http://www.greenbuildingadvisor.com/blogs/dept/musings/cold-climates-r-5-foam-beats-r-6
This has me questioning what to do. I was going to get polyiso, but now maybe 1.5" of xps is in order (zone5 2x6) Any comments?

It depends on the climate and where the insulation is located (interior or exterior). But how about 2" EPS?

The problem I have is if I go over 1" there begins a problem of matching siding to brick and whatnot. There is a corner where brick ends and siding is on the other wall (90 degree). Since the siding is so thick, I didn't want the brick to overhang more than an inch or 1.5" over the edge. With 1.5" rigid, and 0.75 furring, I am looking at 3" overhang. Can't have that. So what I'm thinking about doing is having the siding trim panel (corner piece) sit over the brick slightly but not come to the face of the brick. If I double it up, to 1.5", this covers the siding as well as the furring. So if I do this I am planning to have the brick hang over the foundation edge only the thickness of the foam. If I go 1.5" the brick should be okay (norman style).


Per the article the XPS seems to be better for cold climate so I will most likely use that. I was more interested in whether the blowing agents of these insulations have changed to once again favor the polyiso in cold climate. As I stated I am in Chicago (one of the locations simulated) and being right at the edge of the minimum R value for external, I don't want the rating devalued due to cold air.


The other idea is put 1/2" polyiso on the inside with open cell SPF in the 2x6 wall cavity and call it a day. This would still much better than code minimum.

Is 2" EPS on the interior difficult? Or perhaps even switching from exterior to interior where you have brick.

I think I figured out a good way to handle that corner and do exterior rigid at 1.5". With 1.5" rigid + 0.75" furring + 0.75" trim/siding I have 3" total. What I am thinking about doing is letting the brick hang over the side edge 0.75" which should be fine with no support under it (again norman brick - this should be fine). I will need to make up 2.25" - three 0.75" thicknesses. To do this I will lay one 0.75" trim piece on top of the foam, across the brick's side face 0.5" or so covering the gap behind the brick. I will then lay two more trim pieces staggered over and use this 1.5" "L-shape" indent to put a gutter running down the corner there which will cover up the fact that the siding will protrude further out at this corner. My parents live in a townhome that has this on one corner (no gutter and only one 0.75 trim piece over the brick's side face) and in the last 8 years no one noticed it until we were out there this week looking at how they matched the brick and siding. So with the gutter in front it will probably be invisible.

If you can get 1.5" XPS in there you'll be at R7.5, the dead-minimum for dew point control with just latex as the interior vapor retarder.  With polyiso you'd be at R9 nominal.

The highest R knee in the curve is when the center-foam temp is at +15C/59F, at which point polyiso is running R6.7/inch.  With the mean January  outdoor temp in Chicago running around ~25F/-4C.  With R20 cavity fill and a 20C/68F interior the mean temp through the foam is going to be around +5C/41F, so according to the curve it'll be average R3.3/inch (R5 @ 1.5") during the dead of winter riding on the steep part of the derating curve, but much higher on the warmer winter days and in the shoulder seasons. Your sheathing would still be OK if using 1.5" polyiso rather than 1.5" XPS despite the R5 average performance during the very coldest month since it's R-value rises SO rapidly with higher temps. The derating curve for iso in that article is much more severe than others I'd seen elsewhere, but if that curve reflects the current state of off-the-shelf polyiso, the 1.5" XPS would edge it out on heating season thermal performance.

A layer of half-inch polyiso plus 1 inch of XPS would have a nominal R comparable to the 1.5" XPS, but the polyiso would be in the better-performance range nearly alway, since the modest gains in R at low temp from the outer EPS would keep it there. (edited to add) It would appear that BSC also recommends that approach- see the recommendations at the bottom of this page:

---------begin quote

For cold service temperatures the following recommendations are offered:

• Use thicker layers of polyisocyanurate insulation to ensure that the performance meets expectations. NRCA’s most recent recommendations are to assume that polyisocyanurate has R-5.6 / in. when designing for warm climates and R-5.0 / in. when designing for cold climates.¹¹

• Use a hybrid insulation approach – install cold temperature-tolerant insulation over top of the polyisocyanurate insulation to increase the mean temperature of the polyisocyanurate.

----------end quote

It would be useful to know more of the specifics of the polyiso sample tested- it's pretty bizarre to have such a hard knee and steep slope to the derating over such a narrow temperature band. The derating curves on the above BSC page are different from the four different manufacturers, but don't have a severe a hockey-stick appearance of the sample curve in that blog bit:

I was thinking about that, use a 0.75 XPS over a 0.75 sheet of PISO. Your suggestion of 0.5 PISO and 1.0 of XPS makes more sense. to save labor and scheduling issues, I was also considering just one layer of 1.5" XPS. This would greatly simplify things for me since the framers will be waiting for ME to install the rigid and furring before they place the housewrap over the furring and install windows. I am hoping I can start installing the rigid insulation as they start framing the second floor, and get all this done while they finish up the roof framing.

Making a single pass with only one thicker sheet will be a huge benefit to me with hanging the rigid and dealing with installing the furring (finding the studs). I admit I might just do one sheet of XPS...

Posted By Surfsup on 27 Dec 2013 07:55 PM
I think I figured out a good way to handle that corner and do exterior rigid at 1.5". With 1.5" rigid + 0.75" furring + 0.75" trim/siding I have 3" total. What I am thinking about doing is letting the brick hang over the side edge 0.75" which should be fine with no support under it (again norman brick - this should be fine). I will need to make up 2.25" - three 0.75" thicknesses. To do this I will lay one 0.75" trim piece on top of the foam, across the brick's side face 0.5" or so covering the gap behind the brick. I will then lay two more trim pieces staggered over and use this 1.5" "L-shape" indent to put a gutter running down the corner there which will cover up the fact that the siding will protrude further out at this corner. My parents live in a townhome that has this on one corner (no gutter and only one 0.75 trim piece over the brick's side face) and in the last 8 years no one noticed it until we were out there this week looking at how they matched the brick and siding. So with the gutter in front it will probably be invisible.

Just return the brick 2' around the corner and then butt the siding into it. It will look much better.

Just return the brick 2' around the corner and then butt the siding into it. It will look much better.

I'd have to change the concrete and framing plan. I've seen what I want to do. You don't even notice it. It is also at the corner that no one will ever go to.