Chainsaw retrofit
Hello, I’ve been reading and researching on how to make our 1960’s house energy efficient. We live in zone 3a. I am retired, and doing all of the work myself. I have a hip roof with black fiberglass shingles over 15 pound black felt paper. The rafters are 2×6. The roof decking is 1×6 tongue and groove pine, all nailed by hand. The roof pitch is a 4/12. The house is 30 feet deep by 60 feet long. My plans are to cut off the two foot overhangs, put two inch polyiso foam board on the exterior walls, over 1/2 inch blackboard, then 3/4 strapping, rainscreen and roughcut Cypress. Back to my roof. My plans are to put two layers of two inch polyiso rigid foam over the shingles, or remove shingles, please advise. Then install new rafters on a 5.5/12 pitch, with a metal roof. Also change the roof to gable. The change in pitch will raise the ridge height to about 44 or 45 inches higher than it is now.My question is, have I left off any details or steps in this process that I have laid out? Thanks for all help and advice.
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I'd recommend removing the shingles before install of the two-inch polyiso. I don't know what building code your area uses, but for example my interpretation of the 2015 IRC is that it's required (R806.5.5.1.4 "...rigid board or sheet insulation shall be installed directly above the structural roof sheathing..."). I'm sure it's not enforced evenly.
If you look up the US Department of Energy's March 2018 study "Attic Retrofits Using Nail-Base Insulated Panels" you'll see that in all the examples the shingles are removed before installation. It's just a better assembly this way - less surface height variation during install.
Thanks for your help. So, remove the old shingles, reapply 15 pound felt tar paper for wrb, install two layers of 2 inch polyiso, tape all seams, install new rafters at higher pitch, install 1 x 4 lathes on top of rafters, install new metal roof.
Depending on how your roof is framed, you might not be able to chainsaw off the overhangs without major structural work. At that point, you are better off taking the whole roof off and building a new roof. To me, this would only be worth it if you want to convert the attic to living space.
With generous overhangs, taking them off might not be worth the headache. You can get a pretty good efficiency and decent air sealing just by running the wall foam up to the roof deck and spray foaming the joint from the attic for air sealing.
For the roof, you also have to figure out where your insulation will be. If you are going with exterior rigid foam, it should all be near the roof deck. This means taking out the existing insulation on the attic floor and turn your existing attic into a conditioned space. If this your plan, check bellow for the proper ratio of fluffy and exterior rigid:
https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies
This is a decent vid: https://www.youtube.com/watch?v=JmsosK2My6o
And here's GBA's standby cathedral ceiling reference: https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
Michael Maines makes a pretty good case for removing shingles, in my opinion: https://www.finehomebuilding.com/2018/06/21/retrofitting-foam-insulation-roof
I'm not sure I understand your roof assembly after that. You're adding new 'rafters' on top of the foam, at an entirely new pitch? S you're basically reconstructing the entire roof?
If you're building an entirely new roof over an existing, (and I can't really comment on all that might go along with that) you will likely need to vent that upper roof.
Typically, rigid foam is placed exterior to the roof sheathing, treated similar to the exterior wall with strapping or another layer of sheathing screwed through into the rafters. This is an unvented assembly and requires a certain ratio of foam to interior 'fluffy stuff' for condensation control (based on your climate).
What insulation exists in the rafters?
Yes, Installing new rafters over the old roof, raising the pitch, so the new ridge will be around 45 inches higher than the old one. Keep the old roof assembly, remove the shingles, install new felt tar paper for wrb, install two layers of 2 inch polyiso, tape seams, seal and tape 2 inch layer of polyiso on walls to roof polyiso, then install new rafters over all of above, use 1 x 4 lathes on top of rafters, then install new metal roof. Will this retrofit process work? I am doing all of this myself, so the labor is free.
Tommy. Maybe. But I don't understand how you're attaching new rafters above 4 inches of polyiso.
Without knowing more about the structure and existing insulation, it's hard to say, but it almost seems like building a new roof would make some sense.
Where is your existing insulation? On an attic floor, or tucked in the hip rafters? Any venting currently?
My existing insulation is between my ceiling joist on the attic floor. Yes, the old roof has soffit vents, and a ridge vent. I was planning on removing all of the vents, and changing to an unvented attic. I'm also removing all of the brick on my exterior walls, so I thought this would be a good time to remove the overhangs, ie chainsaw, and connect the foam on the walls to foam on the roof.
