Palmer AK's annual average is about 10,500 HDD, but it's spread over the entire year. Even the summer months run ~300 HDD.

Winter's aren't particularly severe when looking at weather averages- it's similar to Green Bay WI or Minneapolis, MN, and considerably warmer than Fargo, ND or Duluth, MN.

See:

http://www.idcide.com/weather/ak/palmer.htm

http://www.climate-zone.com/climate/united-states/alaska/anchorage/

With R12-16 of interior cellulose and R6-12 of closed cell foam in the cavity the sheathing will need to dry toward the exterior in winter through the 1.5" (R7.5) of XPS, but it can. The worst-case stackup would be R16 cellulose, R6 of closed cell foam, but the condensing hours at the foam/fiber interface will be few, and the vapor retardency of the interior foam will be nearly the same as the exterior foam (but air-tight.) With a rainscreened exterior you'll be golden, but even without you'll be OK.

awesome, thanks. definitely am seriously considering the rainscreening. since i plan on being in the house "forever" it will easily pay for itself and give me piece of mind. so r4/inch of cellulose is the figure to use? manufactures and installers have claimed 5 but i was wondering if this had been stretched some. either way this gives me right around the r30 value i was shooting for and still being able to stay with 2x6 construction. that is a relief, and is encouraging. any issue of using wet blown cellulose over closed cell foam? would consider just going with dry blown if i go with the closed cell but i just feel more confident that it won't settle over 30+ years with the wet stuff. maybe that is just me uninformed? i could essentially get to around the same r-value with just cellulose but i think the closed cell would be well worth the money for the additional air tightness.

You'd be better off ixnaying the spray foam, and using your exterior sheathing layer (plywood right.. ;) ) as your air barrier. Tape/caulk that sucker into submission and you'll have a nice tight cavity. You really don't want to be trapping sheathing between two vapour closed substances. You should be able to densepack your walls with cellulose with no sagging issues. Blow from the inside, saves you from blowing your drywall off the studs. Don't ask me how I know. ;)

Posted By pura vida on 09 Mar 2011 12:34 PM
awesome, thanks. definitely am seriously considering the rainscreening. since i plan on being in the house "forever" it will easily pay for itself and give me piece of mind. so r4/inch of cellulose is the figure to use? manufactures and installers have claimed 5 but i was wondering if this had been stretched some. either way this gives me right around the r30 value i was shooting for and still being able to stay with 2x6 construction. that is a relief, and is encouraging. any issue of using wet blown cellulose over closed cell foam? would consider just going with dry blown if i go with the closed cell but i just feel more confident that it won't settle over 30+ years with the wet stuff. maybe that is just me uninformed? i could essentially get to around the same r-value with just cellulose but i think the closed cell would be well worth the money for the additional air tightness.

R3.5-3.6would be closer for 2-2.5lb spray cellulose. In a 2x6 cavity with 1" of foam you have 4.5" left for cellulose.  4.5x3.5 gives R15.75 (call it R16).

Wet sprayed cellulose feels only slightly damp to the touch, but it's sticky enough to hang on just about anything- close-cell polyurethane included.  The perm-rating of the spray foam is high enough at 1" that the cellulose will still have some drying capacity toward the exterior (even through sheathing and exterior XPS), but it'll be mostly drying toward the interior. With 2" of closed cell + OSB + XPS the total outward drying capacity will be under 0.5 perms, but the cold edge of the cellulose stays much warmer and drier in the first place.

Dense packed dry blown (3.5lbs density) would still be a non-sagging option in that stackup in your climate, but for sure an inch of ccSPF makes the air sealing job easier.  You still need to caulk any doubled-up plates and headers, and it's better to caulk the OSB/ply as you go too. Don't count on sill-gaskets sealing (much)- a foam insulate & seal over foundation sills & rim joists stilll count.

With 1" of ccSPF, 1.5" of XPS and 1/2" of OSB and 4.5" of wet-sprayed cellulose you're looking at ~R29, center cavity, but the whole-wall-R (with the thermal bridging of the framing, studs & plates included) comes in around R23-24-ish, depending on actual framing factors.  That's about R3-4 better than case 2a. (See table 6, and figure 1. ) Your moisture profile is much improved with the 1" of ccSPF compared to case 2a. 

