# insulating a flat roof



## Rodney Cox (Jul 25, 2015)

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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## Windows on Wash (Aug 30, 2011)

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?


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## Rodney Cox (Jul 25, 2015)

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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## Windows on Wash (Aug 30, 2011)

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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## rustyjames (Jul 20, 2008)

Rodney Cox said:


> 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?
> 
> ...


The EPDM can be glued right to the insulation, no need for sheathing on top of the insulation.


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## Rodney Cox (Jul 25, 2015)

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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## Windows on Wash (Aug 30, 2011)

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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## Windows on Wash (Aug 30, 2011)

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/EnergyCodeReqs/?state=Virginia


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## Rodney Cox (Jul 25, 2015)

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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## 1985gt (Jan 8, 2011)

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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## Rodney Cox (Jul 25, 2015)

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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## 1985gt (Jan 8, 2011)

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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## rustyjames (Jul 20, 2008)

>>>>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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## Windows on Wash (Aug 30, 2011)

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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## Rodney Cox (Jul 25, 2015)

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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## Windows on Wash (Aug 30, 2011)

Sure you can. Only area of concern for me would be the overhangs and fascia boards that will not be as insulated and therefore cold spots.


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## Gary in WA (Mar 11, 2009)

Polyiso ages and losses R-value; to get the safest R-value from it, IMHO- figure R-5 (Implications) per inch; http://www.buildingscience.com/documents/information-sheets/info-502-temperature-dependent-r-value

Reducing the temperature of the first condensing surface (roof sheathing) requires R-25 (5" Polyiso) for your location in Zone 6; http://www.buildingscience.com/docu.../irc-faq-conditioned-attics?searchterm=attic+ This would eliminate any vapor barrier requirement on the sheathing, or under it. The previous answers covered it, I just wanted to point out the vapor barrier as it appears you may still be having difficulty there. Hope this helps you with vapor barriers and your set-up.

Though you can figure it (as said already by 1985) at R-5.6 (to meet code minimum) or 4-1/2"... per your energy requirements per climate Zone 6; http://energycode.pnl.gov/EnergyCodeReqs/index.jsp?state=Vermont

Gary


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## 1985gt (Jan 8, 2011)

Rodney Cox said:


> 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


There is no reason you can't do as you described. How does the roof drain? if 3 sides have a parapet then I assume the slope goes to the open side and it goes in to a gutter? You would have to stack up 6 layers of 2x's on teh front edge for a nailer so you have a place to attach your edge metal. So keep that in mind there will be extra money in covering that.

Do you have a picture of the area?


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## Rodney Cox (Jul 25, 2015)

i will posts some pics in a little bit i am just trying to get the best roof i can without breaking the bank. If i would put the cellulose back into the roof would it need to be dense packed i know it would settle over time and leave a void between the insulation and the roof decking would that cause issues later.I would still put 6 inches of iso on top of deck


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## 1985gt (Jan 8, 2011)

There shouldn't be a problem with the void. IMO


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## Rodney Cox (Jul 25, 2015)

I will attempt to post some pictures later today. My first attempt the pics are posting upside down.


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## Rodney Cox (Jul 25, 2015)

but if i was to leave the cellulose i would have to remove all of it seal the perimeter and where all walls are and any ceiling penatrations such as lights with spray foam then reinstall cellulose


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## Rodney Cox (Jul 25, 2015)

another question i will have to buy a heavy duty tarp when i am doing the roof could i also use the tarp under the polyiso for vapor barrier


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## Windows on Wash (Aug 30, 2011)

You don't need a vapor barrier in this climate. 

Seal up the drywall by sealing up the top plates, box and seal out the can lights, seal up any plumbing vents, penetrations, etc. 

I am guessing there is no ductwork in the ceiling but if there is, seal it up well too. 

Insulate the perimeter properly and you should be good once the polyiso goes down.


