Foil face iso board inside of SIP panel
Calling all vapor barrier experts!
I have a question about the suitability of putting a 1 inch layer of Thermax/Tuff-R/whatever manufacturer calls it, on the interior face of the SIP walls of the house I’m slowly completing.
I need to space the drywall inward to accomodate wiring that will be on the inside face of the SIP wall, so I figured I’d upgrade the R value with the spacer and kill two birds with one stone, adding a radiant layer and a vapor barrier. Cost of heating went way up since I started this project, so this upgrade seemed like a good idea.
Can anyone out there imagine any possible negative consequences to this setup? I know there’s debate about vapor barriers but this seems harmless. Would unfaced insulation make more sense?
Any help would be greatly appreciated.
Thanks,
Nick
Replies
Faced polyiso is inexpensive enough. You could do EPS, but for R-value, pay your money take your choice, I think.
Vapor retarder isn't really an issue as the SIP panel is a VR in itself. I thought wiring and the like were installed w/ some technique that didn't require what you describe. Is what you are doing what the mfg recommends? I don't know much about the details of SIP installation ... remember seeing some video demos and such, but don't recall the specifics of wiring, piping, etc.
I don't think there's any problem skinning the inside with another inch of t-max (or any other foam), however, I don't see how this solves your need to have space for wiring. If you were planning to rout channels in the foam for wiring and then skin the whole thing with drywall, I don't think that's a good idea at all. You might consider framing 2x4 curtain walls on the interior side. This would give plenty of space for any conventional wiring, and you could then stuff this wall with batts and drywall as usual without worrying about dinging any wires.
I also would use EPS.
You will get zero benefit from the foil facing. In order for a radiant barrier to be effective, you need at least 3/4" of airspace.
Thermax (polyiso) will give R6.5/inch
XPS will give R5/inch
EPS will give R4/inch
The higher the R value per inch, the higher the cost (typically).
Jon Blakemore
RappahannockINC.com Fredericksburg, VA
Where did "they" come up with the 3/4" requirement?
I don't know where "they" came up with it. I'm not a scientist, but I'm pretty confident that 3/4" is the minimum space required.
But don't trust me, ask the ORNL and the EERE.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
The testing is done with 1" spacing vs no spacing.So THEY is us when we substitue 3/4" for 1"
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I believe the polyiso and XPS will also offgas over time to end up in the same league as EPS when they are done.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Do you have any data to back that up?I always have heard/read that polyiso will start out at 7.2 and end up at ~6.5.I have not heard that XPS has any degradation issues.If it does, that's just one more reason why I like EPS.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
I'm picking this stuff up by osmosis, so no, not really. My understanding so far is that all closed cell foams offgas a portion of their starting R-value, and the "Aged" R-value many foam companies are using is a 15 year weighted average only.. but I don't have a solid number for particular products.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
I'm not totally sure, but I think that "aging" affects more the foams that start out high in the first place, according to the type of gas used to blow the foam. The degradation in R comes when the diffusion results in slow loss of the blowing gas and its replacement with oxygen and nitrogen from the air.A metal-covered foam loses less R due to aging than if the surface is exposed.Water-blown foams start out at (I'm told) 6/inch and stay there. Maybe, maybe not. I don't know what gas is produced by the reaction.Some R values (per inch) of common gases (if truly stagnant), from their thermal conductivities at 35 F:Air 5.94
O2 5.87
N2 6.05
CO2 9.82
H2O 9.08
Argon 8.68
Krypton 16.25
I believe it's open vs closed cell. closed cell often has higher R per inch, so that sounds about right from what you are saying.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
FWIW, I've never been a big fan of fastening drywall directly to foam insulation. Not sure if that is being considered here.Some screws can compress the foam a bit, then later push back, causing the screw to tear the gypsum face. I much prefer furring strips over RFBI, which gives you your air space, plus it makes the drywall hanging go much faster and more worry-free.And from the manufacturer; unfaced polyiso does lose a bit of R-value over the first few years, but foil faced is more stable. The foil facing apparently slows the process down. Nothing in writing, just a phone conversation.
Hello to all
That's an aspect I never considered, that the iso board could result in "screw pops" in the drywall.
One of you guys offered the R values of various gases, including air at R 5.94 or so.
I consulted Glover's "Pocket Ref", which has a section titled "insulation value of materials", and it offers an R value of R 3.48 for a 3/4" reflective-faced air space. Extrapolated up to 1" that would be approximately R 4.64 for almost no cost, versus $.75 per square foot for iso at R 7 or so, a little less for blue styrofoam at R 5 or so, and a little less than that for white styrofoam at R 4 or so. It would take a long time to save back the cost of 1800 square feet of wall area at $.75 to $.50/sq ft, and the airspace would seem to give nearly the same benefit with one less disadvantage.
I'm so thankful to be able to tap into the institutional knowledge of this forum.
