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Converting a vented flat roof to a warm roof

The roof covering on my 68-yr-old house in Montreal (Zone 6) needs to be replaced and I will be replacing the tar and gravel with elastomeric bitumen membrane. However, at the same time, I want to improve the insulation and energy efficency of the roof.

The surface area is around 680 sq. ft. and it is currently a ventilated “cold” roof.

The layers today are:
- tar and gravel
- roof sheathing
- air space
- insulation
- wood “decking”
- 9” airspace with joists
- ceiling

I don’t have exact measures for the existing spaces, nor do I yet know what type and amount of insulation is installed. The air space above the insulation is probably 2” but may be obstructed in some areas due movement of the insulation or maybe roof sagging. I have 4 ventilators on the roof and 2 air intakes on side of house. There is also a 1 drain pipe and 1 plumbing vent.

in doing my research, it appears my best option would be to convert my roof to a warm roof. I have read the excellent articles by Martin on “Insulating Low-Slope Residential Roofs”, “How to Install Rigid Foam On Top of Roof Sheathing”, “Combining Exterior Rigid Foam With Fluffy Insulation” as well as multiple posts on the subject. Now I just need some handholding and validation of my understanding of the concepts!

Here are my current plans based on research and discussions I’ve had. The goal is to get the R value of the roof to a minimum of 28.

The layers of the new roof would look like this:
- elastomeric bitumen membrane
- 4.5” min of polyisocyanurate (Sopra-ISO) boards (R26.8)
- vapour barrier (needed?)
- roof sheathing
- existing airspace and insulation filled up with insulation (i.e. removal of airpace)
- wood “decking”
- 9” airspace with joists
- ceiling

Obviously I need to find out the actual measurements and the amount and type of insulation I currently have. Also, need to determine if there is a vapour barrier on the wood decking above my ceiling.

As part of this job:
- ventilators on roofs disappear
- air intakes on the side of the house are removed and sealed
- the perimeter of the roof (below the parapets) is insulated with mineral wool.

So here are my questions:
1. Does this plan make sense for a house of my age in the area where I live?
2. What is the recommended R-value for the mineral wool insulation for the perimeter (below the parapet)?
3. How thick should the polyisocyanurate (Sopra-ISO) board be? I was told that the insulation above the roof sheathing should be 2/3 of the R-Value and the insulation below, 1/3. In addition, for zone 6, at least R28 on top? (In Martin’s article, for zone 6, he recommended 51% on R-value on top and R25, but I’d like to have at least 2/3 on top)
4. Should theire be a vapour barrier above the roof sheathing? What is recommended?
5. Is it OK to keep existing insulation below the roof sheathing?
6. Is it necessary to fill up air space above insulation? (People I spoke to said yes). Fill with cellulose?
7. If there is an existing vapour barrier below the insulation, should it be removed? (I presume yes)

A sketch of proposed warm roof and existing ventilated rood is attached.

This work will not be done by me, but I want to make sure I have a solid plan.

Sorry for the very long questions and thank you in advance for your advice.

Roof diagrams.pdf144.34 KB
Asked by Philip18
Posted Mar 8, 2018 3:58 PM ET

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5 Answers

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1.

Philip,
If you are installing new rigid foam above your existing roof sheathing, you don't need a vapor barrier under the rigid foam. (The membrane above the rigid foam is already a bulletproof vapor barrier.) What you need between the roof sheathing and the rigid foam is an air barrier. In most cases, synthetic roofing underlayment with taped seams will serve as an air barrier at this location.

Q. "Does this plan make sense for a house of my age in the area where I live?"

A. The plan will work, although thicker rigid foam would be better. In general, when combining air-permeable insulation under the roof sheathing with rigid foam above the roof sheathing, the air-permeable insulation should be in direct contact with the underside of the roof sheathing. Your illustration doesn't show that.

Q. What is the recommended R-value for the mineral wool insulation for the perimeter (below the parapet)?

A. In general, this insulation should meet minimum code requirements for walls. (You're basically talking about insulating an attic wall.) I don't know what the code requirements are in Montreal, but something like R-20 or R-25 would make sense.

Q. "How thick should the polyisocyanurate (Sopra-ISO) board be? I was told that the insulation above the roof sheathing should be 2/3 of the R-Value and the insulation below, 1/3. In addition, for zone 6, at least R28 on top? (In Martin’s article, for zone 6, he recommended 51% on R-value on top and R25, but I’d like to have at least 2/3 on top.)"

