I’m remodeling a roof constructed from tongue-and-groove cedar planks that serve as a cathedral ceiling
HUDSON124
| Posted in Plans Review on
I’m re-modeling a roof (~1:4 slope) constructed from 2.75″ x 6″ cedar planks (tongue & groove) that serve as a cathedral ceiling, structural support, and roof sheathing. (In Climate zone 5.) I have a tentative plan of adding 7.5″ of polyiso sheet and then a metal roof.
The part of the house this roof covers is 24′ x 28′. The roof has 4.5′ overhangs on the north and south sides. The only penetrations are 1 vent fan, a light tube, and vent for water heater.
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Replies
Robert,
It looks like you forgot to ask a question. You might want to post a comment on this page, letting us know if you have any questions.
Here is a link to an article about the type of roof you are talking about: How to Install Rigid Foam On Top of Roof Sheathing.
Thanks for the link. I was surprised to learn that polyiso loses R when it gets cold.
Anyway, I guess my question got lost in editing. I was wondering if I need venting between the insulation and the metal roofing?
Robert,
Above the polyiso insulation, you'll need to install either purlins (typically, 1x4s or 2x4s, installed 24 inches on center, parallel to the ridge) or solid sheathing (typically OSB or plywood).
If you choose to install purlins, you already have an air space between the rigid foam and the metal roofing.
If you choose to install OSB or plywood, you won't have much of an air space. The air space isn't required, but it can be helpful if you live in a climate with ice dam problems. You can install vent channels above the OSB or plywood if you want; the usual way is to install 2x4s, with each 2x4 directly above a rafter. (These 2x4s are installed at 90 degrees to the ridge.) If you go this route, you'll need another layer of OSB or plywood (or conceivably, purlins) above the vent channels -- so this approach is expensive and time-consuming to install.
Before you decide to install your metal roofing on purlins, talk to your roofer. Some roofers prefer a solid deck under metal roofing.
My roofer was planning on 2x4 purlins parallel to the ridge. If we go that way, do they need to be vented at the edges of the roof? Also, do you think purlins only and not OSB under the metal is adequate support? I don't expect to be up on it much, be will need to on occasion.
The reason that I asked about venting is that I was under the impression that allowing air-flow to the ridge from the lower edge of the roof was important. However, your video seems to suggest that it is less critical than having an airtight ceiling seal.
Robert,
If you are installing the metal roofing on purlins, the air gap under the metal roofing will be adequate. I've installed lots of metal roofs on 2x4 purlins -- the method works well.
Make sure that you install a layer of roofing underlayment above the top layer of polyiso, to catch the condensation drips that will form on the underside of the metal roofing. Don't worry about these drips needing to drain; the water evaporates harmlessly.
You can install insect screening at the gaps between the purlins at the rakes if you want, but most people don't do that, because they think it's unsightly. Even if you cover the gaps with trim at the rake, you'll get plenty of air exchange due to temperature changes, and you won't have any problems.
Three more questions.
First, we're going to remove the old roof entirely. This means that we'll need to seal a lot of cracks between the cedar planks to make the ceiling air-tight. Caulking would take a lot of time. Is there a suitable film for placing between the planks below and polyiso above to get a good air seal? How about a layer of foil-face polyiso at the bottom with tape on the joints?
Second, one of your blog posts mentioned that the R-value of polyiso drops about one third when the temperature drops below 25 F. We have many days like that. So is there any particular insulating material you'd recommend for a top layer?
Third, what do you think of the roof color issue? Our roof is not visible from on our lot, but it is a bit visible across the street. So, I feel okay about using white if that makes the most sense from an energy perspective.
Robert,
Q. "Is there a suitable film for placing between the planks below and polyiso above to get a good air seal?"
A. The answer can be found in the article I linked to in my first answer on this page: How to Install Rigid Foam On Top of Roof Sheathing. (If you haven't read it yet, I urge you strongly to read it.) In that article, I wrote, "Before installing the rigid foam, make sure that the roof sheathing is airtight (or that you install an air barrier immediately above the roof sheathing).
