #3
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Originally Posted by joecaption View Post
Is this going to be a finished attic that's heated and cooled, cathedral ceilings, open attic with no heating and cooling? What type roof is there now?
With questions like this it's best to go back and add your location to your profile. Just go to quick links to edit.
No its not finished. Its a gable roof. I am in MI.
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Old 08-05-2013, 01:15 PM   #4
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Should be insulating the joist not the rafters.
Insulating the rafter will not to a thing to keep the living space warmer or cooler.
An unfinished attic needs to be close to the outside temperature.
http://www.energystar.gov/index.cfm?...sulation_table
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Old 08-05-2013, 02:22 PM   #5
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Originally Posted by joecaption View Post
Should be insulating the joist not the rafters.
Insulating the rafter will not to a thing to keep the living space warmer or cooler.
An unfinished attic needs to be close to the outside temperature.
http://www.energystar.gov/index.cfm?...sulation_table
So, why am I seeing builders putting rigid foam on the roof deck?
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Old 08-07-2013, 02:43 PM   #6
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Originally Posted by detroittigerfan View Post
So, why am I seeing builders putting rigid foam on the roof deck?
A lot of the decision of whether or not to put insulated sheathing on a roof is dependent on whether the space below will be heated and/or air conditioned. With a lot of the green building and energy efficiency trends being used in construction nowadays, it makes great sense to insulate the areas that seperate the areas that will be heated and not heated. This is often called the "building envelope."

There are some great reference materials on line regarding proper building envelope design and construction techniques. I hope this is helpful.
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Old 08-07-2013, 05:54 PM   #7
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If it was a cathedral ceiling, exposed beam ceiling, there was only 2 X 4 rafters and the other side was living space with HVAC them SIP roofing panels may be called for.
But not for an unfinished attic.
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Old 08-07-2013, 11:15 PM   #8
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Originally Posted by detroittigerfan View Post
So, why am I seeing builders putting rigid foam on the roof deck?
I've heard it's actually called a MI Roof. By putting rigid foam under the decking you actually put another thermal barrier (insulation) right under the roofing to keep escaping heat from the home from causing ice dams etc.
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Old 08-08-2013, 11:17 AM   #9
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If the attic is air sealed, insulated correctly, Storm and Ice Shield on the lower part of the roof, and properly vented Ice dams should be a non issue.
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Old 08-08-2013, 01:31 PM   #10
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Quote:
Originally Posted by detroittigerfan View Post
Hello,

I have read some articles about contractors using rigid foam under the roof decking but my question is if you do that then aren't you assisting in keeping the underside of the sheathing warmer? By doing this you would be melting snow on the roof and creating possible ice damns on the eves?
The foam insulates (separates) the interior temperature from the exterior temperature; http://www.buildingscience.com/docum...on?full_view=1

Thick enough to prevent condensation, per location; http://www.buildingscience.com/docum...rchterm=attic+

Effective when the HVAC/ducts are in the attic; http://www.buildingscience.com/docum...46-dam-ice-dam

Boundaries of the thermal barrier; http://www.buildingscience.com/docum...on?full_view=1

Fibrous insulation allow air flow through it/moisture; http://www.buildingscience.com/docum...d-roof-systems

Gary
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Old 08-10-2013, 08:19 PM   #11
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When you think about it, and assuming the roof is properly pitched and can handle the snow load of course, you want to see a roof with snow accumulating on it in the winter. It shows heat is not escaping straight up and through the ceiling and roof to melt the snow.

I often put wide bats of fiberglass insulation between rafters in renovation projects I did in Central Illinois even if the ceiling was insulated. Just one more layer for any cold air that might sneak into the attic space. You do need to be sure you have attic vents working though.
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Old 08-11-2013, 08:45 PM   #12
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Originally Posted by sdsester View Post
I often put wide bats of fiberglass insulation between rafters in renovation projects I did in Central Illinois even if the ceiling was insulated. Just one more layer for any cold air that might sneak into the attic space. You do need to be sure you have attic vents working though.
Works anywhere the daytime drying ability exceeds the night-time wetting of the roof framing, controlling high indoor RH- with an air-sealed attic, and ventilation; http://www.buildingscience.com/docum...ifornia-attics


That would be safe in many parts of the country- best to compare per location, not feasible for Minneapolis for example; http://www.buildingscience.com/docum...d-roof-systems

Gary

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