Knee to vault bay to attic transitions – interior polyiso?
Hi, I’ve been reading and learning a lot, and have a few clarifying questions as I embark on an insulation effort on our 1922 craftsman home, located in northern CO, at the border between zone 5 and 6. Winter indoor humidity is 30-35%, without humidification or dehumidification. Current attic spaces are virtually uninsulated (2-4″ of patchy loose fill fiberglass from the distant past, no air sealing to note).
The house is somewhat interesting in that it has a central half-floor, with a vented attic above, and shed roofs to the sides. But instead of the knee walls going from the interior of the house directly to the attic space, there is an intermediate partial-attic storage area that was framed in and “finished” with hardboard and 3/8″ drywall. There is a narrow door that accesses this area, then a hatch into the “real” attic. So two layers of knee walls. My plan is to make this space part of the conditioned interior of the house, but not the “real” attic area – two devil’s triangles to deal with.
This also leaves about a 5′ long section of vaulted roof bays that join the lower attic triangle to the upper vented attic, which is only partially accessible. However, a benefit is that this vault bay is above the intermediate attic space, not a normal room.
Attached is a sketch of the existing situation, as well as a proposed insulation fix for the kneewall/vault bay area. The upper attic will remain vented, and get air sealed and blown loose fiberglass in a typical manner. The lower attic will get the same treatment. It is the interface between the two and the vault bays I’m most concerned about.
I have three questions about it:
1. Because the vault bays are 2×6, and only partially accessible, how do I keep my fiberglass loose fill or batts up in there, and how do I maintain ventilation (or do I need to)? Does there need to be a clear continuous ventilation channel from the lower attic to the upper attic?
2. To get closer to the code minimums, I was considering putting polyiso on the interior side of the knee wall and vault bays, just securing it over the existing hardboard, and using a product like Thermax to avoid needing a thermal barrier. This is technically “storage” but very infrequently accessed. 4.5″ of fill in the bay + 2″ of polyiso seems to get me ~R27 on the full assembly. Am I missing something here? Is this a silly way to get a higher R-value in this attic storage space?
3. Anything else I should be keeping in mind while up here? Old house, tight spaces, trying to do this one time.
Thanks – Sam
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Replies
Sooo.... When did you sneak into my house and eyeball my own current renovation project? :-D I'm dealing with almost this EXACT same issue, except that I have 2x8 rafters instead of your 2x6s. Here's what I'm doing to deal with the situation, but first answers to your questions:
1- Don't use loose fill in overheat spaces like this, because it won't want to stay put (unless you use something like Spider). In my case, I want to get up to around code minimum R value, so I'm leaving a 1.25" vent channel (using ripped plywood furring strips as spacers), then filling the rest of the space with polyiso. That will allow for around R39 worth of polyiso (6") in the rafter bays, and maintains a vent channel to connect the upper and lower attics which you DO need, unless you want to add a bunch of roof vents. On the interior, I'm using a layer of 1/2" polyiso over the bottom of the rafters with seams taped to act as an air and vapor barrier, then 1/2" drywall under that. This will be a lot easier to do than the 2" you plan on, because it's easy to hit framing through 1/2" polyiso when fastening the drywall. I'm going to give you another idea later on though, since you have less rafter depth to work with than I do.
2- I'm not sure Thermax's exposure rating applies to horizontal applications like ceilings the same as it does to vertical applications like walls. I'd double check that before continuing with your plan. In my case, I'm putt R15 mineral wool batts in the stud bays of the wall, and 3" polyiso against the studs on the attic side. This about R34 total in the wall, and keeps with my other upgrades that I've done in stages in other areas of the house. I use MemBrain over the mineral wool on the interior side, and 5/8" drywall over that. I used 1/2" drywall on the ceiling to make installation a little easier (less weight), and since I'm already dealing with a low ceiling, gaining an extra 1/8" makes me feel better :-)
3- You need to maintain ventillation between the lower and upper attics. That means continuous vent channels over your closet space and up into the upper attic. In my project, I'm putting 1/2" polyiso baffles up into the upper attic using furring strip spacers, and tying those into the polyiso in the rafter bays over the closet space. The wall polyiso for the closet will tie into the lower edge of the rafter bay polyiso. Depending on how the angle works out, I might glue some polyiso onto the top plate of the upper wall too, so that the upper attic baffle has insulation between it and the top plate in that area (otherwise there is limited space for loose fill insulation there).
To address your reduced rafter depth, you might consider running 2x2s perpindicular to the rafters, then filling the rafter bay with polyiso like I did leaving a vent channel on top (1" is code minimum for vent channels like this). You could then run 1.5" polyiso between the 2x2s this gives you about 7" to work with, leaving 6" for insulation after the 1" vent channel. If you fill that 6" with polyiso (4.5" in the rafter bays and 1.5" between the 2x2s), you get R38. You could then do what I did with a layer of 1/2" polyiso under the 2x2s to bring you up to about R41 total, and you'd be able to drywall more easily since you'd only have to get through 1/2" polyiso to hit those 2x2s. I like the continuous vapor barrior provided by the foil faced polyiso here to help limit how much moisture can get up into the polyiso filled rafter bays of the rest of the assembly. Since you have a vent channel on top, you have less to worry about, but it's still best to be careful.
If you want to try to just use batts in the rafter bays, you can support them with wires or strings stapled up under the rafter bays. That's the usual way to support batts. If you use mineral wool, it might stay in place by itself. Either way, you'll want to use high density batts here to maximize R value in the limited space you have available.
I would check that your soffits aren't plugged up while you're working up there. I found fiberglass stuffed into every rafter bay, presumably by the original insulating contractor to keep the loose fill out of the soffit area, so I removed all of that, and reworked the soffit to use a continuous soffit vent instead of periodic vents. I have found periodic vents, even if they meet the required NFA for the attic, tend to concentrate airflow in their immediate area, resulting in mold in the areas between the vents. Continuous soffit vents avoid that problem.
Bill
Bill, thanks for all the detailed information, glad to see I'm not the only one dealing with this situation! I hadn't considered using polyiso up in the rafter bays, but it has some appeal. Lots of cutting, but cleaner to get around than the fiberglass or mineral wool.
Good point on hitting the rafters through 2" polyiso. A thinner piece as the interior barrier does make more sense. I was hoping to avoid drywall, but I should probably just bite the bullet and do it. I'm pretty handy with mudding, but I don't like maneuvering big sheets of it around in tight spaces. I keep hoping someone will make something like Zip R but for interiors - 3/8" drywall over 1/2" polyiso, prebonded. I'd buy it.
I'll make sure that the lower vents get opened up with a baffle on them before the rest of the blown in insulation goes in.
Best of luck with your project - I'm seeing a lot that needs to be done in the attic, but the good news is that means a lot of room for improvement. The house is not terrible for 100 years old, blows 8.6 ACH50 with 53 original windows, and is fairly comfortable, only a few cold spots. Hoping to see some nice improvement through all of this.