was thinking about 2by8 construction with 2 inch spray foam in cavity filled remainder with 2by six insulation, outside having 2 inch pink styrofoam to prevent thermal bridging. All that said could also consider icf at what they say r27 on premium line. What would yo do or something better. Thanks alot

With the 2" SPF + 2" of exterior XPS you're already at ~R22 clear-wall, and adding batts or cellulose would get you to R40+, which is clearly better than an R27 SIP if you build it air-tight.

But with your stackup you could potnentially run into moisture-trap issues unless you can get a variance from the code requirement for a poly vapor barrier or use more permeable foam. The combined vapor retardency of 2" of XPS + 2" of SPF will be under 0.5 (maybe WAY under, depending on vendors), and any moisture that found it's way into the cavity by whatever path might take years to dry. Depending on where you are in Ontario you may be able to skip the poly and use vapor-retardent paint (or even standard latex) on the interior instead with our ~50/50 ratio of foam/fiber. If there is a weather station listed near you we might be able to assess more closely if skipping the poly could work: http://climate.weatheroffice.gc.ca/climate_normals/stnselect_e.html

If the poly IS required, going with the mid-density Icynene rather than a standard polyurethane would give you more than twice the permeance on the spray foam (at roughly R2 less R value than SPF), and going with unfaced Type-II EPS (bead board) would similarly double ther permeance of the exterior foam (at the cost of another R2), which would give the assembly adequate drying capacity. It would be R4 less than your stackup however, but the R4 would recovered if you dense-packed (to 1.8lbs +) Johns Manville Spider or Certainteed Optima ultra-fine fiberglass in the cavity rather than batts or spray-cellulose.

In general going with blown or sprayed fiber is easier to get the rated performance out of it than going with batts. If you use wet-sprayed cellulose and no interior poly, the cellulose can safely buffer signficant wintertime moisture diffusion into the cavity, protecting the wood, and may be the preferred material if you used pink-board and polyurethane foam without interior poly. Wet sprayed cellulose won't sag or settle over time the way low-density dry-blown can, due to the water-activated adhesives in the mix. JM Spider has similar adhesives, but no moisture buffering capacity.

You may can also get there (possibly more cheaply) using 2x6 framing and 4" of rigid EPS (2 layers of 2") on the exterior, and 2" of SPF + fiber in the cavities. The exterior foam is ~ R16 (higher at colder temps) to which you'd add ~R12 for the SPF, and R13 for the fiber for similar total R value. With more R on the exterior the sheathing stays warmer, and with ~2/3 of the total R value in foam the need for interior poly would disappear in all but the coldest parts of Ontario. (If the interior face of the foam never stagnates below ~+2C the risk of mold or rot from condensation is near zero. Do you ever get weeks where the average outdoor temp is -40C?) For a thinner, slightly higher-R wall you could also use 3" of polyiso rigid board instead of 4" of EPS. With either of these stackups you may use rigid foam with protective facers (aluminum, vinyl, poly, asphalt-fiber, etc) without risk as long as you skip the interior vapor retarders. Even the sheathing can dry adequately toward the interior through 2" of SPF, as long as you don't block the path with poly. And with R16 on the exterior the sheathing is warmer, than with the R10 XPS option, accumulating less winter moisture in the first place.

i appreciate your vast knowledge and in depth response. Ive heard the local icf contractors have created some horror stories there fore I am probably going the last option you gave. I get alot of my building materials at cost while i dont get condrete or blocks at cost. Thanks again for all your help

If you don't mind me asking, in what region of ontatio are these horror stories located?

Yes really a good forum.Here I find many necessary information.thanks

I'm in a corner of Ontario where it does get to -30c to -40c for weeks on end and really appreciated the info above as it will inform my building decisions.
What I don't know but hope to hear is DOES THE ONTARIO BUILDING CODE allow me to omit the poly (interior) if I build the 2x6 wall with 2 x 2" foam on the exterior and blown wet cellulose?
Anyone know for sure!

and so I don't seem too dumb....can you confirm the math on the R value of that wall.. 2 x R10 + R20 for R40?

and...

what is SPF?

edit; I searched the Ontario Building Code website and found  numerous references to builders being required to have one regardless of the wall construction type.... that would be a deal breaker I assume on this wall type in my climate?

Also if I can add..... with 4" of foam outside the sheathing; how does one attach siding (wife wants Hardie Board)

SPF : Spruce, Pine and Fir. It was created to market the different softwoods generally used for dimensional lumber in our canadian markets.

Check out the following videos from youtube, they explain quite well the 4'' outsulation and required drainage plane required :
http://www.youtube.com/watch?v=UtoxcKVsXfE

http://www.youtube.com/watch?v=12DvyHjqu4Y&feature=relmfu

http://www.youtube.com/watch?v=dSqpKyiWbsA&feature=relmfu

Try and meet your local building inspector or inspectors and guage there thoughts on insulation and different stackups.

I to was looking into Hardie Board, however it is very uncommon in my area and also quite expensive. The local lumber does not seem to willing to sell it to me. However he encourages NatureTechKWP, and I also think that LPsmartside seems like a viable option and a good looking product.
http://www.youtube.com/watch?v=YJ-YM1P53i0 or http://www.lpcorp.com/smartside/

Good luck on the build.

Thanks for the reply...

I feel dumb since I knew what SPF was but got messed up here....

