After reading your article, I may need to take a different approach. Your input would be greatly appreciated.

I’m insulating a new cabin build. Zone 7, northern WI. Main cabin is 20 x 26; Monolithic slab; electric baseboard heat; 1.5 stories; 2×6 wall construction. Exterior walls are flash & Batt with 2″XPS and R19 compressed into the rest of the cavity. I used attic trusses with a 10/12 pitch in order to give me storage in the attic and an over-flow sleeping room upstairs. The upstairs attic is 11′ x 26′ w/ a 7 ft ceiling. A Bessler pull down stairway(these are awesome) will provide access to the upstairs attic/room. Yes, I am insulating the “devils” triangle. As you can see from the attached pic, blocking has been installed, and R38 batts are in the ceiling on each side of the knee walls which are above. You can also see the attic floor which is already installed. Its a vented roof, site-built 1/4″ OSB baffles with 1.5″ airspace to run from the soffit to the ridge. I plan on filling the vaulted ceiling with at least 2 layers of fiberglass batts and then polyiso on the interior to provide a thermal break. With the 1/4 OSB baffles, batts will be enclosed on all 6 sides. No dormers in the roof plane. On the 1st floor all the exposed insulation in walls & ceiling have been covered with drywall for an air barrier, and all insulation batts have been taped at the seams. Eventually a tongue & groove ceiling will be installed on the 1st floor. HERE’s the QUESTION !…and I already know the typically answer which would be to insulate at the thermal envelope; which would make my sleeping loft part of the conditioned space. HOWEVER, the upstairs attic/loft space will probably have less than 15% usage for kids sleeping, or extra place to getaway, etc. During the winter temps can get 20 below. I plan on keeping the heat at 50 even if no one is at the cabin during the winter. Would there be ANY benefit to insulating the attic floor/1st floor ceiling while the bottom truss cord is still exposed? This is the 11 x 26 area between the upstairs knee walls. The thought is to keep electric/heat cost down when most of the time the upstairs attic space doesn’t need to be conditioned. Then…if the upstairs is used, with the pull down stair case and the insulated roof line, upper half of the cabin can be heated or cooled.

@JC, returns are generally
@JC, returns are generally not allowed in an encapsulated attic. Hopefully someone else can be more specific, but I think you would have to cover the foam with drywall.

Supplies are not required but are sometimes added to manage moisture levels (although I would argue it’s better to ensure the attic tight and thus keep moisture out). Without direct conditioning, the attic should stay within about 10F of the house. In the winter, it’s important for the attic to stay warm enough to avoid condensation, especially if rafters are not fully insulated. But in that case, moisture would be coming from the house, not from the outside. The ceiling should remain uninsulated. In retrofit encapsulation projects, ceiling insulation is often removed because it’s dirty and may contain rat droppings (and worse), but aside from that, it’s important to remove at least some of the insulation so the attic can be indirectly heated by the house.

@Charles, there’s a logic for
@Charles, there’s a logic for putting less insulation on the roof. When you look at the heat flux across the ceiling, it becomes obvious that it takes less R-value at the roof of an unvented attic that’s not directly conditioned to end up with a similar ceiling load, compared to an insulated ceiling under a vented attic. Whether an R-21 roof equates to an R-30 ceiling as Allison described depends on several factors (roof pitch, ceiling construction, ceiling-to-roof ratio). The final trade-off depends largely on whether HVAC is in the attic. As Allison said, whenever you can incorporate HVAC into conditioned space, it’s better to go with a vented attic, assuming ceiling is tight, even ignoring the savings in first-cost. That’s always my recommendation to clients.

Code officials will usually accept a letter from the foam manufacturer certifying the equivalency of lower roof R-value (I know that Icynene has issued these letters). And as Allison mentioned, you can always use the Total UA Alternative path to demonstrate compliance via trade-offs (Section R402.1.5).

Also note that the IECC for commercial buildings has a lower prescriptive R-value for insulated (unvented) roofs as compared to insulated ceilings under a vented attic (Table C402.1.3).

I just remember, having been
I just remember, having been to about 3 lectures by Joe Lstiburek, he always said that insulating at the ceiling level is better than at the roof.

Corwin, sorry for the

Corwin, sorry for the confusion. No, you don’t need more insulation at the roofline than you do on the attic floor. You can get good performance with less insulation at the roofline. If you have ducts and air handlers in the attic, R-19 at the roofline usually beats R-30 on the attic floor. But, if you do a really good job with air sealing the attic floor, getting full insulation thickness over the exterior walls, and prevent wind-washing, that method beats the insulated roofline when there are no mechanicals in the attic.

And as you mention, there are other benefits to encapsulated attics beyond energy savings.

If you want to take a deep dive into this issue, read Martin Holladay’s article and the comments (311 of them as of 5/11/17) on this topic:

It’s OK to Skimp On Insulation, Icynene Says

This discussion goes a bit beyond the point of my article so I’ll probably write about it at some point, too.

Hello Allison,
Hello Allison,

Thanks for all the articles and everything that you do to help educate everyone about building science.

Your response to James has me a little confused (I confuse easily). Are your your point simply that you need more insulation at the roof line that at the ceiling line? Do we know if the additional insulation at the roof line (due to greater square footage and to achieve higher R-Value) and the related costs are justified by the energy savings?

While I’m focused on the co$t$, I have not lost sight of how valuable it is to be able to enter (without melting) and storing things in a conditioned attic (additional useful sq/ft).

Bob, yes, the same applies

Bob, yes, the same applies for encapsulated crawl spaces. I did mention them briefly at the end of the article. The more above-grade wall area you have in a crawl space, the more heat gain and heat loss you’re have to deal with. Whether that increases the load or not depends on how well insulated and air-sealed the enclosure is.