Howdy all, fairly new to the forum so i'll try to be brief...

come upon this forum when researching ICF construction so some may have already seen some of my questions on the ICF forum

background:

building a new house in alaska (southcentral, anchorage area) this summer. this will be our home indefinitely, 25-30+ years.  would like to see a wall value between r25-30.  single story with full walkout basement and a large attached garage (attached to main floor) main concern is we will be on heating oil and winter time heating cost could be excessive if not addressed before construction begins.  which led me to looking for alternative building techniques (icf, sips, etc).  was looking at doing an icf house but foundation to rafters blows the budget.  so, as of right now, this what i'm leaning towards,

 icf for the basement with some sort of stick framing for the main floor and garage.  i want something tighter/more efficient than just standard 2X6 construction.  a couple of options i've considered:

1) 2X6 with cellulose and 1-2 inches of foam board on the outside

2) 2X4 offset wall with cellulose.  also possibliy with some foam on the outside.  probably an 8" wall??

things i am concerned with for option #1
is it possible to install cellulose that will be "guaranteed" not to settle?  having an air gap after a few years at the top of the wall defeats the energy efficient efforts
being in a cold climate, moisture/dew point/mold issue inside the wall with the foam on the outside

concerns for option #2
finding a framer who is comfortable and competent with this technique
same cellulose and moisture issues as above

i believe either of these options could reach the r values i am shooting for?  i am also considering spray in poly but that gets expensive fast.  need to get an estimate so i know if it is even an option.  i have done some reading in a number of other post but i seem to still be a little unsure on the moisture issue.  specifically the steps/methods to deal with it when using foam.  i think there is a way to do it but i think my lack of specific construction knowledge is limiting my understanding.  can someone break this down for my in a simplistic step by step?   there are other questions/concerns i have but this post has become long enough as it is :)  and those questions will be address as the thread, hopefully, develops.  thanks as always

pv

maybe this will hep you to take a decision:

http://chandlerdesignbuild.blogspot.com/2009/12/increasing-attention-is-being-drawn-to.html

it helped me to take a decison regarding my own project. you can check my blog to see the way I want to implement it on a ICF foundation.

good luck
adi

There are two ways to eliminate settling with cellulose:  Dense-packing to a density appropriate for the annual hygric cycling you'd see in your climate (in Anchorage I'd take a WAG that it would need be ~ 3.7-3.8lbs/ft3.  A Danish guy named Rasmussen did the science on that one- it works.) The other is to use wet-spray (dense-packed or not), which relies upon adhesives in the mix to eliminate settling over time (which also seems to work, but there's only ~30-35 years of data on it so far. )

Putting foam on the exterior of a studwall reduces the number of condensing hours in a season.  If you put enough foam on the exterior that the mean January temperature at the structural sheathing  is above the dew point of the conditioned space air (typically 37-40F, if you keep the interior RH a healthy not-dry 30-35% RH, no higher, and temps between 68-70F) the mold risk drops to near zero using only standard latex for an interior vapor retarder.  If you put a 3/8"  rainscreen gap between the siding and exterior foam and use semi-permeable foam you can get away with an even lower R in foam.  In Anchorage the mean January temp is ~+15F, which isn't all that bad compared to the interior.   If you assume R20 center cavity on the cellulose and 70F indoors, 2" of XPS (R10) on the exterior would make the mean January temp at the sheathing ~33F- not quite going to be enough to get away with only latex on the interior, but with vapor-retardent latex &/or a rainscreen gap you'd be fine.  Going with 3" of unfaced EPS you'd squeak in there though (and it might be cheaper). With 2.5" of exterior iso (polyisocyanurate) you'd be there too.  For better air-barrier characteristics, use two layers with seams offset, taping and sealing the seams & edges of both layers.

If you use spray foam on the exterior rather than rigid, 2" of generic closed cell SPF would get you there, but 1" wouldn't.  If you went with a cheaper and more-permeable mid-density foam like the 2lb Icycene MD-R-200 you'd need ~2.5", but it would give you better drying capacity toward the exterior. 

