# Flat Roof Construction- insulation



## Ed the Roofer (Jan 27, 2007)

With no ventilation in the ceiling joists, I don't think it would really matter unless there was a vapor barrier inhibiting the permeance of the moisture migration from humidity.

6 " of rigid isocyanurate would be a significant amount to attach with very long screws. I have done it, but it was usually on the thick side of a tapered insulation system.

No facts readily handy to back me up on this, but my instinct tells me that you should pack the attic joist area with as much R-Value as possible and use a standard 2" isocyanurate insulation board on top of the roof deck. 

If this is a fully adhered EPDM membrane, you have to ensure that the top surface of the insulation you are using on top of the roof deck will hold the bonding adhesive or add an additional layer of hardboard for gluing down the membrane.

Ed


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## EricC (Nov 27, 2007)

Thanks, Ed, but i've read that if I insulate the joist cavity, then it's no longer a "warm roof" and will get condensation unless I ventialte the attic space. All the websites for flat roof indicate that you only insulate on top. For example: http://www.celotex.co.uk/flat_roofing/decks.php. There is very little information on-line about flat roof construction and what there is is mostly from the UK.

When I calculate the R values I need for a total of R38, I need about 6inches of foam on top(?). My first inclination was to use batting in the studs until I read up on flat roof construction methods on-line and found the warm roof approach. It's supposed to eliminate condensation in the joist space. Since the room below is a kitchen, I thought this was a real issue to address. If water vapor does get into the joist cavity, but is too warm to condense, I'm not sure what happens to it (?) Or should I assume that if my ceiling is tight, no water vapor really gets up there?

thanks again.


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## Ed the Roofer (Jan 27, 2007)

The only thing that I can suggest, is to add the batts under theceiling and ensure a completely tight vapor barrier on the warm under side of the batt insulation.

Ed


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## EricC (Nov 27, 2007)

Thanks again, Ed. I think that makes sense to me. Keep an air gap between the ceiling board and the insulation batting which is affixed tot he underside of the roofing decking. I'm just not sure how to make a vapor barrier between ceing joists however.

Am I making something out of nothing? Clearly flat roofs in New England have been done before. What is standard practice for insulating a flat roof and does that standard practice keep condensatioon out of rafters and provide enough R value? My gues is that most insulate the heck out of it and ventilate with soffit vents.


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## rodeo (Dec 1, 2007)

that's a very important point, eric, and I too am wondering how to handle this situation. Stuffing batts is a bad idea for the reasons you say. A VB seems like it oughta help but I feel that it too has its drawbacks for a flat roof. Moisture could find some other way to get up there and then it won't have a way out. Another bad thing about VB's in a flat roof ceiling is that if you ever get a leak, they can hold the water up for a longer period of time and soak over a larger area before it becomes noticeable below. Spray foam seems to be the best for insulating the joist cavity. It leaves no place for moisture to seep into and is also a VB. Its pricey and a pain to install it so I'm still hesitating on that. My next best idea is to lay in foamboard strips into the joist cavity but then I wonder if moisture could still seep through the edges - maybe some sealant could fix that. Bottom line is that above the deck is the best place to lay the foamboard. Put as much as you can afford or at least 3 inches. It seems like alot of roofers don't take this very seriously.


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## EricC (Nov 27, 2007)

Rodeo, you're right, the roofers dont seem to know of really care. I dont thik you can lay in strips of foam board and get a good seal. i thought of that too, but I know that they tell you to lay the foam board above the decking and keep a small gap between each board since the foam board expands and contracts. So that seems to me is reason to believe that you cannot use it to seal against the ceiling joists. I think I'm going to try to put two layers of foam board (1.5" ea x2) as you also seem to have converged upon and fix them to the decking with 4" screws. I have looked for hours on the web and no one tells you how to build a flat roof and achieve the recommended R38 value. what's with that? If you insulate above the deck, and below, then you risk not knowing where the cold interface is and the point of condensation. If you do insulate with foam board between the joists, i wonder if you can install a vapor barrier between the ceiling blue board and the joists bottoms. this would keep moisture out of that joist cavity, then you can insulate it with foam board and above the deck(?)

I think I'm just going to go with 3" of iso board above the deck and have an empty cavity below. If it's cold in my kithen after that I'm just going to be totally pissed since no one has a confident answer. I think with this scenario, my total R value will be about 20-25 assuming the airgap is about R5. I found a few links for the assembled R values of the various materials. Please let me know if you find anything else. Why is this so hard?


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## the roofing god (Aug 2, 2007)

keep the air space at the ceiling joists ,and go ahead w/the iso on top,he air space is necessary to retard the condensationand the iso will also give you a longer lasting roof,and help prevent cold air infiltration


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## EricC (Nov 27, 2007)

thanks- my original question was how do you get enough R value with just the iso board on top. I calculated that you need 6" at least of iso board. I spoke with JMannesville and they thought that seemed a bit unusual. If you agree that the joist cavity should be void of any insulation- how do you get enough R value? Or maybe on a flat roof, you just cant?


