My location is upstate NY, climate zone 6. New construction is timberframe with full dimension 2x5 exterior insulation frame that gets sheathed with 2) 1" layers xps, and dense pack cellulose in stud bays. Painted sheetrock provides air barrier and I'll use Intello-Plus smart vapor retarder where walls get wood paneled. Ive gotten a few insulation quotes and a couple installers are strongly recommending closed cell foam in the walls instead of the cellulose explaining that it prevents moisture issues more effectively...but then I need an exhaust fan devoted to removing moist air. The claim is that "the latest building science indicates that in my climate foam is superior". My plan outlined above was recommended by my engineer who specializes in passive solar and energy efficient design and my research seemed to validate it as well. The installers are contradicting this advice and citing the latest building science. My eyes are starting to glaze over and my frustration build regarding some of the complexities and conflicting advice. I apologize for the length of the post. I'd like to use cellulose. Is my original plan a sound one?

I'd start by using google with "exterior-rigid-foam-double-stud-walls-no-no" (yes, it talks about single stud walls).

All new houses need a HRV or ERV and I recommend that you air seal both sides of your walls with tape and/or gaskets. Consider EPS instead of XPS (a subject by itself).

Sorry, I can't access that article. I'm not building a double stud wall. Single full dimension 2x5 stud wall dense packed, sheathed with 2) 1" layers xps, seams staggered and taped. I'll probably use intello-plus continuous instead of insulweb and though my engineer specced out sheet rock I'll use blueboard and plaster.

Where are you building in upstate NY I may be able to give a few companies that may be able to help. People with an investment in poly equipment tend to want to sell that product.

I'm in the old forge area, about 45 miles north of utica. Thank you very much.

You could try Standard Insulation in Marcy. I just met a contractor in Schenectady but I doubt that he would travel that far.

I just had Standard out on Friday. They were the ones that told me I should install 3" of closed cell instead of 5" dense pack cellulose. The other two companies agreed with my engineer's specs. Have you heard of Snug Planet in Ithaca, NY? They came highly recommended, and seem to be well qualified. Only thing, their estimate came in 100% higher than next one down.

Surprising since they started the same time I did. One thing that happened is the Green Fiber plant in Hagamen NY closed so the supply trail just got a whole lot longer for stabilized celly. I'm tempted to pull my machine out of moth balls since your only about 80 miles from me. I'll be doing my last icf build in Russia soon. Sorry not that Russia, Russia NY which is fairly close to you. I haven't heard of the company from Ithaca but it seems to be charging a premium for travel. One thing that you have is the power of the purse. If you want celly tell a company like Standard that's what you want. I'll contact the company in Schenectady to see if they are interested.

Posted By dfvellone on 07 May 2016 01:22 PM
My location is upstate NY, climate zone 6. New construction is timberframe with full dimension 2x5 exterior insulation frame that gets sheathed with 2) 1" layers xps, and dense pack cellulose in stud bays. Painted sheetrock provides air barrier and I'll use Intello-Plus smart vapor retarder where walls get wood paneled. Ive gotten a few insulation quotes and a couple installers are strongly recommending closed cell foam in the walls instead of the cellulose explaining that it prevents moisture issues more effectively...but then I need an exhaust fan devoted to removing moist air. The claim is that "the latest building science indicates that in my climate foam is superior". My plan outlined above was recommended by my engineer who specializes in passive solar and energy efficient design and my research seemed to validate it as well. The installers are contradicting this advice and citing the latest building science. My eyes are starting to glaze over and my frustration build regarding some of the complexities and conflicting advice. I apologize for the length of the post. I'd like to use cellulose. Is my original plan a sound one?

As I pointed out on that other forum, 2" of XPS is (barely but still) sufficient for dew point control on 5.0" of half-pound open cell polyurethane or cellulose using latex paint on gypsum for the interior side vapor retarder.  Using 2.5-3" of EPS or polyiso would be greener and have more dew point margin than 2" of XPS, due to the much lower environmental impact of the blowing agent (pentane), compared to the HFC134a used for blowing XPS.

In 50 years the performance of the XPS will have fallen to where it might not be quite enough for dew point control (barring climate change), whereas 3" EPS or polyiso would still make it.

Using the Intello Plus (or MemBrain, which is cheaper) everywhere would be better still, but only if using cellulose or open cell foam.  With closed cell foam the vapor retardency of the smart vapor retarder is trumped by the very low vapor permeance of the closed cell foam.

Closed cell foam between the studs is a waste of money due to the performance hit incurred by the thermally bridging framing, and reduces the drying capacty for the sheathing.  If foam, go with open cell foam, which is much greener due to lower polymer use and a MUCH lower impact blowing agent (water, instead of HFC245fa).

If cellulose, caulk the framing to the sheathing inside every stud bay prior to insulating.

