Wall Cavity insulation with 1″ rigid on the outside?
I am building a new home in Oregon, with apx 6000 HDD. My contracter has already installed 1″ of rigid foam insulation outside of the sheathing. I live in a high wind area and I am very concerned with making the home as airtight as possible, and controlling my ventilation with a whole house HRV. Is spraying closed cell foam into the wall cavity a bad idea because of creating an area at the sheathing that can not move moisture out? What about the 1/2 pound foam? We have been very carefull about sealing all exterior penetrations in the wall, but still, stuff happens. If so what would be the best alternative for wall cavity insulation. I am very very sceptical of blown in cellulose. I just believe at some point it will settle, leaving a gap at the top of the wall. I am the first to admit I am no insulation expert, but I have some bad experiance with blown in cellulose, and hate to make the same mistake twice. I also distrust the whole notion of “this new stuff is better, it won’t settle” .
I would rather do spray foam, but I don’t want to rot out my sheathing. Am I being overly concerned with creating a vapor barrier on both sides of the sheathing? (The rigid was not foil faced.) Any advice would be aprreciated.
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It isn't the cellulose that's new, it's all in the way it is instaled. Dense pack cellulose is blown in to a density of about 3 lbs/cu. ft. which is a higher density than what gravity can achieve. Think of dense pack cellulose as being pre-settled... and then some.
Other opinions may vary but if you're trying to build a "green" home, then in all ways I can think of cellulose is far more better than spray foam insulation.
I wish there was an edit function. "...is far better than..." is what I meant to say.
No, you're not being overly concerned. Any time you create a double vapor barrier, you're risking serious moisture damage should any water eventually leak through the envelope (which is a good idea to consider as inevitable over the life of a house).
If the exterior foam is 1" XPS, then it will allow some drying to the exterior, but I would not advise restricting drying to the interior as well. A "flash" of pen-cell foam is an option, but probably unnecessary if you've already taken precautions to air seal the exterior (taped joints on the XPS, a WRB in addition, well-sealed sills?).
And your concerns about dense-pack cellulose are unfounded. Just as with spray foam, the performance of the installed product is dependent on the installer's skill and the quality of the equipment. But dense-packed cellulose is not so much a different "new and improved" material as a new technique.
Loose-fill cellulose blown into an attic will settle by gravity to about 1.4 pcf. When it's installed in a closed wall cavity to a density of 3 pcf, it's at twice the settled density. It's like compressing a sponge to half its normal volume and stuffing it in a cavity. It's always trying to expand, and can't settle. Dense cellulose is also a very good barrier to infiltration as well as being an excellent fire-stop, highly resistant to mold and rodents, toxic to insects, and capable of buffering indoor RH more than any other insulation.
Do not use an interior vapor barrier, however, but only a VR latex primer. Seal all stud cavity and interior surface penetrations and you'll have a very tight house.
How do I ensure I am getting Dense pack cellulose?
Is that the same thing as the "wet installed" cellulose? Or do i simply need to specify a density ie. 3lb/ cu ft?
Cellulose doesn't get installed "wet", but slightly dampened to activate the starches in the paper to act as a glue.
There are four methods for installing cellulose insulation:
1) in an open attic, loose fill at gravity settled densities
2) in walls or other enclosed cavities:
a. dense-pack closed-wall blow (behind drywall, eg)
b. dense-pack open-wall blow, though insulweb before drywall is hung
c. damp spray open-wall using moisture to create a self-supporting insulation mat (which requires 1-3 days of drying time before drywalling)
All reputable cellulose installers use one or more of the dense-pack approaches. What you need to do is find a reputable installer who offers a warranty.
Robert
I too live in the Great Pacific Northwest and am considering blown in cellulose - thank you for the clarification re settling. Sam's concerns are what I have heard as well,,,,,"that stuff will settle."
For my house, I will not be using the exterior rigid, but a cedar clad, taped wrb, and am attracted to a CLOSED cell flash (to avoid the dew pt in the interior of the wall cavity) for it's air sealing properties. But after reading your posts, it sounds like I would be better off with cedar, taped wrb, 1/2 cdx sheathing, cellulose in walls, drywall applied using the Airtight Drywall Approach, and a vapor retarder latex primer. Is this a superior assembly as compared to a closed cell foam flash approach for a durable wall assembly that will allow air drying to both the interior and exterior, have above average R value with the blown in cellulose, and have a tight house because of the airtight drywall approach?
Frank,
There isn't a yes-or-no answer to your question (although I have no doubt that Robert Riversong will provide one).
The "flash and cellulose" method can really improve a wall's airtightness. The usual caveats apply:
1. Don't forget to seal air leaks elsewhere in the building envelope -- air leaks that the spray foam doesn't address.
2. Be sure the foam layer is thick enough to avoid condensation problems.
Martin
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Frank,
Either will work. Personally, I would go with just dense-packed cellulose -- assuming, of course, that we're talking about a double 2x4 wall that addresses thermal bridging.