Nick
Air at R5+ per inch might work in the laboratory, but not in the real world. If that were true, why would we be adding insulation with lesser R-values per inch to wall cavities? Convective loops have an effect.In real world stuff, figure air at about R1 per inch. Actually, R1 is a pretty good estimate for an air space from about 1-4" "thick". And that's not per inch, that's for the total depth. 1" of air, R1. 3" of air, R1.
I'm a little embarassed at my lack of critical thinking. Further research after my last post confirmed exactly what you just said. A big oops! on my part. Looks like for out for drywall so screws go into wood and install iso or styro between the furring strips. If I go the styrofoam route, do you know of a brand name for a reflective material that can be installed under the drywall? Someone referred to kraft-faced reflective sheet? Maybe a reflective layer doesn't have a huge benefit, it would serve a vapor retarder/barrier function, I think.
I threw out the tabulation of ideal R values of various gases for comparative purposes. Those values are for truly stagnant gas, but as has been pointed out temperature differences cause density differences and thus convective looping within a volume of gas. That's why we don't see insulating glass with more than about half an inch between panes. Even then there is some convective looping at work.An insulating material such as FG, cellulose, or foam works by creating tiny volumes of air or some gas. The effectiveness of the insulation is partially a matter of how well these pockets of air are isolated so as to make them truly dead gas space. With truly stagnant gas pockets, the overall R of the material is then some mix of what the gas provides and the R of the tiny fibers of solid material separating the gas pockets from each other. The solid particles conduct heat more readily than the gas trapped, so of course the R (per inch) of the insulating material will be less than the ideal R of the trapped gas.
"If I go the styrofoam route, do you know of a brand name for a reflective material that can be installed under the drywall? Someone referred to kraft-faced reflective sheet? Maybe a reflective layer doesn't have a huge benefit, it would serve a vapor retarder/barrier function, I think."
Why do you want a vapor retarder? Maybe I've missed it earlier in this thread, but what is your climate?
If you have SIPs that are correctly installed, you would not need a vapor retarder in most climates. The OSB skin on your SIPs already has a perm rating somewhere in the neighborhood of .75.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
Ironick,
Where is the house? I'm not an expert but I've been reading up on these issues, and knowing the climate is critical to building the right wall, especially when it comes to vapor control issues. What's the overall wall construction (cladding, air barrier, etc?)
Hi
This project is in upstate NY, 6500 degree days give or take a little. -20 F is possible but not common. Structure is 6" SIP walls, no framing, just OSB-styrofoam-OSB walls. Roof is 10" SIP plus 2-1/2 " polyiso without foil backing, just paper facing, R60 or so total.
My biggest concern is getting vapor in the wrong place mess. I think the OSB on the interior face of the SIP is a good vapor retarder, I'm just unsure that Thermax/Hi-R foil will form vapor barrier and trap moisture somewhere other than inside the living space. It's likely that I'm worrying about nothing, but I'd like to know if there's a precedent.
Thanks,
Nick
If you strapped that foil face with 1x and attached the drywall to that, you would maintain the needed standoff, plus have the spacing you need for fasteners.
Tu stultus es
Rebuilding my home in Cypress, CA
Also a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
Your SIPs already are a VB.
You will not get and radiant bar bennies unless you furr out to create a space in front of the foil.
can't hurt, but if this is a properly built SIP structure, it likely won't help all that much.
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Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!
Nick,
1. No problems from a vapor transmission point of view.
2. Your idea will improve the thermal performance of the SIP wall, and that's good.
3. A 3/4-inch airspace would provide better performance because of the foil, but it's not necessary — there's nothing wrong with just accepting the benefits of an additional R-6.5 from one inch of polyiso — that's still a useful benefit.
4. If you have no airspace, you are still at risk of hitting the Romex with drywall screws. Check with your building inspector before proceeding.
5. If I was doing it, I would be sure to tape the seams of the polyiso with contractors' tape.
Hi
I thought about a furred-out airspace, R-2 or so, and decided to spend the bucks for an inch of Iso, R-6.5. Seemed like there would be definite benefit from extra R-value and so far responses and re-reading and old Fine Homebuilding article "Air and Vapor Barriers", FHB # 88 indicate that vapor-barrier effect is positive, radiant effect might or might not have a benefit, and if good detailing is done, vapor shouldn't try to get into the cutouts in the foam and OSB and get stuck in the wall trying to get to the outside world. I plan to tape the seams and foam around the boxes.
As for wiring, I wanted the freedom to put wiring where I want it and to be able make changes in the future, even if it meant drywall removal/replacement. Airspace was always going to be the intentional byproduct, additional insulation and foil face idea came later. I was going to fur it out with 1/2 iso to accomodate the channel that will protect the Romex, then decided an inch would be better insulation at little additional cost.
I built a model of the wiring method and got it blessed by electrical inspector for my area. Romex will be covered by .060 x 1/2"d x 1-1/2 wide channel per NEC. I got a pile of this channel at an industrial salvage place. This method makes for a flexible placements. Inspector also said steel BX or EMT would be acceptable but that aluminum shield BX would not. Inspector used to build SIP houses.
Nick
Riversong will probably disagree, but in my opinion it should not cause problems.