"There are at least two ways to do this:
If your roof has board sheathing, install an airtight membrane (for example, Solitex Mento, a product available from 475 High Performance Building Supply), a layer of synthetic roofing underlayment, or a peel-and-stick membrane.
If your roof has OSB or plywood sheathing, the panel seams can be taped (for example, with Zip System tape or Siga Wigluv tape); after taping the panel seams, install the roofing underlayment of your choice (for example, asphalt felt)."
Q. "One of your blog posts mentioned that the R-value of polyiso drops about one third when the temperature drops below 25 F. We have many days like that. So is there any particular insulating material you'd recommend for a top layer?"
A. EPS. More information here: Cold-Weather Performance of Polyisocyanurate.
Q. "What do you think of the roof color issue?"
A. In Climate Zone 5, where heating bills are higher than cooling bills, you don't want a white roof. Choose your roof color based on aesthetics or cost.
Thanks. Questions...at 475 I came across some discussions of the fact that foam degrades over time, but here on GBA I found indications of the opposite. What's the latest on that?
Also, I'm puzzled by adding the Mento at the base of the foam stack, since it allows water vapor to go through it relatively easily, won't that cause condensation in the insulation during winter time?
Thanks again for your responses. I've tried to incorporate most of them into the design, which reflects my understanding of what my contractor plans to do. I have a couple of questions:
1) The manufacturer says that the metal roof will be noisy unless we put OSB or plywood under the metal. (Purlins would make it noisy). Is this your understanding as well? Previously, we discussed the possibility of not using this OSB layer...will it support a large man walking on it?
2) If I read your comments correctly, we should install i) a membrane (Mento) below the insulation and ii) underlayment under the top OSB layer. Correct? I still have the question that I raised previously about why I should use a material that allows wate vapor to pass through it.
3) Note that in this plan, the rigid foam supports the entire roof. How well will the foam hold up over time? Will it contract and cause the roof to sag?
4) Is it important to have a metal edge between the roof and the supporting wood at the eave edge?
I've attached my drawings of the plans as they stand right now. Please comment. Note that the house dates to about 1960 and the walls are poorly insulated.
Robert,
Q. "At 475 I came across some discussions of the fact that foam degrades over time, but here on GBA I found indications of the opposite. What's the latest on that?"
A. The owners of 475 advise builders to avoid foam insulation. Many of their arguments are philosophical rather than technical. If you want to read a technical summary of the known information about so-called "thermal drift" in rigid foam insulations, you should read this GBA article: Thermal Drift of Polyiso and XPS.
Q. "I'm puzzled by adding the Mento at the base of the foam stack, since it allows water vapor to go through it relatively easily, won't that cause condensation in the insulation during winter time?"
A. The Solitex Mento acts as an air barrier. It vapor permeance is irrelevant. If you want, you could install a peel-and-stick membrane like Ice & Water Shield (although some people worry about the smell of this product) or taped OSB in this location to act as an air barrier instead of the Solitex Mento. Don't worry about vapor flow -- the interior face of the rigid foam will be warm (making condensation impossible), and the rigid foam is already a vapor barrier (which means that the water vapor won't travel outwards beyond the interior face of the rigid foam).
Robert,
Q. "The manufacturer says that the metal roof will be noisy unless we put OSB or plywood under the metal. (Purlins would make it noisy). Is this your understanding as well?"
A. I think that the multiple layers of rigid foam will greatly reduce the noise transmission. I wouldn't install OSB or plywood simply out of noise concerns, if I were you.
Q. "Previously, we discussed the possibility of not using this OSB layer. Will it support a large man walking on it?"
A. Yes, if you are talking about typical types of steel roofing. The same cannot be said for aluminum roofing.
Q. "If I read your comments correctly, we should install i) a membrane (Mento) below the insulation and ii) underlayment under the top OSB layer. Correct?"
A. Yes. You need an air barrier of some type above the tongue-and-groove boards, and you need roofing underlayment under the roofing. (Building codes require the roofing underlayment.)
Q. " I still have the question that I raised previously about why I should use a material that allows water vapor to pass through it."