Posted By dfvellone on 08 May 2016 02:39 PM
I just had Standard out on Friday. They were the ones that told me I should install 3" of closed cell instead of 5" dense pack cellulose. The other two companies agreed with my engineer's specs. Have you heard of Snug Planet in Ithaca, NY? They came highly recommended, and seem to be well qualified. Only thing, their estimate came in 100% higher than next one down.

The thermal bridging through 3" of wood framing is substantially more than through 5" of wood framing. (The framing fraction would run about R3.5 stead of about R6.)   Even if the center-cavity R-values are about the same, the net  performance would be quite a bit lower due to the higher thermal conductivity of the shorter path through the wood.

At 3" the closed cell foam would have to be installed in two lifts with a cooling/curing period between lifts for both quality and fire hazard during curing reasons.

At 3" closed cell foam would be well under 0.5 perms, with very minimal drying capacity toward the interior.

Open cell foam can be applied at 5-6" in a single lift, and trimmed flush with the studs, and would have higher thermal performance, but would need a class-II or "smart" vapor retarder on the interior unless you bumped up the

Cellulose would have about the same thermal performance of open cell foam, but also protects the structural wood by redistributing and sharing the moisture load.

Posted By Dana1 on 09 May 2016 04:00 PM

Posted By dfvellone on 07 May 2016 01:22 PM
My location is upstate NY, climate zone 6. New construction is timberframe with full dimension 2x5 exterior insulation frame that gets sheathed with 2) 1" layers xps, and dense pack cellulose in stud bays. Painted sheetrock provides air barrier and I'll use Intello-Plus smart vapor retarder where walls get wood paneled. Ive gotten a few insulation quotes and a couple installers are strongly recommending closed cell foam in the walls instead of the cellulose explaining that it prevents moisture issues more effectively...but then I need an exhaust fan devoted to removing moist air. The claim is that "the latest building science indicates that in my climate foam is superior". My plan outlined above was recommended by my engineer who specializes in passive solar and energy efficient design and my research seemed to validate it as well. The installers are contradicting this advice and citing the latest building science. My eyes are starting to glaze over and my frustration build regarding some of the complexities and conflicting advice. I apologize for the length of the post. I'd like to use cellulose. Is my original plan a sound one?

As I pointed out on that other forum, 2" of XPS is (barely but still) sufficient for dew point control on 5.0" of half-pound open cell polyurethane or cellulose using latex paint on gypsum for the interior side vapor retarder.  Using 2.5-3" of EPS or polyiso would be greener and have more dew point margin than 2" of XPS, due to the much lower environmental impact of the blowing agent (pentane), compared to the HFC134a used for blowing XPS.

In 50 years the performance of the XPS will have fallen to where it might not be quite enough for dew point control (barring climate change), whereas 3" EPS or polyiso would still make it.

Using the Intello Plus (or MemBrain, which is cheaper) everywhere would be better still, but only if using cellulose or open cell foam.  With closed cell foam the vapor retardency of the smart vapor retarder is trumped by the very low vapor permeance of the closed cell foam.

Closed cell foam between the studs is a waste of money due to the performance hit incurred by the thermally bridging framing, and reduces the drying capacty for the sheathing.  If foam, go with open cell foam, which is much greener due to lower polymer use and a MUCH lower impact blowing agent (water, instead of HFC245fa).

If cellulose, caulk the framing to the sheathing inside every stud bay prior to insulating.

(been reading this forum and finally joined :) ) Why go through the additional step of an internal vapour retarder? Would it not make more sense to keep moisture out of the wall to begin with and use a 2lb closed cell SPF as both a vapour and air barrier? Keep the XPS (or EPS) on the outside of the wall, shoot closed cell SPF against that in a 1.5-2" thickness, and fill the remainder of the cavity with essentially anything (SPF, cellulose, pink batts...) Bringing the vapour barrier inside makes little sense to me. I want everything inside of the siding closed-off from the outside world, not just sort-of closed off.

If you installed 1.5" of closed cell foam (R6- R10) on the exterior it would be sufficient R value for dew point control at the sheathing at about R9-R10 at a cost of ~$1.50. But the same $1.50 would buy you another 3.5-4" (R15-17) of EPS (installed), which offers huge dew point margin and a significant thermal performance uptick.

With very few exceptions, 2lb polyurethane is blown with HFC245fa, which has a 100 year global warming potential of ~1000x CO2. EPS is blown with pentane, with a GWP of ~7x CO2, most of which is released and recaptured during manufacturing to be burned for process heat, reducing it to CO2 and H2O.

There are much cheaper ways to air-seal a wall or waterproof the assembly than closed cell foam.

To meet code vapor retardency with closed cell foam on the inteior you would need 2" of closed cell foam, at a cost of USD$2.00-2.25 per square foot. Since it's displacing some of the fiber insulation, call it a ~$1.75 per square foot premium. The sheathing could still dry toward the interior, but slowly.