If your wall framing is thin -- that is, just a 2x4 or a 2x6 wall -- then you are making a big mistake to omit the rigid foam sheathing.
Since I am not using rigid wall sheathing, than which system do you recommend?
Frank,
How are you framing your walls?
standard framing, 2x6 walls
Frank,
Your studs will act as thermal bridges. Can you at least strap your wall horizontally on the interior with 2x3s (assuming that for some reason you are still dead-set again exterior foam sheathing, which would be a much better option than interior strapping)?
Martin
...nothing is off the table. However, what you are positing is unconventional in the environment I am operating in with regards to my designer and builder.
back to 12, what do you recommend for standard framing, 2x6 walls?
thanks
Frank,
I don't like stud walls filled with insulation unless:
1. There is exterior foam sheathing to interrupt the thermal bridging, or
2. There are two parallel 2x4 walls to interrupt the thermal bridging, or
3. There is interior horizontal strapping to interrupt the thermal bridging.
Frank,
I think Martin's concern is that without either exterior rigid insulation or strapping to the inside (or some type of double-wall/Larsen truss system) you will have done nothing to address thermal bridging through the 2x6 studs. A flash coat of spray foam between studs will do nothing to reduce thermal bridging through the studs.
If you are unable to reduce thermal bridging, the effect is to reduce the whole-wall R-value by a significant percentage.
Hi Sam,
I live in Oregon (Portland) too and saw your post. I debated this issue for a LONG time with our Live/Work project. A few observations that may or may not affect you:
1) Spray foam is an awesome solution. Unfortunately (for me) it was a budget buster
2) Thermal bridging is a very real issue. Staggered stud construction is great if you factor it in the design phase. We didn't (sadly) and missed this opportunity.
3) Exterior foam is an easy, inexpensive way to prevent thermal bridging. I'm a big fan, though window and door detailing can be trickier for some crews.
4) I hate the idea of poly sheeting on the inside. Strikes me as a mold chamber.
I put together a video on how to detail windows with 1" on the exterior. You may find it helpful:
http://www.portlandbuilt.com/video/video-wall-assembly-design
We went with fiberglass batt on our project (again, a budget issue). I had an excellent installer, so air infiltration is less of a concern. That said, blown cellulose, spray foam, etc. strike me as superior performance options.
Final note: I have a vented ceiling with can lights. I was up installing trim the other day, and noticed cold air coming from around the "air tight" fixture. Needless to say, much caulk was applied. In hindsight, I might stay away from the vented/vaulted can-light solution
Hope this helps.
Shawn,
Thanks for the response. The exterior foam, along with the siding, and the window detailing, is already done. It went reasonably well because the contracter I used had some experiance with the exterior foam.
My main question was given the foam(and "almost" vapor barrier it creates) on the outside, would spray foam inside the cavity be a mistake? I know it is expensive, but I have lived in a cold windy old farmhouse for almost 20 years, so maybe I am a bit more willing to pay top dollar for the best insulation than an average home owner :)
I also wanted to do the staggered wall construction, but I could not find a builder in my area with experience doing that, so I went with conventional 2x6 framing. I don't regret the decision, because I think paying some one to do something for the first time is wasting money when it comes to construction.
Sam,
Paying someone to do a slight framing variation that will save you money for years to come is hardly a bad investment, even if there's a learning curve. Any competent framer can build a staggered stud wall.
Shawn,
Vaulted ceilings are generally problematic and can lights in an insulated ceiling are always a mistake. Vented roofs are almost always more durable and energy efficient.
Frank,
Interior strapping and dense pack cellulose is the second best option after staggered 2x4s on a 2x6 plate. Either one is simple to build, will provide a thermal break and increased whole wall R-value, eliminating the (apparent) need for any kind of foam, which will make your wall assembly less durable by reducing its drying potential and maintaining mold-growth conditions.
If either of those framing options are combined with advanced 24" oc framing techniques, then the wall assembly will have ever more improved performance with fewer forest resources.
Robert,
Do you really think staggered 2x4 studs on a 2x6 plates is a good idea? Sounds like a VERY high wood to insulation ratio in that wall assembly...
Brett,
Good idea? It's a better idea than a wall full of thermal bridges (typical 25% wood in a wall) or the same poorly-designed wall bandaided with foam boards. Even KD lumber has a small fraction of the environmental impact of petrochemical foams.
However, staggered 2x4s on a 2x6 plate is the least efficient, least effective, and most resource intensive method of reducing thermal bridging. A full double stud wall is far more effective, a Larsen Truss wall is better yet, and a modified Larsen Truss wall (such as I've used for 20 years) is the most effective and efficient of all.
Has anyone experienced any issues w/ installing cedar shingles directly over 1" rigid and nailed back to 1/2" sheathing behind the rigid? Any movement to the shingles? Is it necessary to install a building wrap or felt b/w the shingles and rigid assuming the rigid has taped seams?