Closed spray foam insulation in attic
We are renovating our attic to convert it to livable space. The house itself is a Tudor with no overhangs or soffits, built in the 1920s.
The contractor has installed knee walls that admittedly look weird but reflect the fact that the rafters are so uneven. The goal is to add closed spray foam insulation, and then foam board, to reach r-41. The insulation will start at the apex and go all the way down inside the rafters, though obviously the knee walls are going to constitute a thermal bridge since they sit inside the rafters.
My question is this: yesterday his team added 2×6 plates at the bottom corners of the roof, behind the knee walls, about six inches from where the roof meet the floor. Beneath the plates is loose spray foam insulation and batting. When I reach my hand through around the plate and through the insulation, I can feel this big cavernous hole that feels like the cavity wall.
I had assumed to keep the envelop tight that the spray foam would go all the way down to the roofline (and the top plate?). Sorry that I don’t have the vocabulary here, but I’ve read about moisture and air coming from the house or outside the house and causing rot and mold. But maybe these plates are not a threat? Thanks for any advice you all can offer!
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Replies
I don’t like anything about that system.
A couple of issues with the framing. The blocking above the wall plates screams ICE DAM! It needs to be removed and the plates below should be encapsulated.
Odd way to frame a knee wall. As you mentioned, will impact performance. A plate nailed to the rafters, through the ISO, would have been much better. I would have strung the rafters first and planed out the ceiling.
How do you get to r41? What are the specific details of your insulation system? Where do you live?
Thanks for your response, though some of it went over my head. I've googled and don't know what an ISO is. We live in western MA. The rafters are 6" deep, so the spray foam is supposed to get us to r-38. We will then add a layer of foam board and then the sheetrock.
Regarding your main concern, the 2x6 inch plates they have installed could create an ice dam. I hadn't thought of it that way. I was more worried about what the plates would do in terms of preventing the spray foam from getting down inside the area below the floor and up next to the exterior of the house.
In terms of the overall attic, it does have rafter ties about 7 feet up and those are staying. I may be incorrect in calling these knee walls, as they aren't actually weight bearing.
I'd be grateful for any clarification on how to fix this. What do you mean when you suggest nailing a plate to the rafter through the ISO? Should I have ISO running down along the rafter before we install the spray foam, at least in this area where the roof is susceptible to an ice dam?
Thanks for responding! I've alerted the contractor and he is removing the plates. I've told him that he needs to pull the insulation back from the wall and try to get the spray foam all the way down to where the roof meets the wall. But does he need to put a plate or something down inside the wall cavity to serve as a backstop? My brother is a contractor and says that he has crumpled up newspaper and stuffed it into soffits to serve as a backstop for the foam. But this house has no eaves and no soffits. I don't know what is down there below the floor and whether we need something down there to serve as a backstop. Thanks for you advice here.
The usual way to block off a cavity prior to spray foaming is to stuff some cheap unfaced fiberglass batt material into the gap. The spray foam doesn't care much what it's being sprayed against, as long as that material stays in place during the spraying. I wouldn't use balled up newspaper though as that would be a fire risk (much easier to ignite newspaper than framing). Fiberglass is much safer here.
The blocking needs to go. All that blocking is doing is acting to create an inaccessible air space above the outer wall that can't be sealed or insulated, so warm/moist air will come up from the leaking wall top plates and "pool" under the outer eaves. As post #1 states, that is a recipe for air dams!
Since you're planning to spray foam things, you can use the spray foam to air seal those top plates. I would remove the loose fill insulation between the floor joists first though to ensure that the spray foam can fully adhere to everything so that you get a good seal. Since you're planning to insulate the roofline now instead of the attic floor, that loose fill insulation isn't going to be needed anymore since the attic will now be (due to the insulated roofline) conditioned space within the building envelope.
I don't see how you're going to put polyiso panels UNDER closed cell spray foam here. Closed cell spray foam is not normally trimmed flush with the rafter edges, it's usually underfilled a bit. That would leave you with an air space between the spray foam's inner facing surface and the back of the polyiso panels. You want to avoid air spaces like this. You could potentially have them spray closed cell for most of the fill, then finish off with a layer of open cell foam trimmed flush, then put up your polyiso. You could alternatively put some low density fiberglass batts in between the spray foam and the polyiso.
Bill
Not just the plates- remove all the framing and start over. Ask about planing out the rafters and how they intend to deal with a ceiling that could be shaped like a potato chip.
You may not need any blocking / “backstop” for insulation as you do not have roof overhangs. Extend the insulation as far as it’ll go.
It may be possible to put some strapping up and shim it in spots. That might be easier/faster than planing the rafters.
Bill
Thanks Bill!
I wasn’t clear. Did not mean planing the rafters, but rather bringing them into plane.
Ah! That's a lot less work if doable! :-)
Bill
Thank you all for this helpful feedback! I don't think there is the bandwidth or money here to bring the rafters into plane. The contractor discussed adding strapping but I wasn't sure what he was talking about. Perhaps that is what he is doing to ensure level walls?
Thanks for the information about removing the insulation from between the floor joists. Can I assume that pushing it back under the floor and giving myself at least six inches of cleared space is enough? My other takeaway is that we need to push unfaced fiberglass batt material into the cavity.
