polyurethane foam

[rongaudier]

Here is my idea: I am planning a 2-story woodframe house. There will be no external sheathing on walls or roof. Instead I will use bracing as needed. Hardie Plank siding will be nailed directly to wall studs. Metal roofing will be screwed to cross-pieces attached to rafters. I plan to use metal roofing underneath the first floor floor joists as well (I know this sounds strange). After all siding, roofing and under-floor metal roofing is in place, I plan to apply 1-2" of closed cell polyurethane foam. The Idea here is to create a fully sealed envelope from below the subfloor to the attic. The foam will also act as a vapor barrier. I'm aware that the foam is a very expensive option, but I'm thinking that I can offset the cost by not using sheathing (which I don't think is required) or a vapor barrier.
Any thoughts on this?

[Rich]

It really depends on your local building codes. Many older style homes were built in that manner (of course they didn't have foam insulation) without sheathing (they didn't have that either). Instead they would use skip sheathing or 1x6 at a 45. So I'm not going to say it doesn't work.. just that some localaties require some type of sheathing.
Check with your local building department.

[NAIL BENDER]

The Hardie siding needs the sheathing for backing. Without the sheathing, the siding will belly in and out between the stud bays. And it would make for an easy house to break into.

[Rich]

haha.. and besides that it's doubtful that Hardie will honor the warranty if it's installed in that manner.
Might want to look through this page -
http://www.jameshardie.com/homeowner/installation/hardiplank_installation.php

[rongaudier]

My local codes will allow building without sheathing.
According to hardie plank (and certainteed) it is okay to nail planks directly to studs as long as the stud wall is properly braced. I am considering using metal diagonal bracing for this. Are you quite sure that the hardie planks will bow if nailed directly to studs. Have you actually seen this happen? Naturally this is something I would prefer to avoid.
Thanks

[Rich]

Think of this..
you have hardiplank from stud to stud.. and then you're going to spray low-expansion foam into the space.. and you don't think it will bow out? That'll happen before the planks have a chance to bow due to expansion / contraction.

[rongaudier]

Sorry, I just dont' know. Maybe I should do a little mock wall say 16" X 24 " and give it a try. Frankly, I don't think the foam is going to exert any pressure on the planks, if anything it may add stiffness much like foam in a surfboard. Again, I just don't know for sure. I am corresponding with Hardie, I'll let you know what they say.
Ron

[rongaudier]

After thinking about it for awhile, I am drifting away from the idea of not using sheathing. In the whole scheme of things, it appears sheathing does not really add that much to the total project cost.
Additionally, I am considering abandoning the idea of using the polyurethane foam in spite of its advantages. The cost of the product is just too high.

As an option for sheathing I am looking into homasote 440. From what I have read so far it seems like a superior product in many ways. I am interested in hearing from anyone who has worked with this product.

[Rich]

When they first came out with homasote it was the crappiest stuff to work with.. if it ever saw moisture it would near fall apart. But now they actually do have a pretty good product. The stuff has been around forever. I believe it has better insulating value than plywood and OSB also.

[mreynolds]

My wife and I are building our first house and I looked at some of the options you mentioned. I seriously considered foam sheathing in place of OSB, but I was somewhat concerned with structural stength. Then I looked into doing foam insulation, but it seemed like it costs were on the high side. We wound up choosing structural insulated panels, our home will be made from 8 of them, 24 feet long with a 6 inch foam core. The cost is more than standard framing and panel walls. But we will get a very energy efficient home in about 3 hours. Here is the company we chose for our walls http://www.insulspan.com/

[mjpliv]

We have a foam manifacturer up here who produces a product called Insul-Wall. In most cases the wall panels are shipped to the site in pre-built sections but it is possble to install the product on site.

This is probably one of the most energy efficient wall systems I have run across. Regardless if you are trying to keep the heat in or out, this will give the best bang for your buck every time. Its made for 2x4 or 2x6 construction.

Here is a gallery for a house I designed that was finished last year.

http://www3.ns.sympatico.ca/1saltydog/gallery.html
http://www.buildersmatrix.com/info/walls.jpg

Here is the local manufacturer. They may be able to redirect you to a company in your area that produces the same product.

http://www.truefoam.com/insulwall.html

It will be well worth investigating further!

[rongaudier]

Yes it would be ok to put our siding directly to the studs as long as you don’t need any sheer value or wind load requirements. Please check your building codes for acceptable applications. The product will not bow as long as you studs and frame are straight and flat. If you have further questions please call our toll free technical service # below.



Thank you,



Joe Niizawa

Technical Service Rep

James Hardie Building Products

joe.niizawa@jameshardie.com

800-942-7343

[rongaudier]

Not that I intend to do this, just thought y'all might like to know.

[Rich]

That's surprising.. but good to know.