A flash & fill technique with 1" of foam on the interior of the sheathing would also work with a rainscreen gap(no exterior foam, no interior vapor retarder), but would be dramatically lower-R with the thermal bridging unbroken.

Double studwall techniques also leave thermal shorts at the band joists & ceiling/floor joists, so you have to go higher-R to get the equivalent thermal performance of an exterior foam approach.  If you're talking 8" between the sheathing & gypsum that would be lower-R at center cavity than 2" of foam (any foam- even Type-I EPS), and higher thermal bridging.  Both an interior vapor barrier and exterior rainscreen gap would be worthwhile with this approach unless you go much thicker (in which case the hygric buffering of the cellulose may be enough to worry less about interior vapor retardency.)

adi - thanks for the links. some good info and things to consider

dana - great info. thanks. especially on the cellulose. i had heard of both of those options but didn't really think either would completely eliminate settling. definitely leaning back towards cellulose if this is the case. have a number of a local company i need to call and talk to them.

"Putting foam on the exterior of a studwall reduces the number of condensing hours in a season. If you put enough foam on the exterior..."

this is what i was originally thinking but when i first talked with people and was looking at the cold climate center out of fairbanks i understood that they were eliminating the moisture issue by going with more than 50% of their wall in (remote wall system), which ends up being 6" with a 2X4 wall. i didn't want to go with that much foam. expensive and issue with siding. but maybe there is a way around this with proper moisture/vapor protection? i would prefer to keep the foam thickness down to 1.5 inches due to ease of siding installation. as i understand it, if you keep it to 1.5" or less then you can still use a standard nail gun but over that you have to go to screws, which significantly increases installation cost. although it may pay for itself in a short period of time...? also, from what i have read most people do some sort of stuco on the outside of the thicker applications of foam? right now we don't really what a stuco look for the house. can you (or anyone) point me in a direction for alternatives to stuco when dealing with <2" of foam? between the two choices of walls i listed, right now i would prefer to go with the 2X6 with foam due to the local framers being familiar with the construction methods. where as with the double stud wall, not so much. and i'm concerned about finding a good contractor. i was leaning away from the 2X6 b/c of the moisture issue, but now i'm thinking with the correct techniques i can over come this? the other issue is i kinda got the horse before the cart in a way, b/c my house plans are already "finalized" and if i now go to an 8-12" wall (double stud, icf, sips) then i'm going to run into design issues, especially with the roof, i think. although if i put all of the extra thickness to the inside that wouldn't be an issue but then we would end up making fairly significant changes to the interior of the house. if i have to have it redesigned/redrawn then that will cost more money... always something.

was only thinking spray foam on the inside, not exterior. although i am not familiar with spf (spray polyurethane foam?) or 2lb Icycene MD-R-200? assuming these are just different options for stray foam? need to look those up.

can you explain flash and fill techniques? for spray foam on the inside i was mainly thinking this for sealing any/all leaks to the outside, added structural integrity, and higher r-value. then fill the remaining cavity with cellulose/bats and you have a nice wall but the cost goes up. need to get a local estimate for 1-2" of spray foam. would definitely like to break that thermal bridging as well. (hence the reason i was looking at exterior foam and/or double stud)

i do understand the double stud still has thermal shorts but much less than conventional 2X6. plus i could to 1-2" xps on the outside and 1-2" of spray foam on the in side with cellulose (or any number of combination) and have a fairly high r value wall, even at only 8" thick. also one advantage of double stud i was wondering about (if possible or makes sense), is that my vapor barrier could be towards the exterior on the interior wall, hence preventing all of the electrical/plumbing breaks you would find in conventional 2X6, which i believe would eliminate my moisture issue? then add cellulose/bat to that and sheet-rock. issues with this idea/thought process? again though, i have the wall thickness issues to deal with as described above.

