Using Rmax as partial attic radiant barrier?
user-5946022
| Posted in General Questions on
I have some 1″ RMax foil faced rigid polyiso left over from a project.
A portion of my home has a vented attic that gets far too hot. It has been suggested to me that installing a radiant barrier on the south facing side could reduce the heat transfer into the house.
My questions are:
1. Would foil faced Rmax fastened to the underside of roof joists act as a radiant barrier?
2. Would it work if I only did part of the area? This seems like a better use of the leftover product than putting it in a landfill. Other than not reaching optimal performance is there a downside to only doing part of it?
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Replies
"could reduce the heat transfer into the house." -> is it the house that is getting hot or the attic? You said attic at first, but then changed to house. If your house is hot you should check air sealing details at the attic floor level and evaluate the amount of insulation you have along that plane as well.
1.) It would work better if you could cut-and-cobble it between the rafters. If the foil has an air gap on both sides (i.e. one towards the open attic and one towards the open rafter bay), it will reflect the radiant energy back to the rafter bay, heating up that area of the roof. Conduction would send some of that heat into the attic, but probably a lower amount than if you had no radiant barrier. It would be better if the board was tight against the underside of the sheathing with the foil side facing the open attic, or better yet, above the sheathing but obviously that would require reinstalling your roof and other detail headaches. Just to be clear, the foam itself is doing nothing for you here. You would have the same effect if you just installed aluminum foil or a dedicated roll of radiant barrier foil. But since you already have it, I understand wanting to use it.
2.) Yes, it will still have some proportional effect with only part of the roof being covered. Radiant heat does not move like conduction, it applies relatively evenly to the entire surface of the roof (assuming same slope/azimuth of course). Air movement and conduction take the path of least resistance so it's more critical to have a continuous, uninterrupted layer with those forces. Vapor diffusion is more like radiant heat too, which is why Lstiburek says your under slab vapor barrier doesn't have to be perfect without any holes since the amount of moisture it will let in will likely be very small.
@matthew25:
Thank you! Some clarifications to the questions you asked:
This vented attic is over a first floor space and adjacent to a 2nd floor space. There is a second floor whose footprint is smaller than the first floor - where there is no second floor footprint there is a vented attic.
The attic "floor" has 15" of blown cellulose, so more than R-50, which seems to be doing its job. The portion of the 1st floor ceiling under the attic feels about the same temp as the portion of the first floor ceiling under the 2nd floor.
The issue is at the 2nd floor. The lower portion of the 2nd floor walls are adjacent to this attic. The walls that get warm are on the south side of the attic - the walls facing east do not get so warm. The south walls are all walk in closet walls so it does not impact comfort that much, but when the attic gets hot, the lower portion of the 2nd floor walls on the south side gets warm, the closet adjacent to that wall gets warm enough to be uncomfortable, so closet doors get left open to increase closet comfort (yeah, I know, but its what I'm dealing with) so the heat goes to the living area so there is then more HVAC load. The HVAC system handles it just fine.
No air sealing is needed - the house blew a 1.1 ACH/50. The walls also have 5.5" of cellulose (R-19) with minimal thermal transfer as I used 6" top & bottom plates with 2x4 studs at 12" oc, staggered in and out, with sealed Zip Sheathing.
I've already installed an extra turtle vent into this attic, thinking part of the problem may be insufficient quantity of ridge vent, and no ridge vent in the portion where the 2nd floor footprint is where the ridge vent would continue. That does not seem to have made much difference. I found some vents especially made for where a wall intersects a roof that would act like a ridge vent where there is no ridge, but neither the original builder or a roofer I've used in the past would install them, so that option got tabled.
Mainly I have a bunch of this 1/2" and 1" RMax and instead of putting in the garbage and having it go to a landfill, I'd like to use it to potentially improve this situation.
Responding to your post made me think of a different possibility - instead of using the RMax as a radiant barrier, would the best results of using this stuff be realized if I attach it to the attic side of the south 2nd floor walls, foil side facing the attic? That would give the portion of those walls where the "outside" of the wall is in the attic an added R3.2 (1/2") or R6 (1"). That would also be the simplest install...
If I use this stuff as a radiant barrier, you note it would work best to cut & cobble between the rafters, tight to the underside of the sheathing (with no air gap between the RMax and the sheathing) with the foils facing the attic. The question is how literal is that no air gap issue? I could cut the pieces the width of the rafter bays, push them up there, and keep them in place with a bead of canned foam at the 90 degree angle where the underside of the RMax butts the side of the rafter. But there might still be an air gap; the only way I know to totally avoid any air gap is to smear some sort of adhesive to the entire back of the RMax. I'm not crazy about that idea.
I agree with Matthew25, and just want to add that radiant barriers only work if the surface is shiny and clean. A radiant barrier has roughly the same impact as adding R-1 insulation, aside from the differences he pointed out.
Michael, thank you that is useful info to know.
What do you think about my idea of applying the RMax to the attic side of the wall as explained above - that would seem to get the full R value of the stuff. I could apply it with the foil facing the attic for some sort of radiant effect...
I'm surprised that you're noticing that large a difference in temperature with R-19 cellulose in the walls. Adding foam insulation to the attic side would help but you need to be careful about the ratio of impermeable, foam insulation to fluffy cavity insulation, to avoid potential condensation issues. What is your climate zone? (https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing)
Yeah, it gets VERY hot in that attic. I'll stick a device up there to measure the temp.
Good catch on the ratio of impermeable to fluffy. THANK YOU.
Climate Zone 3A, about 3/4 of the way up between the 3A warm warm humid line and 4.
UPDATE 5/20/24: I've been measuring the temperature in this attic space for about a week now. It regularly reaches 128-132 for about 2 -4 hours in the afternoon on a sunny day.
So back to the question: I have leftover 1/2" & 1" R-Max. Instead of throwing it in the landfill, what is the best way to use the R-Max to mitigate how hot the adjacent closet gets:
1. Apply directly to underside of roof deck between rafters with foil facing down? If so, is it ok to fasten with spray foam inside the 90 degree angle between the underside of the spray foam and the side of the rafter?
2. Apply +/-2" below underside of roof deck between the rafters, essentially just continuing the vent baffle. I could use small pieces as a shim an install as per item 1 above.
3. Apply /fasten to underside of rafters, with foil facing down. Fasten with cap nails.
4. Apply one or more layers to outside of zip sheathing on the lower portion of the wall that is in the attic. If I do this what is the minimum thickness needed (Climate Zone 3A) and how should I apply it? Based on this post:
https://www.greenbuildingadvisor.com/question/exterior-xps-rigid-foam-in-climate-zone-3a
it seems like exterior insulation is ok in my climate zone. The attic is not exposed to bulk water. I'm considering applying dabs of sealant (what type?) at about 1' oc letting it dry, then applying more to the dabs and tacking the R-max to it. That will leave a minimal gap that can function as a drainage plane to prevent vapor from getting caught between the Rmax and the outdoor green side of the zip sheathing. Do I need the gap only at the Zip, and can I apply more than one layer of Rmax one tight to the next?