A. You don't have to use a vapor-permeable layer for the air barrier, as I discussed in my previous comment. You can choose from a great many materials for this layer -- you just want a durable air barrier.
Q. "Note that in this plan, the rigid foam supports the entire roof. How well will the foam hold up over time?"
A. Quite well. Commercial roofers install rigid foam above roof sheathing all the time.
Q. "Will it contract and cause the roof to sag?"
A. No. Even if it shrinks -- and it shouldn't shrink much -- it should shrink uniformly.
Q. "Is it important to have a metal edge between the roof and the supporting wood at the eave edge?"
A. You don't need drip edge at the eaves with metal roofing, but you should discuss your trim options with your roofer.
Q. "I've attached my drawings of the plans as they stand right now. Please comment."
A. Whenever you have ceiling boards that pass over the exterior wall and create an exterior soffit, you have the opportunity for hundreds of air leaks at the rakes. These leaks are hard to seal, but you can try to reduce air infiltration with interior caulk at the board seams. (This is an unsightly and imperfect solution.) The best solution is the "chainsaw retrofit" solution -- lop off the overhangs at the rakes and create new applied overhangs. Not a fun subject to think about, but forewarned is forearmed.
As I was thinking about the bathroom vent fan that goes into the new roof, I noticed that you recommend using whole-house ventilation where possible. Here's my situation:
1) My house has a forced air heating/cooling system.
2) I have a return air "duct" that would make it very easy to add an external vent/damper near the roof ridgeline.
3) I have a variable speed furnace fan, although I'm not sure exactly how low it can go.
4) My house isn't super tight, but we'll be making it tighter in the near future.
5) Right now I have a standard water heater (no forced vent), an oven vent fan, and two bathroom fans. One is very near the top of the roof in our tri-level house.
Would it take an HVAC contractor to hook up that AirCycler® g2 Supply + Exhaust Ventilation System? Would you recommend doing so?
The water heater is about 20 years old and of standard efficiency. Would you replace it? What with?
Again, thanks very much. I'll get rid of the OSB on top.
Back to the issue of venting the roof...the contractor originally didn't plan on a vent space. We do have cold winters with snow. The roof surface will be metal, but the slope is modest (1:6). Previously you said that venting isn't required, but it might help with ice dams. Do you think we should vent even with the metal roof?
You recommended roofing underlayment. Would you recommend Mento? Would a water and ice product be better? Could we use asphalt felt?
I'm thinking of going with 2" of foil-faced poly-iso at the bottom of the insulation stack. Still seem okay to you? If we use foil-faced polyiso and tape the joints, is the Mento layer still needed at the bottom? Elsewhere, you said that taped OSB would be adequate (Please see attachment).
There will be screws going through the membrane (Mento) layer. Is Mento better than standard underlayment even with screw holes?
The insulation stack will be surrounded by 2"x8" lumber stood on its edge. Should this box plus underlayment and bottom and top be air-tight? Or just water tight? Do we need to seal the cracks between the lumber and the planking?
The planned roof will be up to R-45 with XPS/polyiso, up to R-40 with EPS/polyiso. I suppose we could go thicker, but the rest of the walls are so terrible (maybe R-10) and we have so many windows that my intuition says it isn't worth it. Do you agree or disagree? Why?
I looked at the Owen's Corning website this morning. They claim the global warming potential of the gases they use to blow the XPS have been reduced by 70%. They also claim that EPS is much more prone to absorbing water. There's also aesthetic reasons to not want to make the roof any thicker. Thoughts?
Concerning the "chainsaw retrofit"...the grooves in the tongue+groove boards run parallel to the rake-edge walls (see photo at this link https://goo.gl/photos/98KZGaTEamhbYcEG8). If I understand your comments, that should help on those edges. If I had to, I could remove the trim and add caulk. Most of the rake-edge walls have windows like those shown.
At the eave-wall junctions, I've previously re-insulated some of the exterior walls on the east side of the house and sealed gaps there and can do more of that in the future. On the west side, the eave-wall junctions are either in closets or behind trim, so I can caulk them without being visible.