thanks agian, and please, hit over the head if i am missing something. especially if it is basic or fundamental! also, could you please link me to where you are finding the average temps for my area and someplace that explains the whole temp/moisture/dew point/RH thing. i really feel as though i not full understanding the details of this. basics yes, details no.

pv

unfortunally the answers are getting very complicated when you start to throw in different types of foams on the exterior and on the interior. foams have different permeance rates at different thicknesses and they can do as much harm as they can help if they are not properly used. I think you should download WUFI software (free and easy enough to use) and do some simulations with your prefered stack up. that'll give you an ideea how a specific wall will work.

nature doesnt care about the ease of instalation or about the carpenters skills. if you don't use enough foam on the exterior to bring the sheating above the dew point you will still have moisture problems and at the same time you'll limit the drying capacity of the wall to the exterior. double walls have their own down sides and you can find a lot of disscusions on the subject on the internet - greenbuildingadvisor.com - is just one example.

whatever you do - make it at least work "on paper" first. if it looks good in a WUFI simulation there is a good chance will work ok in real life as well.

I really think the choice gets really easy once you get to price all that foam :-)

good luck
adrian

Regarding the thermal bridging remaining in a double stud wall, the extent of that really depends on the implementation details. In my own house, nearing completion (central NH, zone 6, 7500 HDD), the exterior walls are double stud. Floor joists and rim/band rest either on the inner of two sills on the foundation wall (uphill side) or on the inner framed wall (downhill side). The rim board is insulated on the outside as part of the wall cavity. The only thermal bridging is through the edge of the subfloor, which ties the floor system to the top plate of the outer wall where there are framed walls above and below the floor. The cavity in this case is 12", giving a nominal R40 wall with dense-packed cellulose.

Regarding the location of the air barrier, you have options there also. True, putting it on the outside of the inner wall frame gives you more freedom regarding plumbing and wiring (although many argue against plumbing in any exterior wall). You would have two cavities to fill with cellulose. The alternative is to put it at the drywall layer, using ADA (airtight drywall approach), gaskets, vapor retarder primer (<1 perm), etc., or, as I did, using MemBrain "smart vapor retarder." Either way,use polypans or special electrical boxes (eg. Airfoil) on the exterior walls and ceiling under the attic, so you can seal up against air leakage. Finally, if you do decide to go double frame walls, I would advocate higher than a 2x8 plate will give you. Besides the higher R value/lower heat loss up there in AK, the added space gives you the freedom regarding connections at the floor level and insulation outside of the rim.

No doubt double studwalls and careful framing designed to minimize thermal bridging is a great way to go high-R, but for an 8" wall underperform the foam/studwall approach.

FWIW, despite dramatically more annual heating degree days than DickRussel's NH location, the mean January temps in Anchorage are similar to (or even warmer than) some NH locations, and the vapor retardency and foam/fiber stackups that work are very similar. The climate in Fairbanks is a heluva lot colder than in Anchorage (-10F as opposed to +15F mean January temp), and much higher exterior foam would be required- building for Fairbanks weather in Anchorage would be an expensive waste. In Anchorage if you have ~35%+ of the total R on the exterior and cellulose in the walls the sheathing is inherently protected from interior vapor drives, and the vapor retardency of the exterior foam can be quite high.

If you're limiting it to 1.5" nothing is going to deliver sufficient R value, and you'd have to use something semi-permeable (2 layers of 3/4" XPS would give you R7.5). If you then did a flash'n'fill with 1" of 2lb closed cell SPF (spray polyurethane foam) on the interior of the cavity you'd then have ~R13.5 outside the condensing surface (the interior of the SPF), and ~R16 inside for a center cavity R of ~ R29-30, and ~45% of the R exterior to the condensing surface. The sheathing could still dry in both directions (seasonally), and you could use standard latex on the interior. Done that way you neither need nor WANT a vapor barrier anywhere in the assembly, since that would impede the drying of the sheathing.