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## roofboy (Jan 18, 2007)

Hello,
I am in Ak and we build roofs with an R-38 all the time.
First you should install a substraight over the joists (osb) then build a 6" perimeter around that, usually 2x4s. Install a vapor barrier (usually a heavy mil plastic) make sure the corners are tight. Install the ISO boards. After the ISO you will need to install densdek or 1/2" CDX plywood. We use 3" plates and screws just long enough to penetrate the original substraight for this step. We usually use the double diamond pattern. Last, install your roofing material.
You should end up with an R-38 roof.:thumbsup: 


Keith


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## EricC (Nov 27, 2007)

Hi Keith,

In the name of absolute clarity (since I am NOT a roofer), let me reiterate what you said with more detail.

Starting from the inside of the house:
1. Ceiling blue board/plaster
2. Joists w/o any insulation, no ventilation
3. OSB on joist
4. poly 6mil vapor barrier ontop OSB
5. 6" high roof surround from lumber
6. 6" of iso board (probably 2 layers of 3" board) laid inside the wooden frame
7. Hold down iso with 6.5" screw/plates
8. 1/2" CDX on top of isoboard and wooden frame (cdx covers wooden frame)
9. Hold down iso CDX with 7" screws/plates

Do I have this right? Only issue is that this proposed roof is thicker than current one. How do I terminate one (15') edge that drains to gutter. More the gutter up and have a bigger facia below?

see pix attached. thanks for your help with this. I really appreciate it!


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## rodeo (Dec 1, 2007)

Another idea I had is to stuff blueboard into the joist cavity but first spraying a thick bead of "great stuff" or "hilti spray" around the perimeter so that when pushed up it will spread into the deck and seal it. Also before inserting the blueboard, seal up all edges on on all rimjoists with glue to prevent infiltration from the outside. If that works, then I'd see no problem with stuffing a batt below the blueboard and then sheetrock - and skip the plastic VB. Now is 3 inches of ISO on top of the deck enough? It may be enough to warm the deck past the dewpoint temp. but its still not a whole lot of insulation. I'm guessing that with 3 inches on top, you may have warmed the deck enough so that batts can then be stuffed from below without worries - but, I have not been able to find hard data about this. if you want to keep the joist cavities empty, I'd go for the six inches of ISO on top.


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## rodeo (Dec 1, 2007)

Keith from AK - thats quite a beefy roof you described! 2 points

1. Is the poly VB atop the joist decking necessary? Are you creating the dreaded double VB scenario or is there some other venting going on?
2. Why another layer of plywood on top of the ISO? that seems like alot of weight. I know its stronger but a sandwich consisting of decking-ISO-roof membrane seems good enough. Maybe certain roof coverings might require it or maybe alot of roof traffic. My single-ply PVC membrane if fine laying right on the ISO.


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## roofboy (Jan 18, 2007)

Hello,

EricC - Yes that is pretty much correct with the exception of a few things.
1. I am not sure about the blue board/ plaster. Mostly it is sheetrock here. Then empty joist cavity.
4&5. 6" surround is built first so poly goes up 6" to top of 2x4.
7&9. You do not need the 6 1/2" screws and plates in the ISO level, just 7" screws and plates to hold down CDX and ISO.
Yes the roof will be thicker so you will need to move the gutter and add a bigger facia.

Rodeo- Yes it is a beefy roof but the snow load is different from most of the lower 48.
1. The poly should be the only VB in this situation.
2. Most adhesives will eat the ISO over time. Instead of CDX you could use this product. http://www.aknightcompany.com/pages/sheathing.aspx We use that quite often and it would reduce the weight issue.

Hope this helps.


Keith


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## EricC (Nov 27, 2007)

Thanks again. I get it. Everyone here thinks I'm crazy to look at a 6"think Iso board construction. I think it would be crazy not to. If I do what one roofer told me, 1.5" iso over uninsulated joist space, my total calculated R value is about 10. Won't I get too much heat loss that way and feel the cold air dropping from my ceiling?

If I insulate in the joist space at all I risk condensation in the joist area(?) This is a kitchen ceiling and I can't believe there's anyway I can really seal that joist space from moisture. or is there?

If for some reason I can't get 6" on top of my roof (see pix above) with adjacent peaks and gutters, then I need to make sure I have an alternate plan that gets me close to the R38. maybe the best I can hope for is just to get as much iso brd on the top deck as I can with open joist space(?)


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## roofboy (Jan 18, 2007)

Hello EricC,

You could try using http://www.fomo.com/. We have used this from time to time. There is a company here that all they do is poly spray foam insullation. You can fill your joist cavity with this stuff and it also acts as a VB, plus you will not have the extra height of the build up.

Hope this helps,

Keith


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