I am building a timber frame in zone 4. I would appreciate some input on the design.


My foundation is in and is built from the outside in as follows:

2" xps + Rubberwall + 12" poured concrete walls + capillary break (Deco) between the footer and wall + 2" foam on floor

My thoughts were:

1.) XPS for insulation. I considered Closed Cell Spray Foam (CCSF) but was not comfortable with the responses I received  from the distributors for a foundation application. So I went with the tried and true method.
2.) Rubberwall great waterproofer
3.) 12" walls = strength
4.) Deco avoid wicking
5.) 2" foam under floor for insulation

I am considering my wall build up from the outside as follows:

Stone + 1" air space + 2" closed cell spray foam (CCSF) + 3/4" plywood + 2x4 studs 24"OC. + ?? high perm insulation +5/8" drywall + latex paint

My thoughts are:

1.) 1" air space for the drainage plane
2.) 2" CCSF will be my air barrier (AB) & water resistive barrier (WRB) and a R-12 which will alleviate thermal bridging and will also eliminate virtually all of the gasketing, caulking, air sealing, taping issues. This will tie into the XPS in the     foundation.
3.) 3/4" plywood sheathing = superior structural integrity versus 1/2"
4.) 2x4 24" OC  advanced framing details
5.) ??? a high perm insulation to allow drying to the inside due to the CCSF
6.) 5/8" drywall due to 24" OC spacing
7.) Latex paint for interior drying capabilities

My roof build up will be from the outside in as follows:

roofing (TBD) + synthetic underlayment (SU)+ 3/4" plywood + 1" air space + 5" CCSF + 2x6 T&G

My thoughts are:

1.) Roofing is to be determined
2.) SU 
3.) 3/4"plywood sheathing = superior structural integrity versus 1/2"
4.) 1" air space I believe in cold roofs
5.) 5" CCSF  gives me a true ~R30+.   Eliminates issues as stated in the wall system. It is virtually air tight. This will tie in to my wall CCSF.
6.) 2x8 T&G is the interior finish ceiling


Next steps:

1.) Doors and windows. I will  be looking hard at the European style windows.
2.) HVAC I am looking at a water to water pond loop geothermal system vs mini-splits. I am a huge radiant fan! This is a   tuff decision especially if you throw in some PV and solar possibilities.

Hopefully the house will be like an igloo with a monolithic foam exterior. The project is happening as we speak. Any input or critique would be appreciated. Be gentle, I get my feelings hurt easily!


Thanks,
Ron

A couple of comments regarding insulation choices:

XPS is blown with a mixture of HFCs, the predominant component of which is HFC134a, which has a global warming potential (GWP) of about 1400x CO2.  The same polymer as EPS is blown with pentane, at about 7x CO2 GWP, and has a better closed cell structure.  The higher R/inch of XPS is due to the blowing agent, but as that agent leaks out over time it's thermal performance falls to that of EPS of the same density.  The decay in thermal performance is logarithmic, and the R5/inch labeled performance is something like the average performance over the first 20 years.  After 50 years it's pretty much the same as EPS. In Europe XPS is blown with CO2 (GWP= 1x CO2) and at 1.5lbs density has an R-value of R4.2/inch, just like 1.5lb density EPS, and like EPS, has a stable R-value over decades.

Almost all closed cell foam is blown with HFC245fa at about 1000x CO2 GWP.  Like XPS the blowing agent leaks out over time (though I don't know the time constants).  It's likely that the fully-depleted R-value of 2lb polyurethane is about R5/inch, which is about the performance of 2lb density WATER blown polyurethane (eg Icynene MD-R-200 or MD-R-210).

Where ever it's possible to design out XPS or HFC-blown closed cell polyurethane, it's going to be considerably greener. Note that the water-blown Icycene is higher permeance than most HFC-blown 2lb foams, and EPS at any given density is 2.5-3x more permeable than XPS, but in your climate and stackups this is not a problem- if anything it is a benefit.

