In a roofing app EPS is usually cheaper because the other associated costs of a thicker layer insulation are small. In walls iso still wins as long as you can keep the wall thicknesses bounded to standard thicknesses, keeping the labor costs for window installation details. (Another reason to go 2x4 framing + thicker iso to hit ~R25.)

Both EPS and iso use pentane as the blowing agent which is pretty benign, but in the US XPS is typically blown with HFC134a, which has a heavy greenhouse gas footprint. In Europe XPS is blown with CO2 (which is comparatively lower impact than even pentane) but the resulting goods are about R4.2/inch rather than R5.

I have a house built in 2010 with 2x6 framing 24" on-center covered with OSB, then 2" of XPS, and then HardiPlank siding. Neither the interior nor exterior walls exhibit any waviness. Interior walls and ceiling are 5/8" Sheetrock.

Posted By Lee Dodge on 25 Sep 2012 03:44 PM
I have a house built in 2010 with 2x6 framing 24" on-center covered with OSB, then 2" of XPS, and then HardiPlank siding. Neither the interior nor exterior walls exhibit any waviness. Interior walls and ceiling are 5/8" Sheetrock.

You are definitely not a tract-house builder using lo-end subs for the framing.

In my house there is an addition build in the 1990s framed by the latter using 16" o.c. spacing and it's the least-plumb/square/flat part of the house.  (The rest was built in 1923.)  The floor had a wave deep enough to surf on (OK, it was less than 2" but still...) before I bit the bullet and flattened it out substantially by splitting & jacking the low joists, sistering on some new ones.  (There are some real hacks out there.)

Even custom home builders aren't immune to sloppy framers, and there's only so much "correction" you can do on the finish end. If going with OVE framing take pains to find & use a quality framing contractor.

What OVE looks like with insulation details on a gable end wall. 24 oc framing On the left side you see the nailing blocks to attach kitchen Cabinetry. All places wiring goes through walls or studs foamed. The window header is a single lvl which leaves 3.75” of insulation. Here I used isocyanate insualation with the aluminum facers pealed off on both sides . Here it was critical that the window remain centered so I had to use an additional stud on each side to keep the 24” oc. The top plates are foamed in between for air sealing Yes I used double top plates. The trusses are scissor trusses with a 5” energy heel. On the bottom cord of the truss you can see the netting for the cellulose insulation in the attic Once the cellulose was installed, I cap stapled 1” of iso to the bottom of the cords, I sealed the iso with zip sheathing tape, then strapped it perpendicular to the trusses with 1x3 16” OC then hung regular ½” drywall. The ceiling light fixtures were foamed to the iso, no recessed can lights Window is facing east and is a Kohltech triple pane, low solar gain double low E Krypton filled window foamed in with low expanding foam. Sheathing is Zip Panels sealed with zip tape, then 1” foam on the outside glued on and sealed with zip tape, then 1” strapping and then vertical shiplap siding. Walls got wet spray cellulose ceiling dry cellulose with a binder Walls ended up nice and straight, inside and out Trusses were not perfect so I ended up shimming the strapping perpendicular to the truss cord to get a nice flat ceiling.

Posted By eric anderson on 26 Sep 2012 08:25 PM
  What OVE looks like with insulation details on a gable end wall. 24 oc framing

Do you have any pics of the finished home? Exterior pics?
How big is the home and what state is it in?

Have you added up/calculated the framing fraction on each wall of that house? (Double plates and "extra" studs- what a luxury! :-) )

The micro-cavities on the windows seem pretty narrow- did they get cellulose or foam or...?

why didnt you balloon frame the vault wall?
I cant remember if theres an exception for 8' walls, but as built like in your picture would get flagged because of tall wall rules.

For the "green crowd", one misconception about 2x6 @ 24" o.c. is that it is "greener" than 2x4 @ 16" o.c. The opposite is true. 2x4 studs can be standard construction grade lumber while 2x6 studs must be #2 or better grade lumber. This means the center section of a tree must be used for 2x6 studs, which in reality means more trees are harvested for 2x6's than for 2x4's. This is one argument I heard.

Regarding the home in question. Why did they put that gap between the door frame studs? It doesn't make sense. Structurally one should nail the studs together to provide frame strength, no gaps. Plus it's not wide enough to fill properly with insulation. I am referencing the gaps around the door where one can see the OSB below the header and also to the right of the door.

I think that the main interest in using 2x6 studs is that it provides 57% more space for insulation than 2x4 studs. To meet the 2012 IECC code in my climate zone 6, the requirement for wood frame walls in R20 + 5 or R13 + 10, implying an insulated 2x6 stud wall with ~1" of foam, or a 2x4 wall with ~2" of foam. Since foam is much more expensive per unit R-value than cellulose or fiberglass, and since 2x6 at 24" on-center is similar in cost to 2x4 at 16" on-center, the economics favors the 2x6 walls. The 2006 and 2009 IECC requirements (which are still in effect for most counties around here) for walls are R20 or R13 + 5, which favors the 2x6 walls over 2x4 walls even more clearly since rigid foam on the outside is required for 2x4 walls but not 2x6 walls.

The R13 + 10 solution is nicer to your sheathing though, since it keeps it above the interior dew point. R20+5 requires an interior vapor retarder in zone 6, whereas R13 +10 does not.

R13 + 10 has a wall thickness equal to 2x6 construction sans-foam too.

Like anything there are multiple trade-offs to consider. I'm not a strong advocate of either solution over the other, but I lean a bit toward R13 +10 on moisture resiliance grounds. R20 + 12 in zone-6 is nicer to the sheathing than code-min w/vapor retarder.

