Confused about rigid insulation

Started by Pa_Kettle, September 26, 2006, 10:49:14 AM

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Pa_Kettle

I am fairly certain that I want to add a layer of rigid insulation (1/2"??) to the wall surfaces of the little house in the sticks.  http://www.countryplans.com/cgi-bin/yabb2/YaBB.pl?num=1150900056  I've done a ton of searching here and on the rest of the net and I'm more confused than ever about rigid insulation.  Let me preface these questions by saying we are going to sheath the roof and walls with 7/16 OSB and do the batt insulation thing inbetween the studs and rafters and the roof and exterior are going to be metal  Here are my questions:

Do I want to install the sheets of insulation on the outside (over the OSB) or the inside of the walls?

If I put it on the exterior, do I still need a house wrap?  I've seen descriptions putting it in between the osb and insulation OR over the insulation OR none at all.  :o

If I put in on the inside of the walls should I still install a vapor barrier?

I assume if I put in on the outside of the walls, I should mount my windows on top of the insulation.  In other words, cut the rigid insulation window openings the same size as the OSB window openings.

Thanks,
PK

BTW - We got the rafters up, but forgot the camera.   ::)  So I haven't updated the website, but we're still truckin'!

FrankInWI

I'm no expert...but want to get this topic moving....I am interested in the other answers too.  I know some will be contrary to mine.  I know here in Wisconsin a lot of homes are built like my 1990 duplex is.   Drywall over plastic over studs on the inside.  Vynil siding over rigid insulation over the studs on the outside.  (oh yeh, bat insulation between the studs).  I look at newer houses bring built....and still see the same up here as one of the very common if not predominate way.  Mostly the solid insulation is also covered by house wrap too, although if the rigid insulation has a foill covering, I don't know how the wrap adds value.  I like the system.  I like the thermal break between the studs and the outside.
I have been told (by John?) that the rigid insulation on the outside might be looking for trouble.  I think others have cautioned similarly.  I have no problems with my house and have heard of none around here.  I'm going to be doing the same thing on my building in the north woods....lest I finally have my mind changed by then.
Thanks
Frank
god helps those who help them selves


FrankInWI

[size=12][/size] Oh yeh..... I did not forget about the OSB placement part of the question.... it's not on the system I described, no OSB, no Plywood, no sheating of any kind......
god helps those who help them selves

Pa_Kettle

FrankInWI,
I have heard of the system you describe.  In this system you have to brace the walls with diagonal inletted studs and/or use osb/plywood at the corners of the building to get the structure stiff.  That all sounds too complex for me, so I'm doing wood sheathing everywhere on the outside for sure.  It ought to make for a quieter house as well.

I just went to the local supplier that carries 4x9 rigid insulation. They carry Dow's Duramate Plus.  Looks like Duramate is Dow's rigid insulation with a tougher outter surface to take rough handling.  It's R-3 in 1/2" form I think.

Pk

williet

Interesting topic .... I've wondered the same thing and I'm looking forward to the replys.


John Raabe

#5
Structural sheathing is good insurance and adds a great deal of strength to a stud framed wall. In some places it is not required nor is it standard practice. My plans show it however so you can decide ("X" it out and add a diagonal bracing note).

The location of foam sheathing is somewhat optional and somewhat a function of climate.

It is most commonly done on the outside of the framing (and sheathing if used) and in most climates that is fine. Most of the plastic foams are pretty good vapor barriers so you need to recognize that. In some climates a vapor barrier there can be a problem.

In rainy climates wood siding can sometimes have difficulty drying out if nailed directly to foam sheathing. In that case it is better to have a drainage plane such a 1x4s or 30# felt housewrap behind the siding (to let some air circulate).

If you install foam on the interior then you can tape the seams and use its vapor barrier characteristics to your advantage. This does involve some work cutting out around outlets and such that you would not do if the foam were on the exterior.

Additional confusion can be found at the Building Science website where they have model walls based on climate characteristics.
None of us are as smart as all of us.

DemianJ

It is confusing.  I live in a temperate climate (foothills of VA) and ended up doing sheetrock-->studs and fiberglass-> OSB->tyvek->1/2 foil backed rigid foam w/ taped seams->furring strips->painted cedar lap siding.

In hindsight, I'd probably have simplified it to save money (all those layers can add up to a lot of time/money, especially on a 2 story house; I'd guess doing tyvek, foam, and a rainscreen increased labor costs of the dried-in shell by at least 15-20% as compared with a standard wall; most of that was the rainscreen, though).  I'd also probably have used John's idea of folded tarpaper strips instead of furring strips (quicker to install and less padding out) and I'd possibly have put the foam on the inside as the vapor barrier (a lot easier to install, but eats up some interior space).  By the way, if you use foil backed foam, you're not supposed to put the siding right against the foil.  

However, I plan on owning this house for a lifetime (and the house is on a windy ridge), so once the power bills come in and depending on how long the siding lasts, I might be pleased with the investment 20 years from now.

DemianJ

Pa_Kettle

 DemianJ, your build looks like the typical one I've seen described most.  Very well built it is.  Did you do 2x6 walls as well?

I'm thinking, since I've heard no protests to the contrary, I'm going to put the rigid insulation on the interior walls.  That should give me a vapor barrier in the process and allow me to do the sheathing in a step by itself.  I'm building this thing two days at a shot...  ;)

PK

FrankInWI

mmmmm. now you have me thinking.  I like the benefit of being able to make the house weathered in and going about the sheeting on the interior as I can.  I'll be doing it as a week ender too.  Wondering how much I'll loose around the holes for electric boxes, etc. , but I guess I can be extra careful and use expanding foam and tape too.
god helps those who help them selves


DemianJ

Thanks, Pa Kettle.  I ended up using 2x6 for the first story b/c it was required by code; the second story is 2x4.  Part of me wonders whether I should have used 2x6 upstairs as well, but I would have lost some space in a few rooms that were already really small (plus I've read that foam + 2x4 is better than 2x6 alone; and I like my bedrooms cooler anyway...).  

Frank, you can buy insulated electric boxes and foam gaskets that should do the trick along with the tape and expanding foam you mentioned.  I bought some from:
[/url]http://www.energyfederation.org/consumer/default.php[url]

They've also got some really nice attic fans and other energy saving products.

DemianJ

RonDay

There's a good article on rigid insulation in Fine Homebuilding. Could be last moth's issue by now, but should still be available. In any case, we've pretty much decided to stick build the cabin (14 x 28 Little House w/shed roof on post and beam) we're putting up first and have it foamed. We're close enough to a dealer for the system we want to use for that to be practical. Some folks aren't.

We spent weekend before last in Fredericksburg (NW of San Antonio) at the renewable energy roundup (theroundup.org) and we're both still trying to digest all of the info we picked up. The presentations were audio recorded and I hope the PowerPoint files that go along with them are available on this year's CD. Neither the audio or the PPT files alone convey anywhere near the info that both do. I may end up editing the two bits together if the organizers can get the PPT files.

FWIW,
Ron