2x6 outer walls, Tyvek outer seal

Would spraying the outer walls partially with closed cell expanding foam - just enough to seal the walls from air leaks, (added benefit of making the walls a teeny bit stronger too) and then net the walls for blown in cellulose.

I this is a stupid idea?  Since foam is so damned expensive, do you save money by only putting an inch onto the walls and then filling the balance with blown cellulose?  Or is most of the cost of spraying foam in the set up, truck roll, equipment, etc (and you don't save noticeable money by spraying less foam)?

I'm not savvy to the actual costs of spray foam, so go ahead and make fun of me if this is a stupid question.

It would also have the added benefit of rough wiring and plumbing the house being easier. Wiring and plumbing in expanded foam walls is a pain.

You can search for discussions of "flash and batt". My opinion is that you can do better with taped rigid foam, damp sprayed cellulose and a taped inside film like MemBrain.

a stupid question is one that isn't asked.

you're a lot better off using sheet foam on the exterior surface of the sheathed wall, where it will act as a thermal break. It will also help seal the assembly from air infiltration and warm up your wall so that your cavity insulation works better. If you go this route, you need to be careful about the thickness - you want enough so that moisture doesn't condense on the interior face of the sheathing.

That type of condensation has been a huge issue with houses built in the past 30 years, (leads to mold and rot) and is best addressed by paying attention to superinsulation techniques. the flash & batt technique you ask about is a like a bandaid on a broken leg.

Posted By Bob I on 06 May 2013 08:08 AM
you're a lot better off using sheet foam on the exterior surface of the sheathed wall, where it will act as a thermal break. It will also help seal the assembly from air infiltration and warm up your wall so that your cavity insulation works better. If you go this route, you need to be careful about the thickness - you want enough so that moisture doesn't condense on the interior face of the sheathing.

That type of condensation has been a huge issue with houses built in the past 30 years, (leads to mold and rot) and is best addressed by paying attention to superinsulation techniques. the flash & batt technique you ask about is a like a bandaid on a broken leg.

Bob,  Sheet foam on the outside of the house instead of Tyvek wrap? 

How does sheet foam on the outside help "warm up the wall"?

So is 1/2" thick enough to avoid inner wall sheathing condensation?

I assume I'd need to tape seal the seams of the sheet foam?  I've never paid much attention to the installation of it (and it seems to not be a choice used around here anymore, but you used to see it a lot) but how it is installed (attached to the wall sheathing)?

Posted By Bob I on 06 May 2013 08:08 AM
 The flash & batt technique you ask about is a like a bandaid on a broken leg.

I guess my question was is flash and batt any better than - no foam (other than sealing holes going out and around windows) and standard blown in cellulose (walls)? 

I would think that flash and batt would be superior (R value and infiltration), but not much cheaper than just filling the walls with foam?

Where are you? the right solution varies with climate.

Flash and batt, IMO is simply a way for the large insulation companies to make more money and make the homeowners and builder think they are doing something valuable. Similar to the old undercoating that car dealers used to sell. (Looked good, car rotted out anyway) It will help air seal the wall, but at the risk of increasing mold and rot.

Coastal property; extreme No Calif. 41 degrees lat; 4400 HDD 40-60 inches of rain a year, high humidity; marine layer fog often; average lows in Dec/Jan/Feb upper 30s (but it can get below freezing. Averge highs in Jul/Aug mid 70s.

No AC in house needed. Ventilation will be designed in with the radiant system

I looked into flash and batt. It is still expensive but it is much more cost effective if you are building a double stud wall or if you have 2x6 framing. That being said with 9inch space for insulation in my wall I was looking at $1200 just for inch of spray foam with a DIY kit. 3 inches would be three times and much and so on quickly adding up. I can do the entire room with cellulose at 9inches for $400. I think I will be air sealing and skipping the flash and batt. You can also save a lot of money doing either yourself. What you will save doing the labor yourself you can spring for the flash and batt. I do suggest at least looking at the DIY kits. I would have saved a lot by DIY. I do a lot of DIY projects including 2 heat pump systems and many other things. Each time I start a project and ask a question on a specific type forum I always get the "hire a pro to do it" or "you pay extra for professionalism and quality of work." This is usually a like given by someone in the trade who wants job security. I know with what I paid for my addition and I see all sorts of corners that were cut and stuff missed even with a very good contractor. At least if I cut corners I know which ones I cut and can go back to fix them knowing up front. That being said, the only issue with my heat pumps (one 3 years old and the other 2) was when the vinal siding crew hit both of them puncturing both refrigerant lines. Needless to say they paid to fix it after I spent a night in the cold. They were pissed afterwards and cut more corners like not removing the old unvented soffit before installing a vented soffit over it.

> I think I will be air sealing

I encourage you to consider some type of double air barrier, one near the outside of the wall and one near the inside. Then test it with a blower door, smoke or one of the tracer gas techniques.