If instead, for the same wall thickness you went with 3.5" of XPS on the exterior and frame it  2x4" 16"on center w/cellulose fill no closed-cell flash in the cavity it would be more like R27-28 for a whole-wall R due to the much better thermal break over the framing.  It's more framing labor, but might be a wash on foam costs, depending on how many window & door cuts you'd have to fit for the exterior foam adding to installation labor, and how badly they'd hose you for a 1" flash of closed cell in your neighborhood.  IIRC EVERYTHING is more expensive in AK, but not always by the same multipliers.  Is the installed cost of another 2" of XPS the same or cheaper installed cost as 1" ccSPF?  It's close in my neighborhood, and it may be in yours... (or not.)  I can buy XPS sheathing for ~50cents/board-foot (f.o.b. the local yard) compared to ccSPF at ~$1.15/board-foot (installed price, no waste factor.)  For large flat areas with minimal labor to slap it up it's an easy call, but if there's a lot of cut'n'cobble to detail with possibly large waste factors it may not be so simple, and AK prices probably differ.

Thicker exterior foam and 2x4 construction also ends up being longer screws for the furring, which can be an issue if using fiber-cement siding or something heavy like that. (I'd think in AK locally milled wood siding would be a better deal, and half the weight or less. Painting the back side with primer prior to installation (or buying pre-primed goods) allows the finish paint to hold up quite well on rainscreen-backed wood siding, since there is lower moisture content in (and lower vapor drives from) the wood.

I am also at 10,500HDD in Colorado.

The joke here is that it can snow on the 4th July.

I would suggest you look at the payback of increasing your insulation levels and downsizing your heating system. It would not surprise me that spending more on the envelope has a pretty good payback.

I think you mentioned oil as your heat source, that would make it especially attractive.

At a high enough altitude it can snow any day of the year. I've skied some pretty nice fresh snow on Independence Day in WA in years past, but the snow/rain line was at ~7000'. I'm not sure how often it snows in July at sea level in Palmer, but I'd bet it's not super-rare.

With whole-wall Rs in the mid-20s or higher the total size of the glazed area can easily dominate the heat-loss numbers. Going with smaller/better windows and optimizing the south facing window area & type for passive solar gain can sometimes be more effective (and more cost-effective) than simply going higher-R. Modeling helps sort that stuff out.

The 97.5% design temp for Anchorage is -18F, which isn't really all that cold. (Compared to the -50F ish design temps for the interior it's downright tropical! :-) ) With R22-28 whole wall Rs it's possible to build reasonable sized houses with design-condition heat loads within the output of the smallest typical US style oil fired burners. (I could move my antique New England home as-is to Palmer and still be able to run with any of the smallest gas-fired mod-con boilers.) You'd have to take it to PassiveHouse levels to reap substantial savings on heating systems, rather than running the NPV calc on fuel cost savings alone. It may still prove cost-effective in high-priced AK with 10K+ HDD climate but that's a whole other project beyond what actually fits in a 2x6 framed wall.

I live in a ~6800HDD climate, and it's a pretty reasonable investment to build to PassiveHouse spec here if heating with $4/gal propane or $5/gal oil (but a tougher calculation at $1-1.50/therm natural gas.) This house runs ~R75-80 whole-wall values, and is about 5 crow-miles from mine:

http://www.telegram.com/article/20110227/NEWS/102270448

http://www.builderonline.com/blogs/postdetails.aspx?BlogId=tcushmanblog&postId=95103

http://www.thedailyshrewsbury.com/Articles-c-2010-07-13-69223.113122-This-New-House-television-crew-films-at-Beaton-house.html

as always i have to thank everyone for the information. actually snow one the ground anywhere in alaska except for the north slope june-august is unheard of. i've live here my entire life (33 years) and never seen it. (obviously i'm not talking about mountains or high elevations, just where people live) the whole state is just really close to sea level. there aren't any areas that are much over 1000 feet until you start getting into the mountains. this is why mckinley is so big, from base to top it is actually larger than everest. anyway, beside the point...