I was planning to put in a light tube in the center about 2/3 of the way from eaves to ridgeline. It would illuminate an interior hallway with no windows. Does that seem like a good reason to violate your minimal roof penetrations priniciple?
Robert,
It's getting hard to keep up with your stream of questions. I'll do my best. But I'm beginning to think that you might need to hire an architect, engineer, or experienced general contractor -- someone to help steer you through your project, to answer your regular questions and to reassure you at times of worry. If you are feeling a little bit out of your depth, it may be because you are out of your depth.
Q. "Do you think we should vent even with the metal roof?"
A. As I wrote before, a vent channel isn't necessary. I don't think you'll have ice dam problems, unless your roof has north-facing valleys. This is a judgment call -- one that depends in part on your budget.
Q. "You recommended roofing underlayment. Would you recommend Mento? Would a water and ice product be better? Could we use asphalt felt?"
A. You don't need Ice & Water Shield on your entire roof, but building codes require roofing underlayment. Asphalt felt works perfectly well, but many roofers now prefer synthetic roofing underlayment, which is somewhat easier to install. Almost any brand will work. Talk to your roofer.
Q. "I'm thinking of going with 2 inches of foil-faced polyiso at the bottom of the insulation stack. Still seem okay to you?"
A. Yes. It's rare for my opinion to change in 24 hours.
Q. "If we use foil-faced polyiso and tape the joints, is the Mento layer still needed at the bottom? Elsewhere, you said that taped OSB would be adequate."
A. Many building scientists, notably Joe Lstiburek, emphasize the need for a bulletproof air barrier under the rigid foam installed above your roof sheathing. (Lstiburek worries that air can leak through the sheathing, and find "three-dimensional pathways" through the rigid foam seams.) Taping rigid foam works, but the worry is that some types of rigid foam, including polyiso, may be dimensionally unstable. In other words, the foam can shrink.
Taped rigid foam is more likely to be a long-term air barrier if it is installed in multiple layers with staggered seams. To me, the cost of an additional air barrier under the rigid foam is worth it. But this is your roof, so you get to make the judgment calls -- especially if your budget is tight.
Q. "There will be screws going through the membrane (Mento) layer. Is Mento better than standard underlayment even with screw holes?"
A. I imagine so. Some brands of synthetic roofing underlayment note in their technical literature that they can function as an air barrier. If you go that route, make sure that the manufacturer provides or recommends a compatible tape.
When you ask about "standard underlayment," it's unclear what you are thinking of. If you mean asphalt felt, the answer is simple: asphalt felt is never an air barrier.
Q. "The insulation stack will be surrounded by 2x8 lumber stood on its edge. Should this box plus underlayment and bottom and top be airtight? Or just watertight?"
A. Your roof needs an air barrier, but you can establish the air barrier at different locations. If you establish the air barrier directly above your ceiling boards -- say, with Ice & Water Shield -- then your roof assembly is airtight, and it doesn't matter whether the components above the Ice & Water Shield have a few cracks.
All roofs have to be watertight. The usual watertight layer is the roofing (and associated flashing).
Q. "The planned roof will be up to R-45 with XPS/polyiso, up to R-40 with EPS/polyiso. I suppose we could go thicker, but the rest of the walls are so terrible (maybe R-10) and we have so many windows that my intuition says it isn't worth it. Do you agree or disagree?"
A. Only you can establish your R-value goals. Your roof has to comply with local building codes. If you have any doubts about code compliance, talk to your local building department.
Many GBA readers in cold climates aim for an R-60 roof, but that goal isn't always attainable. I agree that if your walls are R-10, you might want to save some money in your budget to improve your walls.
Q. "I looked at the Owens Corning website this morning. They claim the global warming potential of the gases they use to blow the XPS have been reduced by 70%. They also claim that EPS is much more prone to absorbing water. There's also aesthetic reasons to not want to make the roof any thicker. Thoughts?"
A. The information from Owens Corning is misleading. To the best of my knowledge, their XPS is still blown with a gas that has a high global warming potential. EPS works well on roofs, as long as you select one of the denser types of EPS. The EPS won't absorb water.