Posted By Dana1 on 28 Sep 2012 12:27 PM 
...snip...

 R20 + 12 in zone-6 is nicer to the sheathing than code-min w/vapor retarder.

And indeed, that is essentially what I selected for my place, R20 + 10, consisting of 5.5" of wet-sprayed cellulose plus 2" of XPS rigid foam.  Being in a high mountain desert, the vapor flows are less of an issue here than in more humid areas, but for sure should not be an issue with the stack-up used on my house. 

It is relatively humid out today, with a dry bulb of 67 F, a wet bulb of 41 F, for a relative humidity of 38%.  Humidifier is running to keep the indoor humidity at 35% at 68 F dry bulb. 

The IRC calls out R11.25 min in that stackup to skate without an interior vapor retarder for climate zone 6. The presumptive interior air dew point behind that must be something like 37-38F (30% RH @ 68-70F).  At outdoor temps that matter R10 labeled XPS would deliver R11+ since it increases in R value when it's temperature is below the ASTM C 518 temperature range. 

In the same application you'd derate 2" iso to about R11.2  at 0-10F outdoor temps as well, even if its rated R12 or more in an ASTM test.  Inch-for inch iso & XPS performance is not all that different when the average temp within the foam layer is +20F  (0F outside, +40F at the sheathing), but iso has the performance edge at warmer average temps- 20% better when it's above 40F outside.  And with an air gap on the exterior of the low-E aluminum facer it'll do a little bit better even at the low temps than it would with the siding tight to the facer.

Posted By Lbear on 28 Sep 2012 04:13 AM
Regarding the home in question. Why did they put that gap between the door frame studs? It doesn't make sense. Structurally one should nail the studs together to provide frame strength, no gaps. Plus it's not wide enough to fill properly with insulation. I am referencing the gaps around the door where one can see the OSB below the header and also to the right of the door.

What picture are you looking at?

Lbear The house is finished, I have been living in it since 2009. I will post exterior pics this weekend if you want.
House is a ranch 28X40 with a walkout full icf basement
Dana,
I never calculated the framing factor, I did work hard to minimize framing and not just throw studs at a problem. Windows were foamed in. the narrow space between studs got cellulose, but they had a special narrow wand that they used to fill small areas first.

Greentree, I did not balloon frame this because it was easier to just buy a gable end truss and install it with the rest of the trusses. The truss is 2X4 with the 1.5” cord vertically so I added 2x4’s vertically on the inside so the plane of the drywall would be the same all the way up. It is a gable end so the loads are not that great anyway

Posted By eric anderson on 28 Sep 2012 08:49 PM
Lbear The house is finished, I have been living in it since 2009. I will post exterior pics this weekend if you want.
House is a ranch 28X40 with a walkout full icf basement

Yes, it would be appreciated. Where is the home located at?

Posted By greentree on 28 Sep 2012 05:39 PM

What picture are you looking at?

Question answered:

Posted By eric anderson on 28 Sep 2012 08:49 PM
  the narrow space between studs got cellulose, but they had a special narrow wand that they used to fill small areas first.

2 exterior shots. First is wall with iso on it taped. Outside of wall shown earlier second is with siding finished house exterior south side shown. Chimney is for wood stove solar panel on roof is for DHW, large Black pannel on wall is solar air heater. South windows are high solar gain double pane argon filled. zip code 06232

Basement and floor framing 2inches of xps on top of 10 mil Poly ontop of compacted gravel, footing drains outside foundation as well as underslab. All drains to daylight. Box outs are for laly column footings Floor framing Designed for l/480 deflection on whole floor. Basement 9' ceilings (shop space) Walkout with 5' french door. 8" foundation Polysteel ICF

Lifting walls old school style, in pic is myself, neighbors, 2 cousins, and brother in law second pic is a wall detail spacing between windows preserves 24" OC spacing. I used iso to create the space for the soffet vent channels. I then ran the exterior iso all the way up to meet the vent channel on the outside and foamed the whole thing to air seal it. Because of the nature of the scissor truss, even with the energy heel there is not as much room for insulation.

exterior showing zip panel prior to adding iso. It is flashed to the foundation foam so any water behind the iso can drain. Zip pannel is caried all the way down over the rim joist and taped to prevent air leakage at the rimjoist Second shot is the south wall from the inside after insulation and ceiling iso layer taped to wall studs prior to drywall. the Iso is taped to all interior walls and is intended to be the ceiling air barier, drywall is a secondary air barier. Ceiling is straped with 1x3 16" oc so that there is no problem iwth the weight of the cellulose which is 16" thick near the peak

Last 2 shots are the plumbing diagram and a picture of the mechanical room I hate working on houses where the mechanicals are buried All the Plumbing stacks into one area of the house. The pex distribution manifold is located directly under a 2'x4' chase common wall that has the kitchen sink and sishwasher on one side and the toilet and bathroom sink on the other, with the laundry sink, and washer in the basement directly behind it. Longest run is to the shower and that is only 17' from the manifold. Every single plumbing connection is accessable without opening drywall. The shower backs up to a closet with an access port. The boiler is a 50 kBTU propane Munchkin mod con with a min fire of about 14 kbtu The larger indirect tank is 80 gallon solar heated, it chains into a 50 gallon one heated by the boiler if necessary. there is a thermostatic mixing valve before the manifold as the exit temp of the indirect can be ~150 F in the summer when the solar is maxed out. the house is open design so there is only one zone. Generaly I heat the house with a small wood stove