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Old 07-26-2015, 07:09 PM   #1
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insulating a flat roof


I bought a 1945 brick home with a flat roof that has a 1/2 inch X 12 pitch. There was no insulation and a metal galvanized roof. I blew in cellulose insulation, however it is obvious that there is not enough insulation. I took out part of the ceiling in the bathroom and the insulation was soaked.

Can I tear off the metal roof and add 6 inches of ISO insulation on top of the existing roof deck, 1/2 inch plywood and EPDM rubber? or will it still condensate on the sheathing?

This is neither a cold or warm roof application. Flat roof is unvented. Any help would be much appreciated.

So I would have 6" of ISO on top of roof deck and blown in cellulose in rafter bays.

Thank you!
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Old 07-26-2015, 07:44 PM   #2
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Where is the home? Is it a safe bet that the roof is unvented? Any pics.

6" of ISO is going to be plenty. How are you going to tie it all in and attach it?

Last edited by Windows on Wash; 07-26-2015 at 07:48 PM.
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Old 07-26-2015, 08:24 PM   #3
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We are located in Winchester, VA. Yes, the roof is unvented. I have being doing a lot of research and everything says that I need an air barrier between existing deck sheathing and insulation. I am wondering if I can use "used"EPDM membrane for this purpose,(2) 3" layers of poly ISO staggered, 1/2" OSB attached with 8" screws into existing wood deck?

I could put 2 x 4 furring on top of ISO then plywood on top of that to give me a ventilated space between ISO and roof (EPDM), but only if needed.
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Old 07-27-2015, 06:19 AM   #4
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The drywall on the interior would serve as your air barrier.

The framing/rafters are what size again?

The bathrooms need to be vented. If they are not, no amount of insulation will keep the humidity from diffusing into the interstitial insulation space.

What size is the framing?

If the roof is coming off, this is what I would do:
  • Remove existing roof
  • ensure that bathrooms, cooking appliances, etc. are vented outside the roof deck
  • Remove existing insulation
  • Seal around any penetrations in the air barrier (drywall ceiling) - Google Air Sealing and insulation
  • Install proper insulation fill
  • Re-sheet existing roof deck with proper plywood and clips
  • Install rigid foam over top of roof deck (thickness requirement will depend on framing size
  • Install new roof
    • Depending on the roof, you may need to re-deck again, go straight overtop, or put down some purlins/battens.
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Old 07-27-2015, 07:11 AM   #5
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Quote:
Originally Posted by Rodney Cox View Post
I bought a 1945 brick home with a flat roof that has a 1/2 inch X 12 pitch. There was no insulation and a metal galvanized roof. I blew in cellulose insulation, however it is obvious that there is not enough insulation. I took out part of the ceiling in the bathroom and the insulation was soaked.

Can I tear off the metal roof and add 6 inches of ISO insulation on top of the existing roof deck, 1/2 inch plywood and EPDM rubber? or will it still condensate on the sheathing?

This is neither a cold or warm roof application. Flat roof is unvented. Any help would be much appreciated.

So I would have 6" of ISO on top of roof deck and blown in cellulose in rafter bays.

Thank you!
The EPDM can be glued right to the insulation, no need for sheathing on top of the insulation.
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Old 07-27-2015, 08:44 AM   #6
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If I have to take out the cellulose...roof is 32' x 32' with 24" of depth in the front and 6" in the rear.
Could I just leave out cellulose and add more ISO to top of roof deck to achieve at least an R50 and just consider it a warm roof?

All bathrooms are ventilated to outside.

Thank you for all of your comments.
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Old 07-27-2015, 10:25 AM   #7
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No need to throw the cellulose in the trash if its good. You can remove it and reinstall it once you make sure the interior ceiling plane is airtight.

At 6" = R-18-R20'ish with the cellulose. You need to be R-38 so 3" of Polyiso would get you there.
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Old 07-27-2015, 10:26 AM   #8
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No need to throw the cellulose in the trash if its good. You can remove it and reinstall it once you make sure the interior ceiling plane is airtight.

At 6" = R-18-R20'ish with the cellulose. You need to be R-38 so 3" of Polyiso would get you there.

https://energycode.pnl.gov/EnergyCod...state=Virginia
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Old 07-27-2015, 11:52 AM   #9
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How would I ensure the interior ceiling to be airtight without spraying foam around entire perimeter?
Which would actually cost more than the ISO that I can buy for a warm room application.
I can get "used" 3" polyiso in really good shape for $14 a sheet.

I just want the best and most stable roof for my money.

Thank you!
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Old 07-27-2015, 12:22 PM   #10
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EPDM will not condensate like the metal roof will. 1/2" per foot generally isn't enough slope for a metal roof.

If you are fully adhering the EPDM do not try and glue to used iso, you need a coverboard of some type.

The new R value for ISO is 5.6 per inch. The 3" would actually 17.4. Doing multiple layers the insulation needs to be staggered and generally you wouldn't want to go thicker then 2.7" iso.

Using insulation below and above the roof deck is fairly common. If your real worried put a vapor barrier down over the existing plywood. 6 mil plastic, ice and water shield. something along those lines.
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Old 07-27-2015, 12:51 PM   #11
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how would i achieve the r value needed without using 3- 3 inch pieces of iso if i remove cellulose from attic
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Old 07-27-2015, 01:43 PM   #12
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3 layers of 2" and on layer of 1" will get you R-39.8. That's a lot of insulation for a roof of this type. You will have to add wood blocking to the outside edges to attach edge metal to. It's also very expensive, and will change the way your property looks. Insulating from both sides is a better bet. IMO
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Old 07-27-2015, 02:27 PM   #13
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>>>>EPDM will not condensate like the metal roof will. 1/2" per foot generally isn't enough slope for a metal roof. <<<<<

Except for some standing seam, mechanically crimped, there are some that are good for 1/4:12 slope.
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Old 07-27-2015, 02:29 PM   #14
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You can seal from the top down with just a small froth pack, replace the sheathing, use your 3" foam (be sure to band the edges like 1985gt says) and then install the roof.

We have put metal on near flat applications when it is mechanically seamed.
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Old 07-27-2015, 05:26 PM   #15
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i appologize for not being more knowledgeable but is there any reason i could not go with 3 -3 inch layers of polyiso with a vapor barrier between the deck and the insulation and then 1/2 inch osb on top and just get rid of all the cellulose if i woulb end up leaving the cellulose wouldnt i have to dense pack it if i just go with the polyiso i should have plenty of r value and as far as looks there is a parapit wall around 3 sides of roof i did a box cornice around it with a small roof with a 3/12 pitch so i can make it look ok even with 9 0r 12 inches on top of deck the place im getting the iso from only has 3 inch thats why i want to use 3 inch for 14 dollars a sheet
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