Roofing insulation question - Flat roof

[Tom Sestito]

I'm in the midst of designing and sourcing materials for my flat roof build in NW Ontario. I'm looking at a warm roof design - meaning the roof is not vented. The roof is approx 2/12 pitch.

My question is related to insulation - I'm trying to get to about R60. I've looked at SIP's and am getting quotes for very high dollars ($12k) for my ~900 sqft roof.

I have a potential good deal on some 2" thick insulation, but it's recycled meaning that is has tar on one side from a previous installation.

I was thinking of stacking this up in layers until I reach around R60. I realize the roof will be quite thick at that point (~14") but that's ok.

The product looks a little rough but it's actually in very good condition - the edges and corners are mostly intact. I like the idea of reusing a building material and cost is a major factor. I bought some from the seller before and he's reputable and reliable.

I'm not an expert here and I'm trying to make a well insulated/built house as economical as possible.

Just looking for a little feedback or foreseeable issues by doing it this way. I would appreciate any feedback!!

Thanks!!

 

[The Cobbler]

I don't see issues if you're doing it yourself. if you're contracting it out, it may be an issue as contractors don't usually tend to like using used or blem material

 

[Tom Sestito]

I don't see issues if you're doing it yourself. if you're contracting it out, it may be an issue as contractors don't usually tend to like using used or blem material

Ya good point - I will be doing it myself.

 

[billconner]

No experience but I would wonder about gaps and air moving around. I guess if each layer was sealed, it would be ok.

How will you attach/anchor it to structural framing?

20" of cellulose would get same R value. Would be interesting to compare total cost (laying and sealing 6 or 7 layers of foam sheets versus a blower for a day).

 

[Tom Sestito]

No experience but I would wonder about gaps and air moving around. I guess if each layer was sealed, it would be ok.

How will you attach/anchor it to structural framing?

20" of cellulose would get same R value. Would be interesting to compare total cost (laying and sealing 6 or 7 layers of foam sheets versus a blower for a day).

I was wondering the same thing about air movement. I was thinking I could wrap poly up the sides of the entire assembly before I put any fascia on.

To attach it to the structure, I'd use 1x4 furring strips and very long screws. Need to source the screws before I start this whole thing

I've been thinking about cellulose as a potential solution as well. I'd need to build a structure to support it so I don't think there would be much time savings, and from what i can tell the cost will be about 2x what I'd pay for these used foam sheets.

 

[billconner]

I don't know how inexpensive the foam is.

I think then I'd just gap the foam 1/4" and foam in a can the gaps. Probably worth one of those kits with tanks and hoses.

I was surprised to see so many options for 16" screws. Still some skill to get them to hit rafters. That will be a fun day.

I just finished a large new room with 20" of cellulose and it's very easy to heat. The science works.

 

[Tom Sestito]

I don't know how inexpensive the foam is.

I think then I'd just gap the foam 1/4" and foam in a can the gaps. Probably worth one of those kits with tanks and hoses.

I was surprised to see so many options for 16" screws. Still some skill to get them to hit rafters. That will be a fun day.

I just finished a large new room with 20" of cellulose and it's very easy to heat. The science works.

The foam boards are $5 each. I need around 400 of them.

Good idea re:spray foam. I'll look up the 16" screws.

Thanks for the feedback!

 

[Joemctag]

The foam boards are $5 each. I need around 400 of them.

Good idea re:spray foam. I'll look up the 16" screws.

Thanks for the feedback!

I’d try to get advice from a flat-roof contractor. Don’t think joints get sealed at all; just staggered: not aligned over another. 7 layers sounds like a lot. I guess you pick a day where there’s no wind and have help plus maybe a lot of concrete blocks to weight it down. Then cover it before it rains. What were you thinking of using for a cap sheet / membrane? Rubber? EPDM? I know they use long screws and 4” square sheet metal “ washers” to attach insulation to substrate. What spacing? I’d sure try to make sure all the design and details were going to work. Good luck. Used foam like you’ve got is becoming widely available these days. Got some myself, but free. Good luck.

 

[strutaeng]

As long as all the insulation is above the roof deck, should be fine. I'm sure you know the joints have to be staggered.

It's how your typical low-slope warehouse big box store is insulated.

 

[cannuck]

The gold standard would be MIPs, not SIPs. Great that you are heading for the appropriate NRC best practice of R60, but even for MIPs (i.e. closed cell polyisocyanurate bonded in place to metal skins both side - VERY good structural properties) that needs 8" of PU and other than for refrigeration panels, best I can find for building envelope is 6" (=R48). MIPs can be rear fastened and I would expect about $12k in loonies to secure 1,000 sq. ft. (I did 5.5" of spray foam on my 1000 sq.ft. shop last year, and it came out over $10k)

Another vote to find some flat roof people to give you some advice. Friend of mine went 8" of PU rigid board on his new built up flat roof building (co-incidentally, in Winnipeg) and his energy costs for a larger shop are less than 1/2 of his old place (also with built up flat roof). I have very little experience with flat roofing, but I THINK it is the tar that is doing all of the fastening - and if that is true, you should be just fine with the salvage material you have available and a "glue on" plywood skin with sealing membrane (torch on or bonded EPDM). Find a pro.

