Tim Lange is taking on a major renovation of his 1910 home in North Dakota that will include a new roof, exterior spray-foam insulation, and new doors and windows. His quandary is what to do in the attic.
“I think I’ve got a good handle on the exterior insulation process — using window bucks to create an ‘outie’ style window is the current plan,” Lange writes in a Q&A post at GBA. “The third floor and attic are where I need some help.”
In the attic, Lange is dealing with three distinct zones: the area behind the kneewalls, the sloped ceiling in the living space, and the flat ceiling in the living space. He assumes the rafters are 2x6s on 2-foot centers. Judging from the photos he provides, none of it appears adequately insulated.
Lange is considering a number of possible approaches. “For the area behind the kneewall,” he writes, “one option would be to remove the fiberglass, add a layer of spray foam against the roof sheathing and finish with dense-packed cellulose to get the desired R-value,” he writes.
“I don’t see a way to effectively insulate the sloped section of the ceiling,” he continues. “There is just not enough room and it can’t be accessed easily. I suppose we could remove this part of the ceiling, fill the cavity with spray foam, then Sheetrock back over the foam. UGH. One suggestion was to dense-pack the area behind the kneewall and in the sloped ceiling area and pile lots of loose-fill in the main attic area. Not code-compliant, I know. But they do it out East.”
He could gut the third-floor attic to remove all the lathe and plaster, then spray-foam the underside of the sheathing to R-49 — that’s what’s recommended for his…
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Nice write up Scott; illustrates the complexities of the project and the trade-offs. The gutting of plaster and lathe and insulation is almost done. The roof boards are in very good shape, a few areas show moisture presence likely due to condensation.
The hand-drawing showed a layer of rigid foam on the underside of the sloped section. This was considered but current plan is to use the foam as furring strips only...
For the thermal break on the underside of the rafters of the sloped section - planning to use strips of 1" thick XPS, 1.5" wide. Going to choose an XPS with high compressive strength (60 psi) since drywall will be attached through these strips and don't want the rigid foam to compress. The SPF will lap 1/2" over these 1" thick rigid foam furring strips.
Calculated R for sloped roof assembly: Without any thermal break on rafters, the assembly has 4” of SPF (using R6.5 per inch), the overall R for 16” oc is R20.7. Adding 1” foam furring strips to bottom of rafters and increasing to 5” of SPF increases to R27.8. Increasing to 1.5” furring foam strips: R31.21. The framing is 24" oc, not 16" oc. Will soon have to decide how much headroom to give up in order to get more R.
Interesting idea on the kneewall removal and re-install after thermal break installed. Too much effort.
Added a picture showing the settling and shrinkage of the '70s foam.
Thanks as usual for a good article. We run into this in Maine all the time and I applaud the owner being able to do this right. My post is geared towards other ideas on this issue without removing the interior finish.
I ask because typically, a client cannot afford to remove the interior finish in a 2nd floor cape & we are left trying to insulate as is. This usually means working around batting/loose insulation that is already in the slope that cannot be easily removed and sometimes eliminating all ventilation. Side attics are usually exposed like this one and are more easily upgraded with spray foam and are either vented (preferable) or not. The real trick, and my greatest bane, lies in how to handle those damn finished slopes.
We have been dense packing these for ages with cellulose with apparently good results but based on BSC evidence and the one job I investigated, this is risky. I was able to pull apart a nicely dense packed slope once in winter and was heartbroken to see it was full of frost 3" interior from the sheathing. We surmised that the cavity had absorbed moisture during summer and did not have time to dry out before freezing temps set in.
Here are some options I am bouncing around and could use feedback on for finished slopes with no venting:
1.) If we have to insulate a slope with no venting, we start using blown rock wool or the new Spider fiberglass insulation instead of cellulose. This will still choke off venting, but will not be hygroscopic like cellulose and should therefore dry faster if it didn't absorb summer time humidty. This is my theory.
2.) We insulate side attics, but unless we can remove interior finish as the owner did above, we don't insulate finished slopes at all.
3.) We have looked into pourable foam which seems like a good idea, but with the potential of old insulation still in the cavitiy, I have concerns of its effectivness.
4.) That's all I've got!