To meet code vapor retardency with MemBrain costs about $0.15 per square foot, more than an order of magnitude lower cost, and the drying capacity is huge. Intello Plus is somewhat more vapor tight, but more like $0.45 per square foot.

If you spent the $1.75/foot difference on more exterior EPS it would buy you more than 3" worth, and raise the average temp at the sheathing sufficiently to not need any interior vapor retarder whatsoever.

Dana1, I'm just around the corner from starting the sheathing and I'm wondering... you mentioned caulking the sheathing to the framing. To reduce air infiltration? If I'm using 2-3 layers eps, staggered and taped at the seams is caulking every stud necessary?

Where is your air barrier? It should be at a specific location and it must be continuous around the house. Foam sheets are insulation, they are not an air barrier. With or without exterior foam, our AB is the sheathing, and we tape all the joints. (we do double wall construction and use fir CDX sheathing which is more robust than OSB) The problem with taping seams only is that the edges of the wall - at the roof, at the corners, at the base of the wall - there may be nothing to tape to, so in those locations we caulk the framing. So all of our edges are sealed and all of the panel joint are sealed.

Posted By dfvellone on 14 Jun 2016 10:00 AM
Dana1, I'm just around the corner from starting the sheathing and I'm wondering... you mentioned caulking the sheathing to the framing. To reduce air infiltration? If I'm using 2-3 layers eps, staggered and taped at the seams is caulking every stud necessary?

Yes, it's necessary.

Making the sheathing the primary air barrier has much higher reliability & longevity and is easier to detail as an air barrier than the exterior foam.  You still need to tape the foam, but if the sheathing isn't caulked to the framing air leakage between the sheathing & framing become a thermal bypass.  The size of those long skinny holes are much bigger than most people can imagine. No matter how well you seal the foam to the sheathing, the sheathing-to-framing seal still needs to be tight.

You also need to caulk where the bottom plate of studwalls rests on subfloors, and between doubled up top plates, etc. 

Well, I guess I need to clarify. The foam is the sheathing, there's no osb, no plywood.
The construction is timberframe with full dimension 2x5 exterior framing @ 24" centers. The exterior frame gets the eps sheathing and the sheathing gets furred out for a 1" airspace prior to siding. Two to three layers of foam with seams staggered and taped won't be an effective air barrier?

With only an exterior side air barrier (no interior side air barrier), it may be easy for interior air/moisture to travel towards the exterior, dump liquid water and then return to the interior. Maybe you will have enough drying capability to avoid rot, maybe not.

Am I understanding that you are going to span 2' OC studs with 1" EPS? If it doesn't rip at once, it will the first time the house moves. 2" will do better but will have a tendency to crack with movement; not very stable that way. Also the stresses from the frame shrinking & moving around. (Doesn't usually move much, but it will move).

To my mind the key to making a house work very well, rather than just average is the air barrier. Infiltration allows moisture into the wall which can damage the structure, while a good one will keep it out. Infiltration also ruins the R value of any insulation. Your best bet is to wrap the outside of the studs in an Intello product and follow their detailing instructions. Definitely worth the effort.

Posted By Bob I on 14 Jun 2016 09:19 PM
Am I understanding that you are going to span 2' OC studs with 1" EPS? If it doesn't rip at once, it will the first time the house moves. 2" will do better but will have a tendency to crack with movement; not very stable that way. Also the stresses from the frame shrinking & moving around. (Doesn't usually move much, but it will move).

To my mind the key to making a house work very well, rather than just average is the air barrier. Infiltration allows moisture into the wall which can damage the structure, while a good one will keep it out. Infiltration also ruins the R value of any insulation. Your best bet is to wrap the outside of the studs in an Intello product and follow their detailing instructions. Definitely worth the effort.

I was under the impression that an Intello product should be installed on the interior face of the stud walls. Is it recommended to wrap both the interior as well as the exterior? And yes, so far the plan as outlined by my engineer has been to apply the foam board directly to the exterior of the insulation frame. I can certainly modify this plan though my reluctance is partly due to my distaste for plywood, and partly due to my personal sense that wood sheathing might be a tad bit overkill. My stud walls, other than being soundly attached to the timberframe, get nailers throughout, diagonal strapping, and mostly wood paneled interiors. I realize the timberframe will move, but nothings going to stop that movement. Not even plywood.

Go to the source! You need to talk to the good folks at https://foursevenfive.com. Send them a section of your wall and ask for their advice on the best and most robust methods for air sealing.

You are right that plywood will not stop the frame's movement. Plywood (or any boards), however, will not crack & fall apart like 1" foam will. EPS is a very fragile product, and it sounds like a very flimsy wall overall.