I am a new member and the first question I have is how to delete my test posts. Now to the real situation. I will be building a new home in the mountains east of Salt lake City, Ut starting in June. Temperature will range from -10 to 90 Degrees at 7500ft elevation. My architect and I are odds ends with the exterior walls. I intend to do double 2X4 walls with a 5" gap. First thought is 2" closed cell spray foam followed by wet cellulose if that can be sprayed that thick giving me an R-50+ in the walls and R-60/70 in the ceiling. I don't know if the closed cell foam will eliminate moisture on the external sheathing but read in the forum that plywood would be better then OSB. I think I will be using Hardy board for the siding. My architect wants to 2X6 walls with zip/1" foam exterior sheathing, 2" closed cell foam and 3.5" of cellulose(R-33?). I read a report by Building Science in Oregon and they were against having foam against the 2X6s because of moisture but they were using OSB. The new house will be 2500sq ft main and same for basement. It will sit 48" in the ground. My present home has 5.5" cellulose (R-19?) and below 35 and above 90 and it doesn't cut it. However there are to many big windows. I plan on heating with Hydronic in floor pex. I don't like the pex in the 4" bsmt slab and would like to do 2/4" ridged foam on the 4' slab with pex, then 1.5" slurry concrete. All floors will be tile. I read one post where this was going to be done this way but no pros/cons that I can remember. I need some expert advice. Thanks

Your wall section sounds fine - the 2" closed cell will help eliminate condensation on the sheathing, and the remaining cellulose will be fine. Plywood is more resilient than OSB, and Huber OSB ZIP sheathing is more resilient than generic osb due to its formulation. So ZIP will work in your case since the sheathing will stay dry & taping the wall is a good idea as an air barrier. Some of the German and Swiss tapes will seal to bare wood, but many tapes will not. I'd suggest R70 in your ceilings, 6-8" under the slab & triple glazed windows, and hRV & A few mini splits. One caution - with this level of insulation there is a greater chance of mold growth, so pay attention to air sealing. A tight house with these components will be an enjoyable house to be in. Stick to your guns and tell your architect this is what you are building.

Your architect has is right.

If you over-insulate you waste valuable resource better spent on upgrading your glass for example.

In order of importance, and ROI, the ceiling, the walls and far back in the pack, the basement floor--radiated or not.

Tile is hard, no dropping anything without breaking it and cold to the touch unless the panel is energized, very seldom if you are building a super-insulated house i.e. double the current IBC standards.

When thermally bridged by studs the high R/inch stuff is somewhat wasted, and in this instance it's only purpose would be air-sealing and vapor-retardency, and at $2/square foot that's about the most expensive way to go about it. You can air-seal the exterior sheathing to the framing & subfloor for a lot less, and use cheaper/better vapor retarders at some other layer in the stackup. The extra R 4-5 you get with the foam center-cavity with the foam adds up to less than an additional R2 after factoring in the thermal bridging.

In a UT location skip the 2lb foam, and put half-inch OSB on the exterior side of the interior studwall as the vapor retarder, and drill holes in it to dry either dry blow or damp spray the exterior cavity, which leaves the interior studs clean for running the plumbing & electrical. You can then either use rock wool batts or damp cellulose to insulate the interior studwall before closing it in. That's still two trips for the insultion crew, but with a 2" foam + 8.5" cellulose solution you were already signed up for that.

The difference in climate damage from the two approaches is huge, both from the manufacturing footprint of the polymer AND the HFC245fa blowing agent (at ~1000x CO2) of the 2lb foam.

The double 2x4 wall with the mid-layer OSB vapor retarder comes in at higher thermal performance than the 1" ZIP-R + 2" foam + 3.5" cellulose solution too:

At a 25% framing fraction (16" o.c.studs) the R6 ZIP-R wall + 2" ccSPF + 3.5" cellulose comes in between R21 after thermal bridging.

At a 20% framing fraction (24" o.c. studs) it comes in about R23.

The all cellulose double-studwall at a 25% framing fraction with 4.5" of cellulose and 1/2" OSB between the studwalls come in at about R31 whole-wall less the thermal briding hit of the subfloor & joists- call it R29-ish.

Your existing house with 2x6 framing with half-inch gypsum and half inch ply and 5.5" of cellulose fill comes in at about R14 if the framing is 16"o.c., or about R15 if 24" o.c. (If it had R19 batts it would only be R13.)

You'd see some improvement with your architect's essentially IRC 2012 code-min layup, but you'd be at 2x with the double-studwall.

In foggy-dew OR the exterior moisture drives are orders of magnitude higher than you get in UT, and the exterior drying rates are that much lower too. Interior moisture drives in winter are about the same or sligthly lower in UT, depending on your ventilation rates and how tight the house is. Don't mis-apply science of a marine climate zone to your dry continental climate.

Thanks for the inputs from the three of you. My plan is to do the double 2X4 walls with OSB on the external side of both walls air sealed. I like the wet cellulose and will insulate with only this. Is there any reason I can't air seal the OSB with OSI F-38 glue. I glued all my sheet rock to the studs with a few screws and no pop outs.

In seismic zones gluing the structural OSB onto the framing can make it too rigid, and more likely to walk off the foundation in a shaker. There are parts of CA where air-sealing with adhesives are prohibited by code- not sure if that's the case in your neighborhood. The "gold standard" is to air seal with acoustic sealant caulk, which remains flexible pretty much forever, and doesn't stiffen up the wall.

Thanks Dana 1 for the added info. A question, all I've ever seen used between the mud seal and the concrete foundation wall is blue foam which appears to me to be an air leak. Since I believe I should be caulking the 2X6 to the wall, isn't there something better to use between. Also when I put the ridged foam under the basement floor where does it go for the internal load bearing walls. Under the footings, on top or butt up against them

The gold standard is gaskets. Flexible forever and able to expand over a range that would leave any caulk MIA.

Between the sill and the concrete, and between the subfloor and outside stud walls, use an EDPM rubber gasket, available in 3-1/2" (BG63) or 5-1/2" (BG65) from ConservationTechnology.com http://conservationtechnology.com/building_gaskets.html. On the basement footings, you can use a load bearing EPS foam under the footing, or you can place the footing on solid ground and leave the 3-1/2" under the structural columns uninsulated. In the totality of the house, three or four 3-1/2" thermal bridges to 50o soil is usually doable without a huge bite to efficicency.

Your designer has is right.

If you over-insulate you spend useful source better invested on improving your cup for example.

In purchase of significance, and ROI, the roof, the surfaces and far returning in the package, the underground room floor--radiated or not.

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