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## Gary in WA (Mar 11, 2009)

RC, I gave you the wrong requirements (good catch, WoW- thanks for PM); yours are R-15 in foam, and R-38 total, cavity fill made with whatever... http://energycode.pnl.gov/EnergyCodeReqs/index.jsp?state=Virginia Be sure to air seal the (hopeful) blocking between rafters to stop air/moisture coming up from the brick/wood sheathing on wall void. Many times I see they are not blocked off there as soffit material stops at brick because installed after mason finished (easier not to cut top row of bricks to below soffit/rafters). The warm/moist air permeates the brick after a rain and sun drives moisture through to required void space behind brick. Just a quick check with hand over the wall from inside as this would allow moisture to your exposed rafters. http://www.buildingscience.com/documents/reports/rr-0104-solar-driven-moisture-in-brick-veneer

Gary


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## Rodney Cox (Jul 25, 2015)

Should i use a open cell or closed cell foam for air and moisture sealing in attic space or does it matter. So I need to take out cellulose foam where all walls meet ceilings foam all lighting and other penetrations through the ceiling and go around entire perimeter foam all exposed brick in attic space and rafter ends where they contact brick


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## Windows on Wash (Aug 30, 2011)

Most folks will specify closed cell foam when it comes to this application in roof assemblies. Closed cell is far less vapor open than is open cell foam. While I agree, the most important aspect of the air sealing in this case would be to stop the airflow and both open cell and closed cell will do a fine job of that.


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## Rodney Cox (Jul 25, 2015)

My wife tried to post pics. Everytime they post upside down any ideas


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## Rodney Cox (Jul 25, 2015)

Posted pictures in different albums see what you think it just shows how i did the top of the parapet walls and the existing metal roof i am going to figure out exactly what i need and post them with the steps i need to do for the roof then if you all can look and make sure i am not overlooking anything i would appreciate it Thank you all for all the good info


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## Rodney Cox (Jul 25, 2015)

Another question I want to screw 1/2 inch osb over top of polyiso I found some 8 inch screws for a good price just wandering how many i need and what the screw pattern would be for this


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## 1985gt (Jan 8, 2011)

Rodney Cox said:


> Posted pictures in different albums see what you think it just shows how i did the top of the parapet walls and the existing metal roof i am going to figure out exactly what i need and post them with the steps i need to do for the roof then if you all can look and make sure i am not overlooking anything i would appreciate it Thank you all for all the good info


You will want a cap metal for over the parapet. The EPDM will wrap up and over the wall. A cleat will be installed, the cap metal will hook in to that and it will be fastened on the back side. The cap should be sloped to the roof side. 

Why does it look like the metal roof was installed against the slope? 



Rodney Cox said:


> Another question I want to screw 1/2 inch osb over top of polyiso I found some 8 inch screws for a good price just wandering how many i need and what the screw pattern would be for this


You will want to use 3" plates also for wind up lift. The standard fasting requirement is 16 per 4x8. All the manufactures will have the screw pattern on their web site.

On another note, how big is this roof, this might be a bit much for one person to take on them self.


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## Rodney Cox (Jul 25, 2015)

The parapet wall has the roof on it it sticks to the inside about 10 inches once i put down the foam and plywood i will build up a wall to the bottom of that overhang. The roof on top of the parapet wall will have cedar shakes that slope to the outside then i will cap over i will bring epdm all the way up under. So i have to use plates for plywood wont that show through the epdm membrane


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## 1985gt (Jan 8, 2011)

Yes they will show through. It's not an issue.


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## Rodney Cox (Jul 25, 2015)

I can get 2 inch or 3 inch (deck plates)this will be on top of the plywood and the epdm will go down over top of that which would be best for this application


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## 1985gt (Jan 8, 2011)

The 2" ones are for seams of a mechanically attached roof. The 3" plates are for holding the insulation down.


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## Rodney Cox (Jul 25, 2015)

Thanks


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## Rodney Cox (Jul 25, 2015)

Found a really good deal on number 10 roof deck screws and plates. Many things I've read says I should use #12. Do you think #10's would be okay since they are going into oak decking or should I get #12?


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## 1985gt (Jan 8, 2011)

#10's I've never even seen them for insulation screws. #12's are standard, some projects call for #14, or even #15. If you are going in to thick oak and using a impact driver I would probably use #14's. Be careful not to over tighten the screws, if the plates you are using are plastic they will "pop" when fully driven. The metal ones will depress a slight amount.


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## Rodney Cox (Jul 25, 2015)

Thanks I appreciate all the helpful info


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## Rodney Cox (Jul 25, 2015)

Can you tell me if open cell low density sound barrier spray foam will work for air sealin it has a r value of 3.5 per inch


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## Windows on Wash (Aug 30, 2011)

Open cell SPF foam is an air barrier.


