Interior Continuous Insulation?
tim_william
| Posted in Energy Efficiency and Durability on
This is a general question. Reading the code for climate zone 6, the recommended minimum insulation is, for example, 20+5ci.
The code refers to continuous insulation as “exterior or interior”, but all the assemblies I’ve seen use exterior insulation. Why not build a stud wall with cavity insulation then add the ci to the interior? Most of these assemblies have an interior service chase in the design so it’s not like the details are that different.
I know one goal is to keep the sheathing warm to avoid condensation, but if it’s to the exterior of an R30 wall is that still a concern?
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I guess it depends on where that extra +5 of continuous insulation is. If it's adhered to the sheathing like ZipR, then it will be a barrier to condensation. If it's just R20 with foam at the drywall, then I don't think that would fully mitigate the condensation risk on the sheathing. At least as I understand it, and I fully acknowledge that there are people here who have much more expertise in this area than I do.
Continuos insulation on the interior will reduce thermal bridging to the studs which is nice,
I could see this becoming a thing in the future but the products for exterior insulation are much further along today. How are you going to attach drywall through a layer of CI on the inside?
I wold imagine adding strapping just like people do today with blow-in insulation. Basically the same as a rain screen on the exterior, except on the interior.
This was a code approved wall system used in a commercial retrofit here in Ontario. Zone 7A. We did not want to touch exterior finishes. Heating costs for NG were about $200 for a month averaging -20C ambient. 9000 square feet. The building performed extremely well. All electrical ran through the service area. 2x3s on 16” horizontal were screwed to studs. Very few penetrations to envelope and very tight as a result. Cost on labour was reasonable too as the 2x3”s and foam were pretty easy to install on the inside. No special training needed for labour. Limited sprayfoam was done at the floor joists to carry the thermal break between floors. Exterior was steel vertical panels over OSB sheathing, zero issues over about 10 years now.
As Malcolm mentions, rim joist thermal break is not perfect, however we did carry the interior thermal break foam right down to the insulated slab over-pour.
The cavity was very nice to have as some of the interior space was tenant leased so changing up network and electrical in the outside walls was trivial when bringing in new tenants.
Tim,
The disadvantages are mainly:
- It isn't really continuous if it doesn't cover any floor system or interior partitions.
- It makes hanging drywall, trim, electrical boxes and cabinetry difficult.
- It can't protect the rim- joists the way exterior insulation does.
- The sheathing stays colder so more prone to moisture problems.
https://www.greenbuildingadvisor.com/article/walls-with-interior-rigid-foam
Thanks for the notes, this all makes sense.
Awesome, thanks for sharing (in reply to Dennis Wood)
Just add furring strips screwed to the framing members through the rigid, warm side insulation. The interior finish can then proceed as normal. Place insulation between the furring strips as well. The R-value has been increased and the thermal bridge minimized. Works best for ceilings but would also work for walls.
Doug
Exactly, thanks.
Have you looked into Bonfiglioli walls? (https://www.finehomebuilding.com/project-guides/insulation/breaking-the-thermal-bridge) Sounds very similar to what you're saying: instead of full foam boars on the inside, you cut strips to cover / create a thermal break for the studs then cover those with furring to facilitate future wall hanging while simultaneously increasing the effective cavity bay depth for increased fluffy insulation. We did this on walls we couldn't get permission to do exterior insulation on (in the setback and our city is very particular about setbacks), and the wall system R values turned out to be quite good. I suppose the trade is materials cost for labor cost (ripping the foam into strips for covering the studs).
finePNW,
Another similar option is a Mooney Wall, which is just adding a layer of horizontal strapping to the studs. It limits thermal bridging, while allowing you to forgo foam altogether.