Thanks also for the guidance about what to do with the spray foam stopping short of the rafter edges and then using open cell or low density fiberglass batts between it and the polyiso. I think with budget concerns, we'll have to opt for the fiberglass batts. Is there an easy way to install them? And am I right in assuming that the strapping will go on top of them, followed by the drywall?
I'm not sure I can convince the contractor to rip out the framing and start over, but if you can recommend or share a photo or design of what would be ideal, I can share it.
Thanks again everyone!
John
Strapping would be something like 1x4s running horizontally (perpindicular to the rafters), and the drywall would be attached to the strapping instead of directly to the rafters. The strapping can then be shimmed where it crosses a rafter higher than the others, so you can use the shims to straighten out the strapping so that you get a flat ceiling with the drywall. This is pretty commonly done, but it does eat into your ceiling clearance a bit (a 1x4 is 3/4" thick).
You would need to remove enough loose insulation out near the top plate so that the closed cell spray foam can seal the entire top of the top plate to the underside of the roof sheathing. This typically also means the spray foam will seal to the top of the drywall ceiling under those floor joists. The goal is to have the spray foam tie everything together so that it forms an air barrier in that top plate area that is usually leaky. It's OK to leave the insulation in place further out away from the wall/roof intersection near the eaves.
The interior framing they put in for the kneewall looks a bit odd to me, but should be useable. I would have run a 2x4 under the rafters and tied the studs into that, which would give a better top edge for the drywall to attach to (more even). If they put in strapping you can accomplish approximately the same thing by running a 1x4 across the rafters tight to the top of those studs in the kneewall though. Ideally you want framing on both "sides" of that corner so that the ceiling drywall and wall drywall both have rigid backing there to help avoid cracking. This might be a place to consider using drywall clips because of the unusual way that corner will be framed.
Bill
Thanks Bill! This is very helpful and I'll share it with my contractor verbatim. I did just speak with the insulation installer, bringing up the point that we don't want any air between the spray foam and the foam board. He said that it was his impression that the air actually helped with insulating and that he didn't have experience filling the gaps between his spray foam and the foam board or drywall. He also said that if we tried to add batting, he would have to reduce the depth of the closed foam, eating into the r-value. Is he wrong here? And if so, is there a particular low density fiberglass batt material that might serve to ensure there are no air pockets when we install the foam board?
The usual way to install open cell spray foam is to overfill the cavities and then trip it flush with the outer edge of the rafters (in your case here) so that you have a full fill. You can do closed cell first, then finish off with open cell and trim it. Sounds like your contractor maybe isn't familiar with trimming though, so he might not have the equipment for that.
You do NOT want to skimp on the closed cell fill. Just use thinner fiberglass materials if you want to fill the air gap that way. I would just use cheap unfaced R11 rolls here, split to get the thickness you want. It will be a bit labor intensive, but doable. The low density fiberglass will be easier to compress if you cover it with polyiso, but use thicker sheets -- I'd probably use 1" minimum here, so that the polyiso panels will be strong enough to stay flat while compressing the fiberglass.
Bill
Thanks Bill. I spoke with a different insulation installer this afternoon who recommended skipping the foam board altogether and instead to run 2x4s horizontally every 16 inches up the sides of the rafters and use them to get an even surface for hanging the drywall. He said that not only does this provide a better surface for the drywall, but that he would be able to take advantage of the 2x4 to increase the depth of the spray foam. I suggested this option to the contractor and he's going to pursue it. He's also agreed to pull out the old knee walls and set them on the outside of the rafters they way they are supposed to be.
Really appreciate your feedback on my challenge.
Sorry to keep posting updates on this, but a fresh wrinkle. The contractor has noted that there is bowing in the rafters. I asked around and the unanimous solution was to run horizontal strapping using 1x4 or 2x4s to get a flat surface. But my contractor wants to nail 2x4 to the sides of the rafters to create the level surface. Is this an acceptable solution? When I talked to the insulator about the issue, he said that horizontal strapping was ideal because it would allow him to increase the amount of closed spray foam. But for some reason, my contractor prefers this other method. Should I care?
Running strapping perpendicular to the rafters will reduce the amount of thermal bridging, which is advantageous for getting the best-insulated assembly you can. You can still run strapping perpendicular to the rafters and plane out the strapping so you have a flat surface; it would just require shimming between the bottom of rafter and top of strapping as needed to create a planar surface. Assuming you strap the ceiling with 2x material, you'd gain at least 1.5" more space in which to insulate (in reality, with closed cell spray foam you have the hold it bac at last a half inch from edge of framing anyway so it doesn't protrude past face of framing - or in this case face of strapping).
You could conceivably get deeper insulation in the cavities by sistering on 2x material, and tweaking the 2x up or down to create a planar surface. The more you cut in to your ceiling height, the deeper insulation you could get. Someone smarter than me can run the math on what's a better whole-assembly R-value, 1.5" perpendicular strapping with CCSF vs X-depth sistered cavities filled with CCSF.
Thanks for this helpful feedback!
You can run the horizontal strapping and then just shim it out at the rafters that bow upwards (away from the floor). The depth of the strapping into the room will be set by the rafters that bow inwards the most if you go this route. It's going to be a bit fussy to get everything aligned regardless of which way the boards are oriented.
I see no issue putting in deeper spray foam. Note that you'll want to use closed cell first, then finish off the fill with open cell so that it can be trimmed flush with the inside surface of the strapping if you go this route.
Bill
Thanks Bill!