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## Rodney Cox (Jul 25, 2015)

i found a big kit for a good price is it possible after i tear off roof and remove cellulose to (from the top) spray foam the entire ceiling with a 1/2 inch thick foam before reinstalling cellulose


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## Windows on Wash (Aug 30, 2011)

Anything is possible, however, a half-inch of foam over the entire drywall surface is largely unnecessary in this case. If you just spray foam on the top plates and other penetrations in the envelope, you should be A-OK.


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## Rodney Cox (Jul 25, 2015)

I had found a 600 bd ft kit before but looking today i found a 1350 bd ft kit for 200 dollars more just wandering if it would help with moisture going through plaster ceilings i am going to try to post a step by step later this evening just to make sure i understand the whole process i wont be starting the job for a month or so but i am trying to go ahead and get all the materials needed for the job Thanks


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## Rodney Cox (Jul 25, 2015)

Steps for the Roof:

1. Remove galvanized metal roof
2. Remove decking
3. Remove cellulose insulation
4. Spray foam all wall, light and ceiling penetrations as well as any exposed brick in the attic space and rafter ends.
5. Reinstall cellulose as tight as possible
6. Reinstall decking
7. Vapor barrier or no vapor barrier
8. Install 2 - 3" x 4 x 8 polyiso insulation, staggering joints to avoid thermal bridging
9. Install 1/2" OSB with #15 x 8" w/ 3" deck plates- 16 per sheet
10. Install EPDM rubber with 6" laps at joints and up the walls

Please let me know if this sounds acceptable or if I have missed anything. I would like this roof to last as long as possible. If I get a larger spray foam kit is it acceptable to spray foam upper part of ceiling between joists?

At the lower end of the roof I only have approximately 4-5" of insulation space. Part of it is where the master bath shower is, can I put iso insulation between joists in the lower spaces to increase R value?

Thank you all for all of the valuable information.


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## Gary in WA (Mar 11, 2009)

If using ocSPF, 3-1/2"--5-1/2" is required for an air-barrier- depending on 1/2# or 3/4# SPF. You would also want a vapor barrier between it and the brick cavity to stop moisture or increase the thickness greatly for a moisture barrier there. OR just use ccSPF instead; 1-1/2" for air barrier. 

Did you pull a section of soffit for an inspection access? To see if there is blocking over the wall or not; SPF against the blocking on brick side and into the cavity OR (no blocking; against the rafter rim board and in the brick cavity. Could use some scrap XPS at rim/blocking if less expensive- check cost. Cold also add some poly before spraying brick cavity. Are there exhaust holes in the mortar joints below the top row of units; will you be adding those... since stopping airflow with new SPF. Does the soffit extend over the brick or abut them... if over, just add some rigid XPS there also; no need for SPF. Are the lights rated IC- insulation cover- for temp. gain... light boxes should really be fire-rated foam (pp.42/47, 47/47; http://buildingscience.com/documents/guides-and-manuals/gm-attic-air-sealing-guide/view

Gary
PS, what size a rafters?


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## Rodney Cox (Jul 25, 2015)

So which is better to use open cell or closed cell for this application if I have to spray that much more of the open cell then I would probably be money ahead using closed cell what about putting ISO between rafters at back of the house there is only about 5 inches back there


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## Windows on Wash (Aug 30, 2011)

You don't need to spray everything. If you have the OC foam, you can apply that to the penetrations and top plates. You could sister in some skinned rigid foam at the outer wall where the bricks are and spray it in place.


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## Gary in WA (Mar 11, 2009)

ISO there would work, a lot cheaper than SPF, either open or closed cell. Notice in the link on page provided; open cell is applied *over *the other air-sealing compound- caulk or sealant as they are not full of air holes/passageways in their composition- better and more effective than 4" of ocSPF stopping air AND moisture. Line the rim and some on the soffit board/top plate, between rafters, air sealing (first) as well on the exterior areas. Is the canned foam (cc) an option... figure it both ways to decide. Though GAPS & HOLES or what-ever (cc) is difficult to work with compared to WINDOW/DOOR (oc) that is detail -oriented.

Gary


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## Rodney Cox (Jul 25, 2015)

I dont have any foam yet but will the closed cell work as a vapor barrier for the brick as long as it is 1 1/2 inches thick


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## Gary in WA (Mar 11, 2009)

Yep; http://www.sprayfoam.org/technical/faqs

Little different; http://www.energy.ca.gov/business_m...m_Compliance_Option/Attachment-1_QIISPFv6.pdf

Gary


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