Why are attic knee walls insulated to ext. wall R-value and not attic floor R-value?
In a reply to another question (which I know from experience I can’t link to, otherwise my Q. gets flagged as spam), Martin says that code for attic knee wall insulation is the same as for exterior walls (lower R-value) than for attic floors (higher R-value).
This makes no sense to me. To me, the attic knee wall is essentially a vertical floor, because the other side of the wall is a hot attic. If the attic floor is supposed to be R-49 (CZ-2, R402.1.3), because one side is a hot attic and the other side is conditioned space, then it seems the knee wall should require R-49, for the same reason: one side is a hot attic and the other side is conditioned space.
I know that there’s nothing stopping me from exceeding code. That’s not my question. My question is, why is the standard for knee wall insulation way lower than the standard for attic floors?
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There is an element of practicality to the energy code, they try to make it as cost effective as possible. R38 and R49 are what you get if you fill a 2x10 or 2x12 joist bay with fiberglass batts, those are common sizes for floor joists. Walls are commonly made of 2x4 or 2x6 and the code reflects what can easily be installed in them.
A knee wall is still a wall. The counter argument is why should it matter where in a building a wall is when it comes to how it's insulated?
"A knee wall is still a wall. The counter argument is why should it matter where in a building a wall is when it comes to how it's insulated?"
Because on an exterior wall, the temperature differential between outside and inside in my CZ is only 30°F in summer, but the temperature differential between my attic and my inside is probably about 60°F. Bigger temperature differentials generally call for more insulation. So, it does seem to matter where the wall is.
On the other hand, exterior walls get solar radiation and knee walls don't. But then we're back to wondering why code wants R-49 between the attic and the ceiling below but only R-13 between the attic and the adjacent room.
"But then we're back to wondering why code wants R-49 between the attic and the ceiling below but only R-13 between the attic and the adjacent room"
Do you reject the answers provided here? Why still wondering?
The answers are unsatisfying. Questions like "Why should it matter where in a building a wall is located?" and "Wouldn't this logic basically say that all walls are vertical floors?" seem to miss the point (that attics are hot), and most other comments seem to be speculation. You yourself said that solar gain is "not enough to explain the code discrepancy".
So what *is* the reason for the code discrepancy? Is anyone *really certain* that code is just giving a pass here because it's harder to insulate knee walls to R-49 than to insulate attic floors to the same level?
michaelbluejay,
https://www.greenbuildingadvisor.com/question/why-is-more-insulation-still-required-in-roofs-than-walls-in-very-tight-buildings
To save readers the trip to the link, in the first reply, Martin says:
"The reason is simple: it's usually cheaper (and easier) to install thick insulation in a roof assembly or on the floor of an attic than in (or on) a wall.
No one wants 24-inch-thick walls, evidently. But it's relatively cheap and easy to install 24 inches of insulation on your attic floor."
Michael,
It looks to me like you simply didn't read the responses carefully. They were a bit suggestive but not speculative. My comment that "solar gain is not enough to explain the code discrepancy" was to say that it is due to the other reasons cited.
Frankly your question was not phrased in a way that made a lot of sense and hence people pointing out your lack of logic. You didn't say you think the attic knee wall deserves higher insulation because attics are hot. That would have been simpler to address. You just called it a vertical floor. (Why?) Then you kept making reference to 'common plane with conditioned space' and I doubt anyone could decipher what distinctions you were trying to make.
Well, I did get the highest possible score on the SAT Test of Standard Written English, so neither my reading comprehension nor my clarity has ever been much in question...until now.
And I have been voted best designer/builder on Vancouver Island six years in a row - by my wife.
This could also be a practical consideration based on gravity. Roll-out fiberglass can go across the joists and you can use blown-in loose fill on a horizontal surface, but it becomes an utter fiasco to try and pin the equivalent cellulose to the wall. Weight and sagging might also come into the conversation, acknowledging that batts in walls only perform at their design R-value if they are properly installed, not sagging.
But it's probably the very simply idea of thinking that most houses have the sun "above" them, and therefore a flat ceiling is still considered "normal." For simplicity's sake, knee walls get swept away into the minutiae.
Wouldn’t this logic basically say that all walls are vertical floors? There is a reason vertical elements don’t require the same insulation as horizontal and it has to do with thermodynamics.
This logic would mean that clerestory wall would need roof insulation wouldn’t it?
Maybe I’m missing something….
"There is a reason vertical elements don’t require the same insulation as horizontal and it has to do with thermodynamics."
As DC says, I think it has more to do with installation practicalities then thermodynamics. Solar gain would be the largest counter argument, but still not enough to explain the code discrepancy.
maine_tyler,
Our code gives an indication that it is probably opportunity rather than thermodynamically based, in that we have two different requirements for roof insulation, depending on whether there is an attic or it is a cathedral ceiling.
Interesting
For any new build 1/2 story (or even older retrofit), if you want decent air tightness, you need to insulate along the roof line. Since there is no insulation in the knee walls, it doesn't matter what the code allows.