A. Thicker polyiso makes the assembly more safe, so thicker polyiso is better. R-26 of rigid foam (what you are planning) will work, but ideally, the insulation under the sheathing would be in contact with the sheathing. Your illustration shows an air space under the sheathing, and it's unclear whether you will be blowing cellulose in this air space, or leaving it as shown in your illustration.

If you fill the 9-inch high air space with cellulose, you have to calculate the full value of the new insulation when performing your 51% calculation. If this air space is really 9 inches, then the cellulose would have an R-value of about R-33, so you would need more rigid foam to hit 51%.

Q. "Should there be a vapor barrier above the roof sheathing?"

A. As I already explained, in general you want an air barrier, not a vapor barrier, between the sheathing and the rigid foam. Frankly, the vapor permeance of this air barrier doesn't matter much, because the entire assembly (above the sheathing) is already a vapor barrier. The roofing membrane stops all outward drying -- it's a vapor barrier -- so vapor permeance is irrelevant.

Q. "Is it OK to keep existing insulation below the roof sheathing?"

A. Yes, as long as you have resolved the issues surrounding the ratio between rigid foam and fluffy insulation.

Q. "Is it necessary to fill up air space above insulation? (People I spoke to said yes). Fill with cellulose?"

A. No, it's not necessary. You could add all of your desired insulation -- for example, R-40 or R-50 -- as rigid foam above the existing sheathing if you want.

Q. "If there is an existing vapor barrier below the insulation, should it be removed? (I presume yes)"

A. Probably, but that's the least of your issues. If it stays, it's unlikely to cause problems.

Answered by Martin Holladay
Posted Mar 9, 2018 6:26 AM ET

2.

Thanks Martin for the detailed responses.

The air space I had in the 1st warm roof design is the existing one and I was just wondering if I should fill it in or not. Your answer (and others I have received) say yes, better to fill it in. (Though in your 2nd last answer, you seem to say that it is preferable, but not necessary -- did I understand correctly?).

I don't currently know the exact space under the sheathing, nor the type of insulation already installed, but when I get that, I will be able to determine the R-value and then calculate the thickness of the polyisocyanurate on the top.

I was surprised that no vapour barrier was required below the polyisocyanurate, but your explanation makes sense.

I have revised my sketch based of your answers. Did I get it right?

AttachmentSize
Warm roof diagram v2 2018-03-09.pdf 111.46 KB
Answered by Philip18
Posted Mar 9, 2018 10:52 AM ET

3.

Philip,

You may be able to find reclaimed rigid foam that will cost one-third to one-half as much as new material. Dana or another member may chime in with some possible sources.

Answered by Steve Knapp CZ 3A Georgia
Posted Mar 9, 2018 11:40 AM ET

4.

@Steve: thanks for suggestion. I will probably go with new materials for simplicity and also to reassure my wife, who is even quite nervous (and not convinced) about converting to a warm roof.

What I really need most at this point is to find a local "expert" who knows about cold/warm roofs and who can come to my house and validate the plans and proposals. I just spoke to one person today in Montreal who has been consulting on roofs for 30 years and he is dead set against changing anything. Stay with the tried and true. If it ain't broke, don't fix it. When i asked him what the problems with be with a warm roof, he says that with an old house like mine, I risk getting condensation in my walls. When I asked if he was aware of any flat residential warm roofs in Montreal and he said he was aware of three and they all had problems. (This is not reassuring to my wife!). I am guessing that this consultant does not really know warm roofs and from our conversation, he doesn't really want to know.

So if anyone has any suggestions on a local expert in Montreal, let me know!

Answered by Philip18
Posted Mar 9, 2018 12:06 PM ET

5.

Philip,
Talk to a commercial roofer. There are lots of flat-roofed commercial buildings in Quebec. I know that because I drive to Montreal all the time (and also because I pulled up all the images below of Quebec buildings --- from Google images). These buildings all have membrane roofing over rigid foam.

.

Quebec building 1.jpg Quebec building 2.jpg Quebec building 3.jpg Quebec building 4.jpg Quebec building 5.jpg
Answered by Martin Holladay
Posted Mar 9, 2018 1:41 PM ET

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