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Which Rigid Insulation Should I Choose?

comments (6) May 1st, 2009 in Blogs

FHB_WEB, member

25 users recommend

Extruded polystyrene (XPS)

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by Rick Arnold

Rigid-foam insulation packs a lot of R-value into a thin package, but not all rigid foam performs the same. Choose insulation wisely, and consider the effect its characteristics will have on the performance of the walls.

Expanded polystyrene (EPS)

EPS is the insulation used most widely in insulated concrete forms and structural
insulated panels. EPS has the lowest average R-value of the three types of rigid-foam insulation, around R-4 per inch. At about 19¢ per sq. ft. for a 1-in.-thick 4x8 sheet, it also costs the least. Although EPS is acceptable for ground contact and can be treated to resist insects, it does absorb water. When applied as sheathing, EPS should be used over housewrap. Most EPS is unfaced, which means it is fragile to work with and is considered semipermeable, so it does not create a vapor barrier.

Extruded polystyrene (XPS)

Easily recognized by its blue, green, or pink color, XPS falls in the middle of the three types of rigid-foam insulation in both cost and R-value. At about R-5 per inch, XPS costs around 42¢ per sq. ft. for a 1-in.-thick 4x8 panel. XPS comes unfaced or with a number of different plastic facings. Unfaced 1-in.-thick XPS has a perm rating around 1, making it semipermeable. Thicker and faced XPS is stronger and can have a lower perm rating, but either way, it is considered a vapor retarder, not a vapor barrier.

Polyisocyanurate (ISO)

ISO panels are expensive, costing as much as 70¢ per sq. ft. for a 1-in.-thick panel, and they pay off with (aged) R-values as high as R-6.5 per inch. (R-values start around R-8 and degrade slightly over time.) Because ISO starts as liquid foam and has to be sprayed against a substrate to form a rigid panel, all ISO panels are faced. A few different facings used on ISO affect the performance of the panel in both durability and perm rating. Foil-faced ISO panels are considered impermeable. Because applying these products as sheathing creates an exterior vapor barrier, they never should be used with an interior vapor barrier. More permeable ISO panels are faced with fiberglass and can be used without creating a vapor barrier.

Read the complete article...
Save Energy with Rigid-Foam Insulation
Get the details right, and a small increase in wall thickness can make a big difference in energy performance
by Rick Arnold

posted in: Blogs, energy efficiency, insulation, weatherizing

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Comments (6)

awhiteman writes: I hope this is the right place to ask this question. I am finishing a third floor of a two and a half story house. I have 2x10 rafters. I have at my disposal four inch polyiso board with the foil facing. My plan was to rip one inch strips of wood to put on the back of the roof decking then cut the polyiso board to fit in between the rafters leaving one inch gap for ventilation as well as a 1/4 inch gap on either side to seal after installation. I was going to double up the polyiso board for a total of eight inches. Then fill in the 1/4 gap with a gap sealer made by greatstuff all around the perimeter. In the end I would have the foil facing the air gap for ventilation on the outside and I would also have foil facing the conditioned space on the inside. Any questions or concerns would be much appreciated.
Posted: 11:58 am on March 12th

PGarnet writes: I agree with CMJeff on all accounts in the context of putting 1" foam outside. I would like to add to what he says.

The study in AK with REMOTE walls (insulation/vapor barrier on the outside) was based in the hypothesis that if there were nearly double the R value in the outside of the wall creating an exterior vapor barrier, the dew point would occur somewhere in the layers of foam and therefore prevent the condensing of moisture with in the first third of the wall ( the interior ). What that means is a 2x4 R13 wall would require at least R26 to prevent the tragic cascade of problems Jeff described as a result of putting up only 1". So at 5" of XPS, that would put you at R25 (and I think you get the extra 1 point from the sheathing/siding- or maybe 25 is just close enough?). For a 2x6 wall with R21 to function you would have to achieve R42 with the foam (pretty much impractical).
There have been many REMOTE walls constructed here, and I have not seen or heard of their failures when the theory has been properly applied. I wondered the same about the sag problem with the fasteners but have not heard any first or even second hand stories about that.
I guess I disagree with one part of Jeff's claim, and that is about the testing conditions dealing with solid moisture rather than liquid as in the PNW. South East Alaska has a particular problem with not only high amounts of rainfall almost year round, but also winds driving it sideways allowing it to penetrate siding on a regular basis. The REMOTE walls have been tested in this harsh environment and have been reported to perform very well.
Only time will tell if the properly constructed walls continue to function, but I guarantee the "painful redux" will be seen from the improperly applied theory.