Aesthetics are important. If you aren't certain about the aestetics of your plan, you may want to talk to an architect.
Q. "I was planning to put in a light tube in the center about 2/3 of the way from eaves to ridgeline. It would illuminate an interior hallway with no windows. Does that seem like a good reason to violate your minimal roof penetrations principle?"
A. No. The amount of electricity needed to illuminate a hallway is trivial compared to the energy loss that will occur through your light tube. Most people spend their daylight hours at a job or at school, not at home, so daylighting in residences isn't very important (compared to daylighting in an office or factory).
Robert,
Q. "Would it take an HVAC contractor to hook up that AirCycler Supply + Exhaust Ventilation System?"
A. Either you need to find a contractor who is experienced at installing residential ventilation systems, or you can do the work yourself. Considering the number of questions you ask, I would advise you to find a reputable contractor.
Q. "The water heater is about 20 years old and of standard efficiency. Would you replace it?"
A. Personally, I probably wouldn't replace it until it failed -- as long as your house isn't depressurized to the point that your water heater is backdrafting. But you can always replace it early if you want.
Q. "What with?"
A. I suggest that you read this article: Domestic Hot Water: No Perfect Solution.
Sorry if I've exceeded my quota of questions. Regarding the AirCycler, I meant to ask if you would recommend putting it in while building the new roof given what I've described of the house.
Robert,
This is a Q&A forum -- feel free to post as many questions as you want. There may be days when I can't keep up with your question-asking pace, however.
Q. "Would you recommend putting it [a central-fan-integrated supply ventilation system] in while building the new roof, given what I've described of the house?"
A. Every tight house needs a mechanical ventilation system. Without a site visit, I don't know whether your plan to install a fresh air duct connected to your return-air duct makes sense. For a variety of reasons, most ventilation systems don't draw fresh air from a roof penetration. (One reason -- the smelly nature of hot asphalt fumes -- doesn't apply in your case. Another reason -- the risk that snow will block the intake -- may or may not apply.)
If you haven't read it yet, you may want to read this article that explains your options for mechanical ventilation: Designing a Good Ventilation System.
We do have snow. I think it is possible to come in from the wall, but it's not as simple as through the roof. I get that minimizing penetrations is important.
In your 10-Rules of roofing you say that you like through-fastened metal. I assume you are saying that more expensive, concealed fastener-style roofing isn't necessary. Are there other reasons besides not wasting money? My steel roof will be barely visible and have a relatively low slope (1:6). Unless the low slope makes it helpful to use concealed fasteners, it looks like you would pass on the more expensive option. Are there any down sides to using through fastened other than aesthetic?
Also, from what I can tell, you can find XPS made with Solstice GBA (HFO) in Europe, but I can't find it here. I assume you don't know of any manufacturers or you'd have suggested them.
Robert,
Your roof slope (usually described as 2/12, not 1/6) is right on the border of what you can get away with. I have never installed metal roofing on a roof with such a shallow pitch.
One online reference notes, "Most standing seam profiles are applicable on certain low slope roofs, usually down to 2:12 pitch. [Note: this refers to standing-seam roofs, not through-fastened roofs.] Any roof with less than a 2:12 pitch requires a mechanically seamed profile to help ensure watertightness. Typically used on commercial buildings, availability of these products for residential use may be limited."
Talk to your roofer and the roofing manufacturer before you specify what type of roofing to install.
Thanks for bringing this up.
We were planning on a 3/4"-high standing seam product with concealed fasteners. The manufacturer says it will work on a 2/12 slope for 20'-long panels (mine will be about 16.25'), but noted that adding mastic in the seams would be prudent. I'll read over your linked document further.
Remeasured my slope: 2.01/12. Whew! ;)
I'm getting ready to dive into this project. T&G roof boards exposed rafters. If I'm using Ice and Damn as the air barrier how can I determine if the smell will be noticeable and if any of the adhesive will eventually soften and "run" or drip between the T&G cedar boards.