Thanks Tyler. I've now changed my mind on roofing over my old roof. Too difficult and to much cost. I still plan on doing the retrofit, just keeping my old roof. So, 1. remove old shingles, 2. Add roof underlayment 3. Add two layers of 2 inch polyiso 4. Add 1x4 lathes 5. Install metal roof. Have I missed any steps? Or are any of the steps out of order? Thanks
I just did this (except for metal roof), and I would suggest taping the top layer foam joints, edges, and penetrations with a quality weather barrier tape. I used SIGA Wigluv, which worked great, but there are many options. Also be sure to mark out your rafter locations before you lay down the weather barrier and foam. Screwing through 4+ inches of foam into a 2x can be difficult. You need to determine how you are going to add eaves back on if you cut them off initially.
Yes, Building a roof over a roof. The new rafters will be approximately 45 inches above the old roof at the ridge
Thanks David, So you removed your old shingles, put down a synrhetic weather barrier and taped the joints and seams, then installed your rigid foam. Did you then install plywood or some type decking over your foam? Or did you install your new shingles or roofing directly on top of your foam?
Ours was a partial job in a 1 story section abutting the 2 story section of our house. I tore down and rebuilt the roof as there was extensive rot from lack of step flashing, and I guess because the roof had been changed to a cathedral ceiling with only fiberglass batts and no air sealing long ago. I used prosoco joint and seam sealer to air seal the sheathing joints, and cat5 liquid applied weather barrier on the plywood. I Taped the foam sheathing as well as additional layer of protection. I did all this was because I was doing this rebuild part time in the fall and wanted to protect sheathing from water flowing in the foam joints and also in case the roof leaked. Didn't like the idea of water migrating between foam and structural sheathing and potentially getting trapped there for a period of time. I have no experience with metal roofs, would ask the manufacturer on how to detail your roof, or hopefully others on this site can help you. If you use asphalt shingles then you of course need a sheathing layer of some sort on top of insulation to nail to.
Tommy,
How do you plan to get the balance of your R-value? Are you planning to install fluffy insulation tight against the sheathing? (As an FYI, see https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation).
Yes, I plan on screwing either a 2x4 or a 2x6 to the bottom side of my existing 2x6 rafters in the attic. Then putting at least a 9 inch batt of unfaced fiberglass batt insulation R30 between the rafters. The Polyiso rigid foam board is R11.4 per 2 inch piece. So 4 inches would be R22.8 plus at least R30 between the rafters in the attic would be R52.8. I still ask that someone advise me on if I should put plywood, or zip system sheathing on top of my foam board, and then 1x4 strapping, and then my metal roof. Or could I just use another layer of the synthetic roof underlayment with taped seams on top of the foam board, then metal. Or nothing at all between the foam board and the 1x4 strapping then metal. Thanks for everyone's help.
Tommy,
See this article: https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing
I think your questions are answered near the end of the article.
Thank you Steve, That article was really helpful. I have removed my shingles, and parts of my 1x6 decking are weak and need replacing. So since I am down to my decking, I plan on placing new rafters on the walls and raising my roof pitch from a 4/12 to a 6/12. My question now is, would it be acceptable to air seal my old decking with a synthetic roof underlayment, plywood, osb, or Zip System sheathing? Any of the above for my air barrier, then install my two layers of 2 inch polyiso foam board on top of that. Then, my new rafters will be on a 6/12 pitch, so well above the old decking and foam board, 1x4 strapping on the new rafters, with a metal roof. The metal roof can be vented. Do you see any problems with that?
Hi Tommy,
I'd like the building professionals to comment on your last post. But here are my 2 cents...
If I am understanding your plan, you are framing a new, steeper roof on top of the existing roof. Correct? If that's the case, OSB or plywood are more affordable than ZIP. Plywood is more durable than OSB, but that doesn't mean you can't use OSB--especially if it is protected by rigid foam.
(I'm assuming you are not recycling the existing board sheathing and not using through-fastened metal roofing. Please correct me if I am in error here.)
If it were my house, I would install plywood. I'd tape the plywood seam to create an air barrier, install the rigid foam, and tape those seams. I'd install a second layer of plywood to provide a stable (and co-planar) base for the metal roof. If possible, I would not cut off the overhangs since recreating those would add work and cost.
Thanks Steve for your advice. My plans have changed about leaving the old rafters and decking. We removed the front side of the roof shingles, and discovered that my 1x6 sheathing boards are not sound any longer. My rafters were on 24 inch centers, and the 1x6 boards actually broke through in three places while we were walking on the roof. Some of the rafters are also sagging. So our plan now is to remove the sheathing over the outside walls, set new rafters on the walls, change the pitch to a 6/12 or 7/12, does anyone ever build a 7/12? So a complete new roof. Then remove all of the old decking and rafters. Now, what is the best advice for roof sheathing on top of the new rafters, ,plywood or zip sheathing, all seams taped of course? Then the two layers of two inch foam, and what material if any over the foam, plywood, just strapping then metal? Please advice, all opinions are appreciated. Thanks.