New energycode requires R-5 on the outside
The new Washington State Energy Code (WSEC) is going into effect and our architectural firm is in the process of updating our details. We design a fair amount of resort homes on the East slope of the Cascade Mountains, 70 miles east of Seattle, in climate zone 2. This area is not as wet as Seattle, but can get several of feet of snow, with below freezing temps, while the summers are in the 70’s and 80’s. One of the WSEC requirements is to apply R5 foam between the wall sheating and the siding, to act as a thermal break.
We are wondering about the order of assembly. Logic tells me the following (from inside out): gwb, 2×6 studs with R21 batts, 1/2″ structural sheating, 1″ foam, 60 minute paper, siding. The siding is usually cedar board&batt, horizontal bevel, or shingle ( it seems odd to be installing the latter thru the foam though). At the windows and doors we could show a 5/4 nailer for the nailing flange; at the bottom of the wall also a 5/4 strip, to cover the edge of the foam.
One builder suggested adding an airspace between the paper and the siding and installing the siding over 3/8″ or 1/2″ strips, as a rain screen application. Add a CorVent type product top and bottom between the strips (assuming they run vertical; not sure what we’d do with vertical or shingle siding; we need to keep the vertical drainage channels, so add another layer of strips going the other way, or sheating?).
Part of my concern is how much the siding starts hanging away from the foundation wall with all these layers. It gets trickier if the clients wants thinset stone in places (here we’d be adding another layer of sheating, metal lath, etc). In that case I’d be inclined to create a foundation ledge by placing the mudsill to the inside of the stemwall.
I saw similar subjects discussed in this forum, but not this particular one. So ideas and suggestions are welcome.
Ben
Replies
About 15 years ago, a colleague of mine in Spokane would delete the OSB sheathing and replace it w/ a diagonal brace let into the studs. He replaced it with 1/2 or 1" polyiso. He said that finding a stud was relatively easy ... just poke a hole through the insulation to see if the stud was there. Now literally, this isn't practical/good, but the point is well taken and it would be easy to mark your studs for siding applications. This makes a wall not much thicker than normal ... but w/ R-5 or 1", you may still have some issues here and there. Diagonal bracing still meets structural codes and is inexpensive; and not that difficult labor wise.
http://www.greenbuildingadvisor.com/book/export/html/17157
http://www.buildingscience.com/documents/guides-and-manuals/irc-faqs/irc-faq-cladding-attachment-over-insulating-sheathing/?searchterm=foam
Below is Table 5-1 of the WSEC
The table shows walls (climate zone 2) required to have a U-value of .056, which converts to about R-18.
So, I'm wondering if R-5 on the outside is really needed, or is there some other part of the code I'm missing?
TARGET COMPONENT VALUES FOR SINGLE-FAMILY RESIDENTIAL
Climate Zone
Component
1
2
Glazing % Floor Area
15%
15%
Vertical Glazing U-Factor
U = 0.30
U = 0.30
Overhead Glazing U-Factor
U = 0.50
U = 0.50
Doors
U = 0.200
U = 0.200
Ceilings
U = 0.027
U = 0.027
Walls
U = 0.056
U = 0.056
Floors
U = 0.029
U = 0.029
Slab on Grade
F = 0.36
F = 0.36
Below Grade
Wall R-Value
R-21
R-21
2' Depth: Walls
Slab
U = 0.042
F = 0.59
U = 0.042
F = 0.59
3.5' Depth: Walls
Slab
U = 0.041
F = 0.64
U = 0.041
F = 0.64
7' Depth: Walls
Slab
U = 0.037
F = 0.57
U = 0.037
F = 0.57
Are you looking at
the 2009 code? There are indeed some scenarios in which R21+R5 would be required for exterior walls in zone 2. Whether you need to accomplish that with exterior foam or some other means... not sure. This takes effect 1/1/2011.
I'm pretty sure
it is the '09 code, listed to become effective beginning Oct. 29, 2010.
http://apps.leg.wa.gov/wac/default.aspx?cite=51-11-0502
Also, the WSEC specifically allows trade-offs in any of envelope elements so long as the net u-value for the sum of the components ends up being equal to the prescriptive path.
EDIT: when you open the link, be sure to scroll down to the section that becomes effective Oct. 29, 2010--the top section was effective only up till Oct. 29.
I attended a training
yesterday put on by WSU Extension Energy program, and they covered the various paths to approval. Just grabbed the notes, and I was mistaken about what I wrote above. In Zone 2, there are three prescriptive paths based on percentage of glazing. At glazing = 12% or less, your wall requirement is R-21 intermediate framing, which basically means 16" O.C. with insulated headers @ R-10. Above that and your wall requirement is R-19 + R-5, although I'm not sure if that means that continuous exterior foam is required, or if you could satisfy the requirement with R24 or better in the stud cavity. I tend to think not but since I'm in Zone 1 I didn't ask. I can email you their PPT slides if you want, they are pretty good, and very good when combined with the Gary & Luke lecture.
It appears that Washington has modified the the IECC to allow a little more flexibility in prescriptive paths, so it's a good idea for designers to get familiar with various options available to them. Do you know if the WA code works with DOE's REScheck program?
Dunno about that
but the folks at WSU will. (360) 956-2042
pulled a permit today
RI is 2009 now too... my BI told me i can use res check for conformance
ran some preliminary numbers.....looks good
R5 over the framing
I have attended the same WSU program, earlier this year and we talked about the climate zone 2 wall insulation requirements. The exterior insulation is specifically meant as a thermal break, rather than beefing up the R value between the studs. I thought about putting the foam between the studs and the sheating, but that poses challenges in getting the lateral design to work.
Thinset stone may be applied over paper and metal lath attached directly to the foam board. At least accordind to the El darado seminar I attended a few years ago. just use long enough nails to pentrate through the foam and sheathing to the studs.