Where should I apply polyiso in an attic? South wall?
My budget is $2000. Built in 2002. I have average R40 attic insulation a combination of blown pink and rolled pink. My thought is to add R10 poly iso to the attic but I wondered the best way to do it. R10 would be from two layers of 3/4″ R5. The attic is large and ventilated with 2×6″ joists 24 OC. I has an existing radiant barrier that looks to be properly installed on the underside of the top of the attic I live in 3A W/H Dallas TX.
My first and best solution was to
1- place 1×2 furring strips against the joists on top of the drywall deck and to lay the R10 in the channel and spray foam the joist for air tightness. then add the fluff on top.
2- then i thought maybe i should do the above with one layer of r5 and then lay the second later over top of the joists to stop thermal bridging and add the fluff on top of that.
3- Leave the fluff alone and add R10 to the underside of the top of the attic.
The second part is my south wall. It’s 2×4 construction with R13 pink batts. Brick face. Fiber board used at a “vapor barrier”. Generally during the late spring through fall the south wall interior temps are 7-11 degrees higher than the rest of the house. I had some cathedral ceilings that I addressed with adding R5 Poly iso, a 1″ gap and drywall. Should I do the same on the south wall? Should I remove the existing sheetrock and add the foam on top of the batts? Should I remove the batts? How deep should the foam be? These are already small rooms so I want to reduce their size by as little as possible. Ease is also a factor. Total of south face is 280sq, so manageable either way.
Thank you in advance. This site is so great I can’t even say. I’ve spent many hours reading.
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Replies
"vapor barrier" in quotes as a joke. Ok. I'll leave the attic alone and work on the south wall. I think I will remove the drywall and existing insulation. that should be plenty where I won't have to build out the window box. I had no idea how I was going to do that.
R-40 attic with a radiant barrier... that's pretty good already. But if you want to beef up the insulation of this type of attic, the simplest and best method by far is to blow cellulose right over the top of what's already there: your option 3.
Your idea for the south brick wall will work just fine. Add as much foam as you can stand without losing floor space. 2" of polyiso will stop that heat flow dead in its tracks. Fiberboard isn't a vapor barrier; in fact, it's extremely vapor-open, allowing drying.
However since your problem is heat gain caused by solar radiation, I'd just plant some trees and grapevines to shade the wall if the site allowed for it. Or stick some DIY-ed solar thermal panels there if your water heater is close. Put that sunlight to good use!
The only reason to remove the existing insulation it would be to add something that would fill the stud bays with more than R-13. Options would be mineral wool (R-15) or closed-cell spray foam (R-24.5). Mineral wool is easily DIY-able and easier to install properly than fiberglass batts, but it's still not much of an upgrade.
The easiest option might be to remove the fiberglass batts and simply reinstall them properly.
Nate. If I remove the batts and put in 5 layers of R5 3/4" poly iso thats at least an R25 right? If then add one more layer over the studs and then 1/2 inch sheet rock I've made the room 1.25" shorter and taken the r13 to an R30 right?
sorry. actually 3/4" shorter
You can certainly do that, but you're not getting the full R-25 out of the foam between the studs due to thei thermal bridging at the studs. See this article: https://www.greenbuildingadvisor.com/blogs/dept/musings/cut-and-cobble-insulation
Nate. I guess we are not on the same page or maybe I don't understand. I said I would add another layer over the studs. Understood I cannot stop the bridging on the exterior, but doesn't covering them on the inside take care of that?
It's certainly possible to remove the existing drywall and rebuild the wall. If you are going to do that, adding a continuous layer of interior rigid foam will do more to improve the wall's thermal performance than changing the existing insulation between the studs. Here is a link to a relevant article: Walls With Interior Rigid Foam.
I'm a little worried by your statement that you hope that "I won't have to build out the window box. I had no idea how I was going to do that."
If you aren't capable of building jamb extensions or installing a new stool, you shouldn't be doing this work. It's time to call a contractor.
-- Martin Holladay