At only R10, the foundation stackup just barely meets IRC 2012 code-minimums for climate zone 4, and with foam on the exterior of the assembly there is a discontinuity with the slab foam forming a thermal bridge to the cooler-than-optimal subsoil.  A bottom-of-the-line 2" + 2" ICF would be better (R16, with an R8 of EPS between the slab and wall.)  Alternatively, 3" of interior side EPS, or 1" of EPS + a non-structural studwall insulated with rock wool will get you do reasonable performance.

On the roofing stackup, 1" of vent space isn't usually enough (though it meets code minimums), but if you're applying a 2lb density foam to the underside, that is more protective of the roof deck than a mere 1" of vent would be.  Faith-based cold roofs need at least 2" of gap to be as protective as 2" of closed cell foam applied to the underside of the roof, unless you've got a real lightning-splitter of a roof line (say a 16:12 pitch.)

HFC blown goods at R6.5/inch DOES NOT meet IRC 2012 code minimums for a zone 4 climate- you need R49, and at only 5" you're at less than R35 (best case, using R7/inch polyurethane.)   At 3" any 2lb foam will also be adding significant structure.  To hit code min more greenly & cheaply, you can get there with a minimum of R15 of rigid foam above the roof deck (say, 2" of polyiso with 1" of EPS above that, seams staggered for ~R17, with 8-9" of open cell foam or cellulose directly under the roof deck works.  Open cell foam would provide at LEAST as good (usually better) air seal as the proposed 5" of ccSPF.  Furthermore, rigid foam above the roof deck A: keeps the roof deck average winter temp higher (=warmer), and provides a substantial thermal break on the rafters (that you don't have in your proposed design.)  At only 5" of cavity-fill the thermal bridging of the rafters SEVERELY undercuts the performance of high-R/inch foams, since the framing fraction is only good for about R6.  At a typical 12% roof framing fraction the "whole-assembly-R" of about R22, and that's including a generous R2 for the thermal performance of the 2x t&g interior. You'd get better performance than that out of 2" polyiso + 2" EPS on the exterior, and leaving the rafter bays empty(!).

At about 17-18 cents per R per square foot, closed cell foam is pretty pricey stuff.  Rigid EPS & polyiso run about 10 cents/R-ft², open cell foam & XPS about 13 cents/R-ft²- there's definitely better bang/buck using other methods, independently of the environmental issues.

Dana, thanks for your input. I will add a few things to clarify some of the choices. We are building with 2009 code not 2012. This is a timber frame home so there are no interior framing bays to insulate. The 2x6 T&G is the final finish. Everything is on top of the T&G. Below is the drawing of the proposed roof framing. This is basically a modified version of the wrap and strap method of insulating a timber frame. The roof is extremely cut up, and would be very difficult to implement the wrap and strap method and obtain a good air tight roof.




A few things have changed in the design. I am now using 3/4" plywood and 2x8 T&G.  The 2x6's are only used for attaching the plywood. They have no other use. I have considered numerous other methods of attaching the plywood but I think the 2x6 is the easiest. The spacer under the 2x6 is for a thermal break. This will allow the foam to flow under the 2x6 when the cavity is filled.  I could use 2x8's in place of the 2x6's. Once I have the 2x6's in place for the plywood the spray foam will seal and fill everything with a continuous blanket of foam. With a 7% framing factor (no plates or headers) I calculate the total R for the assembly of ~ R-31. A 2" spacer under the 2x6 would put me at ~ R32.The depth of foam can be varied to achieve what ever R-value I want. I believe that more value is achieved in the initial inches of foam then continually adding foam. Law of diminishing returns. The air seal achieved with spray foam is critical. No caulking, sealing or tape to worry about.

We will be using icynene MD-C-200 , R-6.75 per inch and .9perms at 1.5". I want a lower permeance to deal with the moisture drive from the stone facade. This is why I will be using a high perm interior insulation that will allow for interior drying.

I hope this clarifies some of my choices. Any additional thoughts are appreciated and welcome.

Posted By RGates on 25 Jul 2014 11:11 PM

Sorry, duplicative post.