Paul
Posted: 9:03 pm on December 31st

CMJeff writes: Its too late to save Royread, but for anyone else considering installation of hardboard insulation on the outside of the studs, plese read on. The purpose of the vapor barrier on the inside of the insulation is to keep water vapor from inside the living space from traveling through the sheet rock and condensing inside the insulation in the wall cavity when it gets close enough to the outside wall for the temperature to reach the dew point. Adding an inch of hardboard insulation to the outside of the studs in the Pacific NW does not provide enough insulating value to keep the temperatures inside the wall above the dew point so it is inevitable that this arrangement will cause condensation within the wall space. Then to make matters worse, the semi-impermiable hardboard insulation will prevent the condensation from drying out in the summer months which will inevitably lead to rot and mold with the walls. Then to add further insult to injury, your contractors liability insurance company will probably refuse to cover the damage based on this configuration constituting a sort of home made EIFS system which is excluded from coverage on virtually all contractor liability policies in the Pacifice NW. To make matters worse, there so far, is not a good solution to the problem of sagging fasteners which are likey to allow movement in the siding and open up gaps which can add further water intrusion into the system which will be trapped behind the hardboard product. Some insulation manufacturers have been sending their sales people around pimping this system based on a study in Alaska where they used 5" of hardboard on the outside of the studs and in an area where most of the precipitation falls in solid rather than liquid form. Anyway, for anyone who has been around for a while, this new idea of hardboard insulation on the outside of the studs is destined to be a painful redux of the EIFS debacle from the 90's where the product manufacturers once again foisted off an untested and conceptually unsound system on bulders and the public.
Posted: 5:12 pm on November 27th

roryread writes: We just wrapped a new house here in Portland, OR, with 1" of unfaced xps. The obvious upside is the thermal insulation gain.

The less obvious, but possibly more important benefit in our climate, is that code then allows us to omit the interior vapor barrier between the sheet rock and the stud wall.

The xps, Tyvek, plywood, 2x6 stud wall with batts, and drywall and latex paint, is built to breath inwards and outwards. The science I read suggests that this is close to optimal construction for our marine climate.

You can always build and spend more, but this feels like a much smarter way to build here.

Rory Read
RDG Read Development

Posted: 11:28 am on February 5th

Arbutus writes: Our building department required R20 for all exterior walls - however with 2x4 studs this would have been impossible without the polyiso insulation.

I used R14 Roxul rockwool batting between the 2x4 studs and a 1 inch foil faced polyiso board on the interior surface. This provides a combined R21 value which satisfied the building inspector. All seams were taped with foil tape so the vapor barrier was intact. Interior sheathing was 1/2" G1S fir ply.

This construction also isolates the interior wall from the thermal conduction through the studs, effectively increasing the overall wall R value.

Posted: 1:25 pm on May 5th

captainbil writes: I'm finishing up a 1200 sq/ft addition and energy efficency was at the top of the list. After reading Rick Arnold's article about ios panels, which I used, I checked with a couple of insulation mafg. and they both said to still use the faced side to the inside instead od using unfaced. What i wound up also using was that breathable vapor barrier that Certinteed sells. It is only 2mil thick and I don't see how it works, time will tell. according to Ricks article no faced insulation was used on the inside. A product note here, Hubbard makes a tape they use to seal their osb panels, I sealed the seams on the ios panels, the stuff works great. Better than the foil tape.
Posted: 5:02 pm on May 3rd

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