yeah i was a little high in my estimates for whole wall r value, my mistake. i will be pretty happy with with mid 20s. just have to figure out how exactly i'm going to get there. and you are right, -18 IS tropical when compared to -50. if someone tries to tell you there is not difference, it is just cold, then they have never been in -50. HUGE difference. anyway, drifting again... i'll check out the links when i get a chance the next couple of days. thanks

pv

what is the r value of 3.5lbs density cellulose? it goes up as you increase density correct?

if i do use 1 or 2 inches of spf does that add any support to 24" on center construction?

is there any long term durability/structural concerns with 24"oc 2x6 construction?

as far as pricing between xps and ccspf i just talked to an insulator today and he said a ruff estimate was $1/sq ft/inch of foam. i'll have an exact number next week. bigger jobs can be done for a little less and my house would fall in that category. i have approx. 3300 spft of wall space that needs to be insulated. so around $3300+/- for an inch of closed cell and just taking the prices from lowes for 1"x2'x8' blue board it comes out to about $3100. 2"x4'x8' is $5100+/-. although the rear, south facing side of the house is going to be a PITA to install the xps b/c of all the windows. definite potential for waste which could add to the cost. fyi, 1.5"x4'x8' came out to $4400. the main reason for keeping the xps to 1.5" is for the ease of installation of both the xps and windows/doors, from what i have read. i guess it would be nice to do the entire wall with ccspf but then we are looking in the range of $15k plus the xps. that is starting to get expensive. still waiting on the estimate for the framing, foundations, cellulose, etc. they seem to be taking a long time... in some cases it has been weeks. need to call and checkin...

The R value of 3.5lb cellulose is slightly lower than 2lb cellulose, but not enough to matter- it's less than an R1 difference in center-cavity R, affecting the whole-wall R even less. Figure on a K factor of about R3.5/inch, down from R3.6-R3.7/inch for low density wet-sprayed. If you stuffed an R19 batt into that 4.5" deep cavity you'll have a similar R value if it can be installed perfectly, no voids or compressions, but the inevitably less-perfect less-complete aspects of batt installations makes it worth sticking to sprayed or blown.

A 1" shot of ccSPF will glue the sheathing to the studs, but adds very little rigidity against racking forces. It takes ~3" before it becomes a significant structural element against wind or earthquake forces. The main advantage is ease of air-sealing. An inch of foam adds about R2-2.5 to the center-cavity R as compared to filling that volume with cellulose, but with the thermal bridging it's adding only ~ R1.5 the whole-wall number. (Adding 3/8" of thickness to the XPS would have a similar effect on whole-wall R.)

The structural ratings and durability of 2x6 24" o.c. construction is very similar to that of 2x4" 16" o.c. construction. If you're dense-packing cellulose rather than wet-spraying you'd be well advised to go thicker on the interior wallboard if you want the walls to not bow out a bit in 25 years. 5/8" would be min, 3/4" wouldn't be outrageous.

A buck a board-foot would be a decent price for closed cell foam in my neighborhood. I've paid as much as $1.25/board-food for smaller jobs (before the recession), but $1.10-1.15 is still the middle-of-the road. Sounds like you're paying significantly more for XPS sheathing than me though.

i'll probably go with wet sprayed just for the piece of mind, have never really considered r19 batt just b/c it is impossible to get that perfect seal. just need to decide how much ccspf is justified. will probably only do 1 or maybe 2 inches and the rest wet-sprayed. i'm guessing that will be the best bang for the buck. the earthquake thing is a bit of a concern mainly b/c i wonder if a significant quake or multiple quakes over the years will cause the spf to pull away from the studs/sheeting leading to air infiltration? is my understanding of moisture issues correct in that, if i went with 100% spf then mold/moisture would not be an issue or ever a concern? i'm guess the pricing differences between xps and spf is due to shipping. for the same cost you can ship more spf than xps. but i will do at least 1.5" of xps to break the thermal bridging. what about putting ccspf on the outside of the structure to break the thermal bridging? possible? any "easy" way to make it smooth so you can install the rain screen and sheeting? probably not since i don't think it is done, but just a thought. and dana, thanks again for all the time you spend responding to posts. i know how long it takes to type up a coherent response. probably be better if i just talked to you over the phone. save us both some time, but then no one else would be learning anything. enjoy.

pv