Can I skip blocking under the knee walls with this insulation install?
michaelbluejay
| Posted in Green Building Techniques on
SUMMARY:
(1) Can I skip blocking under the knee walls with this insulation install?
(2) Is R-42 on the knee walls correct?
(2) Is R-42 on the knee walls correct?
(3) Is there anything about my proposal that’s contraindicated?
DETAILS:
Climate Zone 2.
There is no ductwork in the attic space.
There is no ductwork in the attic space.
There is no ridge vent.
Code for insulating an attic floor here is R-42. I think the knee wall counts as a “floor” even though it’s vertical, because it abuts the attic space. So I plan to put 12.5″ of cellulose behind the knee wall (3.5″ in the stud bays, plus another 9″).
I plan to blow cellulose under the floor of the attic bedroom as sound insulation, not for any thermal/energy-saving reason. In the article about insulating knee walls, the recommendation is to put blocking under the knee wall to prevent attic air from entering the space between the upper floor/lower ceiling. Since I’m filling that whole space with cellulose (and then continuing that cellulose install in the joist bays all the way to the exterior walls), I don’t think I need to block under the knee walls. Is that correct?
The joist bays are only 5.5″ deep which gives me only R-19 in the attic space, but I’m loathe to add an extra 7.5″ inches of height, because then the attic space will be too cramped to ever work on electrical, plumbing, etc.
Condensation doesn’t seem to be an issue because:
– The underside of the roof sheathing looks perfect, and it was likely constructed in 1980. Half of the sheathing has fiberglass batts between the rafters, the other half the batts fell down and I removed them.
– There was a huge amount of inside air easily entering the knee wall attics, seemingly without problems, and I’ll be greatly reducing that air infiltration, even though I can’t seal tightly.
– House is in Climate Zone 2.
– 3/8″ of EPS above the roof sheathing
– No ductwork in the attic
One Devil’s Triangle is completely unvented, and has a 12″ gap between the original exterior wall and another exterior wall that was added at some later time, allowing lots of indoor air to get to the attic through that huge opening in the attic floor. Only about 1/4 to 1/2 of the rest of the attic floor is insulated, with deteriorating fiberglass batts.
The other Devil’s Triangle has only a single, small gable vent, though air can move through that attic space to the living area because, ridiculously, there’s about 20sf of open space on the floor of the knee wall, connecting to the floor below. That portion of the “ceiling” of the floor (the knee wall attic floor) below is a bunch of 2x6s spaced 1/2″ apart.
I plan to keep the one triangle unvented, and in the other triangle, to keep the one small gable vent, but seal up the attic floor. I decided to have no venting on one side and “inadequate” venting on the other based on this:
(1) I can’t easily encapsulate because it’s cost-prohibitive to have spray foam installed under the sheathing.
(2) Martin says, “It’s safe to say that ventilating attics in a hot, humid climate is just plain stupid.” https://www.greenbuildingadvisor.com/article/all-about-attic-venting
(3) Forty years with no problems, it should be fine (unless the work I’m doing now adds risk).
I know code is that attics must be vented, but I’m grandfathered in since this isn’t new construction.
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Replies
Bumpity bump bump, de bump.
I am not an expert, but have you seen this article? https://www.greenbuildingadvisor.com/article/insulating-a-cape-cod-house
It may address your question. In short, it's usually easier to air seal and insulate the knee wall space, but your specific case may differ:
"Some builders assume that the triangular attics behind kneewalls shouldn’t be brought into a home’s conditioned space. “Why heat those areas if you don’t need to?” they ask. But if you try to make your thermal envelope follow the kneewalls, you end up with a complicated shape — one that is extremely hard to air seal.
If you want the triangular areas to be outdoors rather than indoors, you will need to install blocking between the floor joists under the kneewall bottom plate, to prevent the movement of air through the joist bays. (Without the blocking, conditioned indoor air can leak into the unconditioned attic, or outdoor air can leak into the home through the joist bays.) Each piece of blocking needs to be sealed at the perimeter with caulk, spray foam, or tape.
You will also need blocking between the rafters above the kneewall top plate. This blocking needs to extend up to the ventilation baffles between the rafters. (Someone remembered to install ventilation baffles, right?) Of course, each piece of blocking also needs to be carefully air sealed at the perimeter.
Do most builders remember to install all that blocking? No.
Is the blocking easy to install? No.
Should you try to do it that way? No."
Thank you, but this doesn't seem to be related to what I asked. I already said I'm not making the attic space conditioned because of the considerable cost, and that the recommendation for blocking under the knee wall assumes no insulation below the room floor.
So again, my question is, since I *am* adding fluffy cellulose below the room floor (with fluffy cellulose completely filling the joist bays from one exterior wall to the other exterior wall), does that mean I can skip the blocking? It sure seems to, but no one is speaking to that.
Come on, man, help a brother out.
There was never any blocking before, or any insulation between the top bedroom floor and the lower floor ceiling, and almost no insulation in the joist bays behind the knee wall, so my thinking is that once I stuff all that with fluffy cellulose, that should pretty appreciably limit the amount of air that could flow through those areas, so I shouldn't kneed to add blocking under the knee wall (see what I did there?), esp. b/c it's hot, uncomfortable work and maybe hard to caulk well. But maybe I'm wrong about blocking not being essential in this particular install? Help a brother out.
Michael, your question seems fairly complicated but I believe the main question you're asking is about the need for blocking under the kneewalls. If that's the case, then Martin's article covers the two ways it can be done--the so-so way and the best way. I've been in a LOT of spaces like this and fully agree with Martin--it's very difficult to do an adequate job when the kneewalls are the thermal boundary, but it's possible.
Even with insulation in the floor system, unless it's filled with spray foam, you need blocking under the kneewalls; fluffy insulation does not stop air flow. If the bays are completely dense-packed with cellulose it will slow air flow, but few installers use a high enough density to eliminate long-term settling, and with any settling at all, there will be an air space under the floor sheathing connected to the cold exterior sheathing and/or kneewall space.
Thank you very much for the reply! In Martin's article on knee walls (btw, how do I create a link in a reply?):
https://www.greenbuildingadvisor.com/article/insulating-behind-kneewalls
He says, "[M]ost builders forget to add blocking between the floor joists under the kneewall, so cold air has an easy path into the uninsulated joist bays that separate the first floor from the second floor." The reason I thought that wouldn't need blocking is that my joist bays would *not* be uninsulated. Also, he continues:
"A leaky kneewall also can provide a pathway for escaping warm air to heat up the roof sheathing, which can contribute to ice dams." But again, air has *already* had that path for decades, and one of the attics is unvented, and the sheathing is fine. Remember, I'm in CZ-2, and there's 3/8" EPS above the sheathing, which is technically insufficient but again, sheathing has been fine. Given all that, does my insulation plan increase risk, decrease risk, or not change risk? Seems like it would decrease risk (less air flow), but even if risk is unchanged, it seems like I should be fine, since sheathing has been fine for decades. What I'm wondering is, am I *increasing* risk somehow by not adding blocking, with something that I'm overlooking?
Also, do I need R-42 on the knee walls?