Spray foam on the interior of a studwall does NOT provide a thermal break on the studs- they're still going to be a ~R5-6 thermal bridge between the gypsum. A 2x6 24" o.c. studwall wall with 1.5" of exterior foam, 1" of spray foam on the interior of the half-inch sheathing and half-inch gypsum and 1" allowance for rainscreen + siding gives you 9" from the interior paint to exterior paint, and a whole-wall R (thermal bridging included) of about R26. Building a staggered stud double-studwall on the same 2x6 plates with 2x4" and the same foam features would only add ~R3 to the stackup for a lot of additional complexity. If you're doing double-studwall, go thicker (a LOT thicker- like double) to make the additional complexity worthwhile.

Alternatively, inserting the 1.5" of rigid foam on the interior of the outer studwall and air-sealing it (skipping the SPF) would put the higher vapor retardency layer & condensing surface in a perfect spot, and you could skip interior vapor retarders. The exterior studs and sheathing would dry toward the exterior, the interior toward the interior. With 2x4 exterior, and 2x3 interior studs (separate stud plates), and the same siding & rainscreen/sheathing/gypsum you're looking at 10.5" paint-to paint, and a higher whole-wall R.

Stick built walls anywhere as rainy and cool as Anchorage will benefit GREATLY with the addition of at least 3/8" of rainscreen back-ventilation of the siding. (It's required by code in Canada, and should be for AK, and for anywhere in zones 6 or higher in the lower 48 IMHO.)

The 2lb Icynene is a somewhat different option than generic SPF. It has a lower K value (R5/inch as opposed to R6/inch for SPF), but a much higher perm rating at any given thickness, which would allow you to put more R-value on the exterior before the drying in that direction is too impeded. At 3" Icycnene MD-R-200 has about twice the vapor permeability of just 2" of most vendors' SPF, and a higher total R. The "right" foam in terms of both permeance and R value can depend on where it is in the stackup, and the local climate variables. Both SPF and 2lb Icycnene are useful for adjusting the permeance for optimizing drying capacity and limiting winter absorption of moisture into the structural wood.

adi - thanks for the wufi link, i'll do that.

dick - thanks, gives me more to research.

dana - wow, thanks! lots of great info as always. i know you posted in my icf thread, thanks for doing it again for me here. lots of different options, that is actually encouraging even though it will take me a while to sort through, organize, rate the different options. i do agree that if i'm going to go double stud then i will increase the wall thickness. probably at least to 12". i think i'll start with the 2X6 options first and go from there. fwiw, the house will actually be build in a town called palmer, which is 45 miles north of anchorage. but very similar temp/precip as anchorage.

"Spray foam on the interior of a studwall does NOT provide a thermal break... "

yup, one of the few things i do have figured out.

thanks again. i'll be back with more once i do a little more research.

pv

The January mean temp for Palmer is a whole 0.1F warmer than Anchorage- better account for that in your hygric analysis calc. :-)

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

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

(Scroll down mid-page for the climate norms.)

Call 'em both +14F- it'll be wrong every year, sometimes higher, sometimes lower...

For a pretty good analysis of alternate approaches to high-R wall assemblies, take a look at

http://www.buildingscience.com/documents/reports/rr-0903-building-america-special-research-project-high-r-walls

In Palmer your winters have slightly warmer averages than the Minneapolis climate on which the hygric modeling was done, so you can use a hint less foam and still keep the assembly dry, but stackups that work for them surely works for you. If you average the DEC-FEB means you're almost identical- within a degree for a whole-winter average.

http://www.idcide.com/weather/mn/minneapolis.htm

awesome info, thanks for the links.

i'll spend some time reading and then i'll be back...

