Roofing Material & Vapor Permeance
On a 1950’s cape in Oreland Pa, zone 4 but near 5, we are retrofitting attic insulation and doing a new roof. Generally, the proposed roof section of this cape will be metal shingles, 1″ foam, original roof deck, dense-packed 2×8 rafter bays. Given the foam-on-roofdeck, it is intended to dry to the interior. Questions:
1) Does anyone have experience with metal shingles? We are looking to avoid asphalt and make the most sustainable decision given the budget. What is the best underlayment? What other roofing materials should we consider? We want to use a shingle manufacturer that will accept a hot roof assembly in terms of their warrantee.
2) With the build up of foam/plywood on the roof deck and no overhangs, what is best way to finish the now-large roof edges? What considerations at the valleys? Best type of foam? How thick for the plywood? Note: This is currently an unvented roof with no opportunity to ventilate because of no overhangs and the presence of dormers and valleys prohibiting adequate and balanced ventilation.
3) Should we use CCSF inside the kneewall attic spaces at the eaves to properly seal and insulate the bottom of the slope and down across top plates?
4) If we dense pack the roof rafter cavities where it is sheetrocked (from the top, since we are re-roofing), what is the best way to deal with the open bays between the kneewalls and eaves? In other words, how do we dense pack the open bays? should we clad the underside with rigid foam and then dense pack? Again, this is intended to dry to the interior, so while CCSF would be easier, it would create the dreaded sandwich, as might the rigid foam panels on the inside.
5) The actual attic above the 2nd fl. ceiling is only 4′ across and 3′ high, it is a 10 pitch slope. Should we fill it with cellulose? There is only 1 insulated flex duct serving as a return jumper in the attic and no HVAC equipment. There are gable end vents that we will seal up. There will be no chimney.
6) There is a history of ice-dams at one section of lower roof. I am hoping that we can remedy this by this treatment. Any thoughts?
7) What considerations if any with the following roof penetrations: 2 bathroom ventilators,1 vent stack; 2 copper antifreeze lines for solar-thermal; 1 wood stove chimney- B vent.
Many Thanks!
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Replies
Here is the home in question:
Robert,
You've got a lot of questions. I probably won't answer them all.
Why did you decide on 1 inch of foam? Any chance you would consider thicker foam? The thicker your foam, the less likely moisture will accumulate in your old roof sheathing. I would suggest that you use polyisocyanurate foam in any case, since it has the highest R-value per inch.
Any good trim carpenter should be able to come up with a trim solution for the rakes and eaves; trim can be layered or offset in a way that makes the trim decision look deliberate rather than tacked-on. Don't forget to install metal dripedge.
You're right that closed-cell spray polyurethane foam is a good way to air seal the perimeter of the kneewall area.
If you want your roof to dry to the interior, you can install gypsum drywall on the underside of your rafters to contain your cellulose. If desired, you can sister or scab on new framing to provide more depth for more cellulose.
I don't think it's necessary to totally fill your attic above the 2nd-floor ceiling. About 16 or 18 inches of cellulose should do it.
Concerning ice dams: the thicker the layer of rigid foam you install, the more likely you'll lick the problem.
Concerning penetrations: every penetration needs to be carefully air sealed.
Martin,
Thank you. I used the chart presented in your article below as a guide. I realize it was intended for walls. What is the optimal thickness of roof deck foam for my climate? I will do what is best.
https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing
Robert,
You've got the right idea, and I think you are probably right the you'll be safe enough with R-5 foam. Thicker foam just gives you a wider margin of safety.
I assume you are in Climate Zone Marine 4. As you probably know, the thicker the framing of the building assembly, the thicker the exterior foam has to be. R-5 foam should be adequate for 2x8 roof framing in your climate zone.
To me, though, most of the hassle of installing the foam is the labor. With the same investment of labor, you could get twice the performance by choosing 2-in. foam. It's true that your materials cost will go up -- but your labor cost won't.
Then would we fur the roof out with 2" lumber stacked over existing rafters and infill with 2" foam or just install contiguous foam sheets and install furring on top of the foam? I suspect the latter to minimize bridging, but I've never done this particular treatment.
Robert,
B-vent is for gas stoves only, and cannot be used for wood-burning appliances which have much higher flue gas temperatures. Stainless steel triple-wall or metalbestos is required.