 

[Joemctag]

As long as all the insulation is above the roof deck, should be fine. I'm sure you know the joints have to be staggered.

It's how your typical low-slope warehouse big box store is insulated.

I think the black liquid on yours was used to adhere each sheet . Then the top layer gets the screws , as a positive attachment. Some, like mine, just have a few big spots of yellow adhesive of some kind for adhering layers as you go. Probably want some kind of treated-lumber parapet to contain foam and give support for whatever edge termination, etc. Maybe you can do separate stacks of foam 7 layers thick with the edges staggered in and out say 6”. No expensive adhesive. Then screw down that stack. Do adjacent stack sliding every other sheet into its 6” “ slot” until you’ve got another stack to screw down. Commercial roofers have a lot of guys and materials and experience, so it’s easy for them. You’re going to have to plan carefully.

 

[Tom Sestito]

I’d try to get advice from a flat-roof contractor. Don’t think joints get sealed at all; just staggered: not aligned over another. 7 layers sounds like a lot. I guess you pick a day where there’s no wind and have help plus maybe a lot of concrete blocks to weight it down. Then cover it before it rains. What were you thinking of using for a cap sheet / membrane? Rubber? EPDM? I know they use long screws and 4” square sheet metal “ washers” to attach insulation to substrate. What spacing? I’d sure try to make sure all the design and details were going to work. Good luck. Used foam like you’ve got is becoming widely available these days. Got some myself, but free. Good luck.

I was thinking plywood or OSB on top of this whole thing then roofing on top of that.

 

[Tom Sestito]

There's a lot of detail in this. Not only the insulation but how to make my overhangs work, detail around the roof connection, soffits, fascia, and so on.

I'm good now on the concept (been thinking on this over a year) and now I'm sketching out all the connections and intersections and doing section drawings of each.

I appreciate all the feedback!!

 

[cannuck]

roofing being roofing, strongly suggest treated plywood over anything else. I had a friend in that business (sadly now deceased) and his 100% recommendation was for rubber membrane as top seal over ply. His 2nd choice was always torch-on. Once more, though: listen to me as a completely unqualified and relatively inexperience layman. Find the real deal. You will find roofing materials suppliers (such as my late friend) somewhere near you, and they can give you a ton of free and expert advice.

 

[Tom Sestito]

roofing being roofing, strongly suggest treated plywood over anything else. I had a friend in that business (sadly now deceased) and his 100% recommendation was for rubber membrane as top seal over ply. His 2nd choice was always torch-on. Once more, though: listen to me as a completely unqualified and relatively inexperience layman. Find the real deal. You will find roofing materials suppliers (such as my late friend) somewhere near you, and they can give you a ton of free and expert advice.

Good call on the PT plywood.

Agreed on the vendor - I"m going for a visit shortly and talk through the ideas I have to see. I'm sure they'll have some good feedback.

 

[Joemctag]

There's a lot of detail in this. Not only the insulation but how to make my overhangs work, detail around the roof connection, soffits, fascia, and so on.

I'm good now on the concept (been thinking on this over a year) and now I'm sketching out all the connections and intersections and doing section drawings of each.

I appreciate all the feedback!!

Good to hear! I understand now that you’re not just jumping into this without much thought, as some might do. The sketches, of course, are great for when you talk things over with others. I’m sure your efforts are worth it, especially in your climate. Again, good luck and let us know how it’s going, next summer or whenever it happens.

 

[Tom Sestito]

Good to hear! I understand now that you’re not just jumping into this without much thought, as some might do. The sketches, of course, are great for when you talk things over with others. I’m sure your efforts are worth it, especially in your climate. Again, good luck and let us know how it’s going, next summer or whenever it happens.

I will do, thank you! I sure appreciate this forum and the shared information/techniques.

I'm practically allergic to paying others to do something I can do myself, so this place is a really good to bounce ideas off of others.

 

[jollygreengiant]

I'm surprised to see that you're going with a flat roof design in NW Ontario, are you not concerned with snow loads?

 

[Tom Sestito]

I'm surprised to see that you're going with a flat roof design in NW Ontario, are you not concerned with snow loads?

That's what engineers are for.

 

[cannuck]

That's what engineers are for.

you got one with a long handled shovel?