Exterior roof insulation system with metal roof
I have a circa 1850 timber frame house near Syracuse NY (zone 5a). We get a lot of snow! This house needs everything – siding, roof, windows so it is ideal for installing exterior insulation. In addition to renovation of all of the exterior I will be constructing an addition. I have a good feel for what I want to do on the addition. The existing roofs are 7/12 and 5/12 slopes. There is one small valley but otherwise are simple gables. Rafters are small and widely spaced. Decking is wide rough-sawn boards in good shape but as is typical for this era house, there are gaps between boards, splits, etc. I have a few questions on the renovations:
1. Is there a chance I can install peel and stick membrane directly to board roof sheathing or should I plan on adding OSB on top of boards? Roofs have lots of slope and membrane will be covered fairly quickly. It seems that one of the lesser grades of peel and stick is ok in this method.
2. I plan to use exposed fastener metal roofing. Per suggested detail I will install 2x4s attached through foam (I plan to use 2 layers 2″ XPS) to create vent space (aligned with rafters). I was going to add 2×4 purlins on top of first layer of 2x4s to attach roof to. I can find no details or photos of roof installed this way. Has this been done successfully?
3. Can I just lap the joints in foam, and tape the joints under the roofing? Felt or some other membrane on top of foam seems like a waste of resources, as taped foam will shed moisture and peel and stick is the last line of defense.
4. I plan to open up ceilings to expose the timber frame where I can. I’ll have to somehow improve the rafters but it may be difficult to add enough depth to put R-30 below the sheathing to get to R-49. It doesn’t seem to matter if I underachieve here, correct? It will be so much better than what it’s replacing (anyone else have vermiculite insulation in their attic?
Enough for now. Thanks in advance for the input!
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Replies
Joel,
Q. "Is there a chance I can install peel and stick membrane directly to board roof sheathing?
A. This is a judgment call. The answer depends on whether the sheathing boards are rough-cut or planed, how dusty they are, and the width of the gaps between the boards. If the gaps are narrow, the boards are clean, and the lumber is planed, you are probably good to go. If the situation is iffy, a layer of OSB might be a good idea.
Q. "I will install 2x4s attached through foam (I plan to use 2 layers 2" XPS) to create vent space (aligned with rafters). I was going to add 2x4 purlins on top of first layer of 2x4s to attach roof to. Has this been done successfully?"
A. Yes. You can probably use 1x4 purlins if you want.
Q. "Can I just lap the joints in foam, and tape the joints under the roofing?"
A. I don't understand this question. How can you lap the joints in the rigid foam? I think you will need to butt the panels together. Of course, you can choose rigid foam with tongue-and-groove joints if you want.
Q. "Felt or some other membrane on top of foam seems like a waste of resources, as taped foam will shed moisture and peel and stick is the last line of defense."
A. I'm a believer in the use of roofing underlayment in all cases. Rigid foam can shrink; if this happens, tape might fail, and water would get between the cracks. While the rubberized membrane is, indeed, a last line of defense, why not include the roofing underlayment? It doesn't cost much; it's a code requirement; and it is required by the installation instructions of most roofing manufacturers.
Q. "It may be difficult to add enough depth to put R-30 below the sheathing to get to R-49. It doesn't seem to matter if I underachieve here, correct?"
A. The answer depends on your performance goals. If you install less than the code-minimum requirements for roof insulation, your roof will leak heat at a faster rate than a code-minimum house. I urge you to contact your local building department in any case, to make sure that your plans comply with local code requirements.
Thanks for the input. Boards are rough cut and gaps up to 5/8" so new layer of OSB it is. 1x4 purlins won't work as rafters are 36" +/- on center. Sorry I sounded confusing on "lap joints"; I meant staggered joints for two layers of insulation.
I agree felt is cheap but where would I add it and how? There's no way to attach to foam that I know of. It seems to call for another layer of sheathing in lieu of purlins so I can install felt in a code-approved manner. I like the ladder effect of purlins as it makes metal roof install easier. Again, has anyone done this installation method with metal roof or is this theory?
Thanks!
Joel,
安装在刚性泡沫顶部的屋顶下层，请使用盖指甲。您所需要的只是足够的盖子指甲，以便在安装贴纸之前将下划线持有到位。
You can buy cap nails in lengths up to 8 inches:
http://www.simplexnails.com/InsulationCapDataSheet.html
Joel,
Lots of houses are built with this method. Look to the the Cold Climate Housing Research Center (CCHRC) for info on it, there are even videos.
http://www.cchrc.org/retrofits
or here are the videos
Part 1 plays right into Part 2 or there are separate links
REMOTE - Walls - Part 1
https://www.youtube.com/watch?v=1leyQtqVV-A
and
REMOTE - Walls - Part 2
https://www.youtube.com/watch?v=9GIrSF26SjA
I'm doing my house and barn this way (only I have 6 layers on the house and 5 on the barn). On the barn to reduce the weight (I've got exposed rafters and barn board beneath) - I put down luan (1/4" sheathing). I stapled it down to the barnboard, so that the ice & water shield I used doesn't show through the knots and gaps.
我也就只有伊都ArmourGard(冰&佤邦ter shield) instead of the Grace Ice & Water Shield, because it was less than half the cost, and because it is going to be covered in LOTS of other layers of rigid insulation, and purlins, and more sheathing and then standing seam metal roof. If I get bulk water all the way down to the ice & water shield.....I've got BIG problems. The less expensive stuff is not as flexible as Grace, but I only used it on the pitched roof, valleys and hips. Everywhere else I used liquid flashing by Prosoco - R-Guard liquid applied flashings and membranes.
For the longest time, I used nothing but Grace, but a few years ago, I was doing a remodel that had used the IKO ArmourGard, and it was so resilient still I have used it on other jobs in areas where the flexibility isn't a concern.
布拉德，谢谢你的输入。你是否运行了两层贴纸（一个带有斜坡和一个平行）？由于空气空间并不是真正的通风通道，并且没有良好的通风金属屋顶脊细节（我设计了几个是头部划痕），我正在考虑立即安装一层平行的贴片岭。金属屋顶将从绝缘层隔离，并且每个肋处的空气空间将为空气运动提供一些机会。
Joel,
Honestly, I'm not quite there. Just like many others have done though - I'll have purlins (which the screws go through back to the framing), and 5/8 sheathing on top of that, then a roofing membrane (not ice and water (except at the first 3 feet)), because even on a cold roof, if the soffit overhang (eave) isn't insulated, the hot air that builds up under the soffit from convective currents can still cause ice damming. I'll then have standing seam metal roof atop of it all.....