Since you're doing a major roof rebuild, I would recommend adding overhangs on all sides. Overhangs are one of the most important elements of durable exterior claddings and the prevention of water leaks.
Robert-
I was mistaken, I am here now and verifying that it is triple wall stainless. Thanks.
Robert,
If you are planning to install metal shingles, they you probably need a plywood roof deck. The plywood can be fastened through 2 inches of foam with long screws.
I am very interested in adding overhangs. Could you point me to any details on this? One challenge is that one windows is right up under the roofline (front left, 1st floor). I might need to kick out the roof with a change of pitch. Thoughts? Of course there is the budget too.
Martin,
I spoke with the owner of Fasten Master recently and he mentioned that they made hardware for this. Thanks for jarring my memory!
Robert,
The standard PERSIST solution is to cover the roof sheathing with thick rigid foam, and then to install 2x4s (parallel to the rakes) on top of the foam. At the eaves, the 2x4s can be cantilevered to create your overhang; a soffit creates a triangle to strengthen the construction.
At the rakes, you can install the 2x4s at 90 degrees to the other 2x4s, to create a ladder extension. The rake trim helps stiffen the cantilever.
The roof sheathing is installed on top of the 2x4s.
There is no simple, inexpensive way to build out and up such a complicated roof. But you could fabricate parallel-chord trusses, extend them out for a new overhang, and do all the insulating outside of the existing roof, even including ventilation if desired.
It sounds like you need a competent local building consultant.
Robert,
Let me see if I get the concept. This would be essentially over-framing the entire roof, adding insulation, decking with plywood, then underlayment and shingles? I would need about 7 inches of polyisocyanurate or 12" cellulose? Do I understand your concept, Robert?
If I do, this would be quite a build up. This dominos into questioning the existing structural capacity given the weight of the proposed solution. While this is a creative approach, and I love the out-of-the-box thinking, the engineering and my suspicion of cost's might put this out of reach.
Martin,
Would you direct me to a relevant PERSIST overhang detail? Could I just cantilever from the lower roof, say 48"up the rafter (in the same plane as the foam), without installing 2x4's over the whole roof?
Robert,
Q. Could I just cantilever from the lower roof, say 48" up the rafter (in the same plane as the foam), without installing 2x4's over the whole roof?
A. Yes.
PERSIST links:
https://www.greenbuildingadvisor.com/blogs/dept/musings/getting-insulation-out-your-walls-and-ceilings
http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/99/991108.html
First of all, you guys are GREAT. Thank you Robert and Martin. I have been building for over 20 years, most of which in a broken paradigm. About 7 years ago, I started making a real effort to learn more about building science, energy efficiency, and durability. 2 years ago I achieved my BPI certification and have made energy analysis and retrofits a substantial part of my business. BuildingScience.com has been a great resource and recently, as you are aware, I have been involved here. This is truly a masters class and I am very grateful to the two of you and all the other informed, experienced contributers.
Martin-
Why is there a soffit vent in the detail above? I can't quite read the words, is there continuous ventilation? Where would I find a better image of this detail? Given my locale, what thickness foam would you use on the roof deck?
Lastly, with that window jammed up under the roof line (front left,1st floor), how would you handle the eave overhang? Articulate the roof just above the window like a shed?
Robert,
The soffit vent allows air to enter at the bottom of the roof, ventilating the space above the foam insulation and keeping it cool during the winter. The air exits at a ridge vent.
A better image of the detail can be found in the July 2002 issue of Energy Design Update, which may be hard to find.
The thickness of the foam depends on your goals. As I always say, the thicker the better.
You have to determine how much overhang you want over your window. Overhang design often involves compromises.
Robert,
Maybe a well done eyebrow could be created in the roof area above that window. You could create the overhang while keeping the window head in view. A slight brow where the snow is laying .
Roy-
Great idea. Thanks for the thought. I just may do that.
Does anyone have an experience or info. on the following insulated panel? I'm considering this for the roof deck insulation. I find it attractive for its labor savings. I have no cost info yet.
http://www.hpanels.com/2009/pages/pdfs/Lit_Prod_Color/H-Shield-NB.pdf
No personal experience with them, but Hunter Panel also makes the same nailbase roof panel in a venting version, called CoolVent.
I saw that as well, however, this cape roof is too complex to attain balanced, continuous venting. The CoolVent may be a good solution on a future project, though.