Combination Rigid Foam / Batt Insulation in Basement
Basement Insulation Detail
Hello,
I added 2″ of rigid foam to an unfinished portion of my basement walls in zone 4 (St. Louis,MO) and then added 16″ oc 2×4 framing along with R-15 mineral wool batts in the cavities. The foam is taped/spray foamed to provide a solid air/vapor seal. One of the basement walls is mostly above grade. If it matters, the rim joists have rigid foam and mineral wool.
Because of obstacles, the framing is set 1.5-2″ off the foam. Does it matter if the batts are pushed directly against the foam vs leaving the batts flush with the framing? I set them flush with the framing and ran the electric wires behind the framing to limit interference. I’m starting to question myself.
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Replies
User...o11,
It's fine either way, but I would have done it as you did.
Technically the fact the two layers of insulation are not in physical contact with one another means the R values are not additive only the one with a higher R value is doing anything.
Dana Dorsett posted the following in response to a very similar question.
“An air gap between insulation layers is a "thermal bypass" that cuts into the thermal performance of the assembly, since any air convecting around the batts moves heat (and moisture) via the air. Installing the batts tight to the foam prevents that.
While there would ideally be a continuous air barrier on the interior facing side of the batts, the thermal performance of high density R15 batts isn't going to suffer much without it. Air tightness of the MemBrain becomes critically important if it isn't installed tight to the batts. Rock wool is extremely vapor permeable despite it's air-retardency, and air leaks around the MemBrain could result in periods of condensation (or frost) and dampness where the rock wool meets the foam.”
https://www.greenbuildingadvisor.com/question/where-to-place-air-gap-on-framed-wall-insulation
Since it is done, the real question is does to matter enough to go back and change anything. Since the earth on the outside of the wall is about 55° and the basement is about 65° R 15 seems like more than enough.
Walta
Walta,
I think you have misunderstood what Dana was saying. His response does not support your contention that "only the one with a higher R value is doing anything." You might be able to say that only the foam was useful - if the gap between the two insulations was open to the interior - but it isn't.
Using R-15 batts, the choice was between a gap between the foam and batts, and a gap between the batts and drywall. Both will have some convective air movement, and both will to some extent reduce the effectiveness of the batts - but neither gap comes anywhere near to completely negating the R-value the batts add to the assembly.
Can we agree the if the insulation layers were in contact with each other R10 + R15 = R25 and with the air gap the R value is something more than 15 and less than R25 depending on the amount of air and flow.
Walta
Walta,
The data I've seen on wind-washing in relatively closed cavities indicates the effects are pretty marginal. See tables 8 and 9 in this link: https://www.rdh.com/wp-content/uploads/2017/10/Van-Straaten-Windwashing.pdf
So yes the result is between R15 and R25, but a lot closer to the higher value.
1st, the ABTG Wall Calculator - while pictorially intended for above grade walls, it may provide informative results for below-grade and at-grade conditions.
Simplistically put:
Evaluate the temperature at the indoor air / foam surface interface.
Of interest is the ratio of R-outer c.i. to R-inner that guides the evaluation for moisture and mold and the need for a vapor retarder.
(Thoughts/ into the weeds on temperature in the wall, R dependent on T and PIR implications as c.i. follow below.
Membrain as VR:
I gather you have installed Membrain. That’s another deep dive.
I had failed to ask myself (and then ask certain someone else’s) “What happens when fugitive water enters the wall assembly if the c.i. is XPS, the sheathing is exterior gypsum, the stud voids are mineral wool and Membrain is the VR behind interior gypsum?”
Empirically observed, the liquid water stays in the stud void, evaporating as the vapor drive between the cavity and the interior (the cave with the occupants) allows.
Oddly, interior paint may separate from the gypsum with water or water vapor filling the paint skin like rhino-rinkles or a painful blister. One benefit of the Membrain still is no staining. All impurities are retained in the cavity. Yippee!
Weeds:
Keep in mind that the published R is based on 50F - 100F (75F mean) test condition. EPS and XPS improve at colder temperatures while PIR becomes less effective in cold weather.
(PIR low T performance is a separate deep dive. Suffice to note, if the external R is 1/3rd to 1/2 of R-total, the mean temp of the exterior insulation is 1/6th to 1/4 of the way from the outside cold to the inside warm. Example: 10F outside - 70F inside would yield mean exterior c.i. T of 20F to 25F. Of course, iterative recalculation with the correspondingly lower R lowers the average c.i. insulation T.
sterilecockoo58,
Martin's article makes much of the guess work and calculations you are worrying about unnecessary: https://www.greenbuildingadvisor.com/article/three-ways-to-insulate-a-basement-wall
Thank you for the info! Glad to hear that the mineral wool will still be fairly effective despite being spaced a bit off the foam. I recently got a thermal camera and the insulation appears to be performing pretty well. This part of the house went from uninsulated and cold in the winter to the best insulated and most comfortable part of the house.
User ...011,
Good stuff!