Challenging Sitka Alaska new build options (ICF ? TSTUD?) Zone 7 ?
Sitka Alaska is a unique climate, most of the year the temperature doesn’t change more than 10 degrees overnight, and sits at 40 – 50. Summer can get up to low 60’s. So it is pretty much a mild winter environment most of the year. It does get down to freezing occasionally in Dec and Jan. Annual rainfall can be 80-120 inches per year.
To further complicate matters items only come via boat, so material costs come into play. Concrete 300 a yard. Lumber and materials about twice of normal.
So Vapor, Mosture, Mold prevention etc. Looking for where to start. I am toying with TSTUD, or ICF framing on a monolithic slab. Due to lack of pumps and such the roof is most likely to be normal rafter stick frame, the design has all wall to wall spans at 24ft.
I was thinking ICF would benefit for tightness and high winds, and give a decent R Value, relatively quick pour. Maybe add an additional layer of eps and a rain screed ? I am early on in the project and just have the rough design, so now would a great time to tweak.
I really really appreciate your experience, I have never dealt with a high humidity year round moderate to low temp environment. Ideas ?
6in ICF ? Tstud with RockWool insulation ? I will look when up there this week, but I doubt that spray foam is any kind of an option.
* High Humidity
* Low Temperatures
* Huge annual rainfall
* Expensive materials (double walls ? can cost ya)
* Coastal Exposure to Salty winds
* Climate map doesn’t show Alaska, I think it is zone 7?
Thanks a million
Steve
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Replies
If I'm in Sitka, with constant cool and dampness then I would totally opt for the REMOTE or PERSIST method. I would do a normal stud wall(2x4 or 2x6), cover with ice and water shield all over, and then add 4-6 inches of exterior EPS. I would then add 1x3 strapping and corrugated metal siding. I would include big overhangs. I would probably opt for an unvented "hot roof" but with thicker exterior foam.
If you are worried about wind, you can consider a closer nailing pattern for your sheathing. Add lots of Simpson connectors and beef up your overhangs.
I wouldn't bother with icfs or t studs.
Thanks a million, I will start researching REMOTE and PERSIST.
I completely agree on overhangs, what size do you recommend ?
I think I will pass on the hot roof due to the lack of spray foam availability and will probably opt for a conventional unconditioned attic.
I am thinking heat pump
Also consider putting your structural sheathing on the interior side (where it stays as warm as a PERSIST wall and warmer than a REMOTE wall), filling the cavity with fiberglass batts (lower cost R value) and then exterior foam (adds R value and keeps the studs above condensing temp).
You can't beat the cost vs R value of an unconditioned attic.
Jon,
If you are already putting the vast majority (or all) of the insulation outside the sheathing, does it really make sense to add the complication of moving the sheathing to the interior?
Jon, I totally agree on bringing the sheathing as close to the interior as possible. In that climate, if the sheathing gets damp then it will stay damp until it turns to mulch and is replaced. No warm, June sun is drying that wall out in a place like Sitka.
I recommended the hot roof with exterior foam because I worried that a vented attic might lead to moisture issues in that climate- especially if the builders can't get the ceiling air barrier just right.
Malcolm, you work in a cool, wet climate- do you guys see issues with vented attics?
While you want to avoid cold sheathing (on the exterior), it's a good question as to what it is worth to have completely warm sheathing (all insulation to the exterior) vs cool sheathing (some insulation on both sides). Interior side sheathing gives both - the cost savings of batts and warm sheathing. Net cost - no idea - Malcolm knows far better.
I agree 0n moving the sheathing in if possible to keep it warmer, as long as the wall assembly remains relatively simple. Interior sheathing just adds too many complications.
Moisture problems don't occur in unprotected exterior wall sheathing here if it includes moderately good air-sealing and a rain-screen. We do get some mold in well constructed and vented attics, but in the absence of other exacerbating factors it is a cosmetic concern, not one that causes damage.
If everything is coming in by boat, are you able to get a concrete truck into the site in order to pour a monolithic slab? That's a lot of concrete to pour. How are you planning on providing heat and DHW?
There is a concrete plant on the island, and I am right off a good road, so no issues there. The remote videos make me think that I am going to have a lot of cost in that method for that location and I can’t afford a mistake. The ICF still seems tempting.