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Old 07-14-2008, 09:08 AM   #1
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Flat Roof Help


I am replacing insulation in a 80+ year old house flat roof. Joists are 2 x 8's. I am using Roxul Flexibatt insulation. Seems the stores only have the insulation for 2 x 6. Should I be filling the 2 x 8 fully or is insulation for 2 x 6 ok. Also, if not good insulation to use, am open to suggestions. Thanks!

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Old 07-14-2008, 09:32 PM   #2
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Flat Roof Help


What did it have in previously?

Some say that you need to ventilate all heated spaces, but on a flat roof, it is much less functional.

Personally, I would add the additional insulation.

Ed

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Old 07-14-2008, 10:18 PM   #3
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It didnt have much, a little loose insulation, about 2" thick. I dont think anything had ever been updated as far as insulation goes in the house. The attic space in the peak roof had newspaper from 1941 under the insulation, which was also old.

I sent the insulation company an email as well (www.roxul.com) and this is their reply:

"With regards to your e-mail, code requires that there bean air space above the insulation to allow for ventilation. With this in mind, I donot recommend filling the cavity. Our R22 insulation is 5.5" thick which will leaveyou a 2" air space for ventilation."

The manufacturer is in the same province as me (Ontario, Canada). I am not sure how this creates ventilation by leaving 2". Is the brick where the ventilation would be from? I assume the brick will let air and moisture to escape so leaving 2 inches would allow the airflow to the brick? If this is the case, I would not be placing a piece of insulation or vapour barrier across the brick at the end of the 2 x 8's?

These old houses are fun to figure out!! lol


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Originally Posted by Ed the Roofer View Post
What did it have in previously?

Some say that you need to ventilate all heated spaces, but on a flat roof, it is much less functional.

Personally, I would add the additional insulation.

Ed
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Old 07-14-2008, 10:54 PM   #4
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Flat Roof Help


you should have vents at each side of the joists in that situation in order to vent properly,I would add 2-3 "Isocyanurate roof insulation on the roof side for added insulation R-22 doesn`t seem like enough R value for your locale,I`m in NY,and it`s R-38 here
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Old 07-14-2008, 11:46 PM   #5
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Flat Roof Help


Hmm, ok, this is definitely a lot more then I am looking to do. I guess my choices for now are use R22 and leave the air gap for venting in the future, or fill the 2 x 8's so there is less air space and have a warmer ceiling in the flat roof area. Is this what I am looking at? Thanks for help.
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Old 07-15-2008, 12:21 AM   #6
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don`t fill the space as this will trap condensation,and destroy the roof from inside---remember adding insulation only saves you money in heating bills in the long run,and allows your roofing to have a "much" longer life expectancy-your call--I know several canadian roofers who say the better sealed and insulated the better,also you may want to look at this alternative-closed cell foam(R-6.5-7 PER INCH)--http://www.soythane.com
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Old 07-15-2008, 12:55 AM   #7
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Thank you very much. Will definitely consider this method.

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