-You may can also get there (possibly more cheaply) using 2x6 framing and 4" of rigid EPS (2 layers of 2") on the exterior, and 2" of SPF + fiber in the cavities.-

I mistakenly thought it was another product like insulation. Still not sure about 2" of SPF though; isn't there just 7/16" OSB (or similar) on the exterior of the 2x6 framed wall followed by 4" of outsulation.

and Hardie board is what the boss wants so I gotta at least try and make that work :)

Pardon me, I thought that the SPF question was for the studs, however in the insulation context it refers to SPRAY FOAM insulation. Sorry about that I replied to quickly.

In most cases, it is possible to omit the OSB or Ply or ZIP sheating, for sheer walls it is possible to use let in braces or even some metals braces (large T to structure them together). However I would prbably add OSB just to make sure.

I agree, those bosses are quite happy when we can make things work for them. In the end it all works out.

Thanks for the clarification.

I am not up to speed on much of the lingo but am learning fast.

2" of spray foam on the interior of a wall without exterior sheeting (with 2 layers of 2" foam) must then (I assume) be for air sealing.

I have already spoken to some local spray foam contractors and learned that the $$$/R is pretty high. If the spray foam is primarily for air sealing would I be better off and $$$ ahead to leave it out (the spray foam) and substitute the OSB which I would take considerable pains to air seal with construction adhesive.

I wonder also if anyone can comment about how one is to fix siding (Hardie board or even vinyl) to 4" of exterior foam.
Forgive the continuing questions, I really want to get this house right.

I believe you will need furring strips attached through the foam to support any type of siding besides stucco.  The furring strips will also provide for a rain screen or drainage plane behind the siding.  Since most people prefer to use nails instead screws to attach the siding, you might want to consider using treated wood furring strips.  Now the question is, should the furring strips be 3/4" thick or 1.5".  Maybe Hardi-board literature can answer this.

Posted By bp f150 on 11 Feb 2011 03:08 PM
Pardon me, I thought that the SPF question was for the studs, however in the insulation context it refers to SPRAY FOAM insulation. Sorry about that I replied to quickly.

In most cases, it is possible to omit the OSB or Ply or ZIP sheating, for sheer walls it is possible to use let in braces or even some metals braces (large T to structure them together). However I would prbably add OSB just to make sure.

I agree, those bosses are quite happy when we can make things work for them. In the end it all works out.

SPF==>Spray Polyurethane Foam, one of the most commonly used spray foam chemistries, different enough from sprayed polyisocynaturate and some other commonly used insulating foams that they can't be considered equivalent from both a vapor-retardency or insulation value point of view.

Hardi doesn't spec a depth, but 3/4" (19mm) would be fine from a rainscreen benefit point of view, and doubling that increases the moment-arm of the heft of the siding on screws fastening the furring to the studs, increasing the number required. (IIRC Hardi documentation does specify different fastener spacings for different fastener lengths/furring depths. Ripping 3/8" ply for furring (for a Canadian code-compliant 10mm rainscreen) may also work.

Hardi Board came into prominence as an upgrade from vinyl, and because wood siding has these problems with paint peeling. The paint peeling is widely regarded to be caused by moisture trying to get through the siding. The rainscreen should eliminate moisture problems with wood siding since it allows the siding to breathe, allows the back of the siding to dry to the air and stabilize the wood. I'd suggest using wood rather than Hardi Board. Excellent product, hundreds of years of trials and it is much lighter than the cement. In Onatrio you can probaly get locally sourced hemlock which makes wonderfull clapboards and holds paint very well.

What Bob said!

To stabilize wood cladding even better to eliminate cupping and minimize checking, paint both front & back with a decent acrylic-latex primer (including cut ends) prior to installing. The stuff can last pretty much forever. Cement-fiber siding has some advantage in forest fire zones however.

I chose (or should I say my wife chose) Hardi Board in the hope that it would be a "forever" siding, needing no or almost no maintenance. Vinyl is very popular around here and weighs almost nothing compared to Hardi board; I may have to look into some of the better vinyl since supporting Hardi board at the end of a 5" nail looks like an issue, vinyl should be almost a non-issue.

I am not familiar with any but the big box store vinyl (HD or RONA) is there something I should look at in a premium vinyl.

I've used Hardiboard/strapping/2" XPS/sheathing. Screwed the straping in with GRK screws - excellent strength & a few different types, so that would probably work for 4"foam. I'd suggest you do some rough calculations of load and perhaps talk to the GRK people. - You might want to use a thicker strapping and heavier screw. On the other hand, paint stays on wood clapboards for decades, but it does fade and needs to be renewed every 15 - 20 years. Painted wood has a bad rap from the moisture problems we've thrown at it, but its actually pretty good stuff, which is why we've been using it for over two hundred years. It also looks a whole lot better than plastic.

you should be able to find some answers here:

http://www.greenbuildingadvisor.com/blogs/dept/musings/fastening-furring-strips-foam-sheathed-wall

good luck
Adi

Posted By adi43d on 17 Feb 2011 11:02 AM

you should be able to find some answers here:

http://www.greenbuildingadvisor.com/blogs/dept/musings/fastening-furring-strips-foam-sheathed-wall

good luck
Adi

Just what I needed, thanks!

Hi I have a problem right now with an ICF addition near Lindsay, ON. It can happen, I have the ugly truth.