ahhh, yes, that study. actually i have already read this report a couple of times when i first started looking at alternative walls for my house. but i came across this when i was mainly looking at icf. hence my first post in the icf forum, where i linked this same study . fast forward a couple of months and i re-read the article again and learned even more. especially since i have a better understanding of a few of the options they looked at and am now just as interested/concerned with moisture control as i was/am in R value. so, considering my current situation (house plans already "finalized"), as dana stated above, i'm leaning towards staying with 2X6 walls with a combination of spf, wet blown cellulose, and exterior xps. the still unknown part is balancing R-value, spf cost, xps cost, moisture control, and over all building difficulty. taking the numbers from the study, if i take the flash and flil (see i am learning, i understand what that is nowl ), which is case 9 and add 1.5 inches of xps i'm right about an r-25. (17.5 from the study and ~5 per inch for the xps). actually it would be even slightly higher due to addressing the vast majority of the thermal bridging. so going to 2 inches of foam puts me in the high 20s and 3 inches put me over 30. all of those options are acceptable. but it does get expensive to add all of that foam and the spf is not cheap either. although i really like the added strength the spf adds and makes me feel much better about going 24" on center with the studs. now i need to run the different combinations through the WUFI software and see which ones solve my moisture issues. still lots to learn and consider but at least i feel like i'm making progress...

i haven't mentioned it before, but i am in a high wind area. considerations to my thought process? (actually i like the idea of the spf to help lessen the wind effect)

if i am reading it correctly, using xps on the exterior eliminates the need for OSB? that is definitely a cost consideration for use of the xps

if so, to attach the siding do you just use long enough screws to reach the studs?  or am i missing something again?

any more details on the 3/8" rain screen? another link perhaps? i tend to be rather visual. i'm assuming this will help with moister and do what it is named and allow rain/moisture to drain before it actually gets to the wall itself?

pv

never mind, found some rain screen info. even a video or two. pretty much what i was guessing. so now i know what it is, are there any specific recommendations for residential? methods? products?

Most XPS sheathing is NOT structural- if to skip the OSB you have to cut in some diagonal bracing or the place will fall down. (Dow SIS sheathing is structural, but many not be strong enough to meet spec in high wind areas without additional bracing. See: http://building.dow.com/na/en/sis/ and http://www.dowsiscalc.com/ )

To mount siding with a rainscreen over XPS sheathing, glue the foam to the structural sheathing with specially formulated foam-board adhesive (that won't degrade the XPS), then long-screw the furring for the rainscreen gap to the studs, and mount the siding to the furring. It'll be far fewer fasteners, less work and less thermal bridging than long-screwing the siding to the studs. Use housewrap-tape to seal the seams on each layer of XPS, and spray-foam seal the edges.

You still need a weather-resistant layer (house wrap or felt) over the XPS, unless you use WRB-rated XPS and methods, which involves using housewrap tape in a Z-flashing method on the exterior layer, eg:

http://www.ujr.ca/EN/PDF_Support_Files/insulation/greenguard/xps/greenguard-fanfold-siding-underlayment-wrb-installation-guide.pdf

Key to making rainscreens work well is to make both the top and bottom of the gap vented to the exterior with a reasonable cross section so that air will convect in the thin cavity. It's important to put some screening on the vent openings to critter & bug-proof the ventilation cavity to keep things from nesting in there, which would impede the air flow. Using wooden siding you can back-primer the siding to minimize warping & checking. Paint durability is much-enhanced on rainscreened wooden siding since the moisture content of the siding stays lower, with less vapor-pressure and solvent action driving blistering/peeling/cracking. (Wood is a lot lighter than fiber cement too, a consideration when hanging the siding on the moment-arm of much longer fasteners.)

ok, that makes more sense now. wasn't seeing how i wouldn't need sheeting. i was just reading/seeing wrong.

again, much thanks for the link and the info.

I'm not sure if you seen this material, but the CCHRC has been putting out some great research in your region.

http://www.cchrc.org/remote-walls

You should check out the Sunrise Home by Thorsten Chlupp as well... I believe he's calling it his ARTIC system, using cellulose & double stud walls, versus the foam exterior that he first used in the REMOTE / PERSIST wall methods.