This has kept me up nights so I could use some fresh advice:)
Response to Bo Jespersen
Thanks for sharing the anecdote about the 3 inches of frost on the exterior side of the cellulose-insulated slope. Anecdotes like yours reinforce my opinion that you don't want to use an air-permeable insulation -- even Spider insulation or mineral wool insulation -- in an unvented roof cavity.
How to Insulate the Attic in a 1910 Remodel
Why Insulate an Old Wood Building it will only be a Total Wast of Time & Money.
Best to leave it alone then?
Thanks for the quick response, Martin.
1.) If you cannot strip the finished slope, would you then leave the cavity as is (empty or poorly insulated) rather than take the risk?
2.) Have you seen or heard of pour-in foam used in these cases?
3.) Do you see any potentially ill effects of insulating the side attics with spray foam directly to the roof deck but then leaving the finished slopes untouched? I don't see a hazard here, but would love another opinion.
Injected Polyurethane Foam (IPF) Method
As a point of information, a *good* spray foam contractor might be able to insulate existing closed cavities using closed cell polyurethane foam and an injection method, through holes drilled from the interior. I note "good," because this is a somewhat finicky method--the installer is working "blind," and needs to do things like "timed shots" relative to the dimensions of the cavity in order to avoid underfill problems. Overfilling, of course, runs the risk of breaking or bulging interior or exterior finishes. Also, I believe a slow-rise formula is required; an infrared camera is often used as quality-control tool during installation.
The main reason I know about this is conversations with Henri Fennell, who is currently a spray foam consultant. This web page talks more about the method.
Injected, poured, blown-in, or foamed-in-place (FIP)
And apologies if this is pushing the edge of commercialism, but Henri is teaching an online course on Heatspring on this method:
THE INJECTED POLYURETHANE FOAM (IPF) METHOD
Obviously, existing closed cavity installations will be have R-values limited by the depth of the cavity.
Response to Bo Jespersen (Comment #5)
To give you enough courage to handle your next interaction with a homeowner, we're going to do a role-playing exercise.
Bo [speaking to homeowner]: "In order to insulate this sloped ceiling, we're going to have to remove the drywall so we can install some closed-cell spray polyurethane foam."
Homeowner: "My goodness! That's going to be disruptive and expensive!"
Homeowner: "I don't want to do that because it's disruptive and expensive."
Bo: "OK. Here's my card. Give me a call if you change your mind."
Here's my point: It's not your job to come up with a cheap way to insulate your customer's house. It's your job to recommend an insulation method that complies with building science principles and best practices. That's how you avoid callbacks and litigation.
If a homeowner wants you to insulate the small triangular attic behind a kneewall, you can do that. If they can't afford to insulate the sloped ceiling section, that's the homeowner's choice. If the sloped section gets a buildup of frost and starts dripping, it won't be your problem as long as you didn't insulate that part of the house.
Two excellent responses gentlemen and I thank you. I will look into the pourable foam a little more and think HeatSpring is a great resource- perfect timing, Kohta.
This is a simple enough dialogue, Martin (My 9 year old and I have been reading my old Calvin and Hobbes books lately and I can think of many ways to lighten this converstation up:) and it's time to make a change. Perhaps my willingness to walk away will be enough for them to understand the consequences of filling a cavity with air-permeable insulation. As Straube says, not every roof will fail using this method, but some will.
I have attached a great BSC report that really hit home for me.
Thanks again and we will keep rockin'!
Just to add to the ventilation issue, the DOE Weatherization Assistance Program (WAP) requires adequate ventilation prior to any attic insulation. Granted, most of what we do is loose-fill cellulose (knee walls and slopes are dense packed), but ventilation is key. Using blown fiberglass where you think there’s moisture isn’t a bad idea, e.g., we require it in mobile home floors for the same reason. It’s a little more expensive, but wet cellulose is basically ruined and then promotes mold. But the bigger issue is providing for adequate ventilation; we won’t install unless that’s addressed first.
BTW: I’m all for a Bill Watterson and Gary Larson reunion tour. :)
Good info, Pat
Thanks, Pat, and sounds like we have the same sense of humor:)
So if you are asked to dense pack a finished slope how do you maintain or create an air space? Do you remove the interior finish and start over or have you come up with a way to thread a baffle up the slope?
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