Not sure if linking GBA is cool or not, but this link has lots of good details. http://www.greenbuildingadvisor.com/community/forum/general-questions/19942/sunrise-home There was a post near the end describing how to use joist hangers to maintain the interior air barrier plane.

cool, thanks for the links.

went to the local home show with the wife this weekend. i really hate it when you ask someone a question (an insulation dude in this case) and he just tries to BS you. i knew enough to know he was full of sh!t but not enough to tell him so. he was clearly trying to just sell me on a more expensive system. so,

spray foam insulation does off gas correct? for how long?

does this cause the r value to drop over time? where does it end up?

as long as you allow it to dry, wet blown cellulose doesn't have any issues due the water they use during the installation does it?

are there better brands of exterior foam? ones i want to avoid and/or seek out?
ie. the blue board you find an lowes/home depo? good? crap? six of one/half dozen of the other?

Different foams use different blown agents, with different outgasing profiles. Most of the outgassing happens in the first 24 hours for most products, but some close-cell stuff takes longer. How long sometimes depends on the applied thickness. Most of the outgassing of stuff on the market today is pretty benign- it's NOTHING REMOTELY LIKE the urea foams of the 1970s- differenct chemistry, different blowing agents, etc.

The R-values specified at various thicknesses in their literature is required by FTC regulation to be the aged-value, not the day-1 (or minute 1) value. 2lb polyurethane ends up around R6 per inch of thickness, as does 2lb polyisocyanurate, but they have dramatically different vapor retardency. 2lb Icynene(tm) runs about R5/inch. Open cell foams end up in the R3.2-R4/inch range depending on product & density.

Drying time on wet-spray cellulose varies with thickness, temperature, and dewpoint of the surrounding air etc, but yes, as long as it's dried to less than ~25% humidity before you close it up, there no ongoing issues with the installation moisture. Wet-spraying 12"+ thick cavites can take quite awhile to dry though, which is just one reason why double-studwall superinsulation projects are usually dry-blown. Wet-spraying a 2x6 cavity in coastal AK should probably be done in May or June, give it at least a couple of days of drying time before putting XPS on the exterior and gypsum on the interior, and if you can hold off painting the interior for a month, the high perm rating of wall board allows it to dry even further. The only people who run into issues with it are usually skimping on drying time &/or putting up an interior poly vapor barrier (in contravention of the manufacturers recommendations or best practices) with minimal drying time. If your stack up is adequate for using a class-II or class-III vapor retarder (standard latex), you can put up the wall board after a couple of days of warmer weather and wait. (Heat the place up with space heaters to speed the process if it's 40F and pouring rain outside.)

XPS comes in a number of densities, all are suitable for insulating sheathing. Blue board is made by Dow, a company with a good reputation for consistent quality, as is Owens Corning, the manufacturer of pink board. Those two probably own 80% of the market, both are good. The XPS from 30 years ago suffered from dimensional shrinkage over time, but the big manufacturers claim to have solved those issues (but let's wait another 50 years, just to be sure, eh? ;-) )

thanks as always dana, your info has been invaluable to me. everything you said pretty much follows what i was "thinking" but you should have heard what one insulator was trying to tell me when i started asking questions. i wanted to call him out but wasn't confident in my knowledge enough. guess who i won't be going with... although i did meet a number of people and am going to get multiple estimates. should be enough to give me a good idea of how much it is going to cost to get the building enclosed. that will determine if our eyes were/are bigger then our wallets. now i just need to figure out my stack up... i downloaded the WUFI software but haven't had a chance to play with it yet. i'm really interested to see what it tells me about 1.5 inches of xps, 1-2inches of close cell stray, with the rest of the 2X6 wall filled with cellulose for my area. hopefully i'll be able to figure it out, and interpret what it tells me. i guess that will be where this forum comes into play. again... thanks

pv

What is the HDD for where you are?