Question on wall insulation

[MountainDon]

This is an original question from mountainmomma. I moved it here from it's original location as it is useful information that others can use as well. Easier to find on its own here.


I also have a question for Raab and anyone else who might have some insight.

The BIBS system is very good at filling all the voids that reduce the performance of batt insulation. I have been using BIBS in custom homes (and the addition to my own) since the mid 1980's. It is a specialty contractor install and its higher performance comes at little additional cost.

I did a study in the
Superinsulated Design and Construction book on the importance of insulation voids. In short, the higher the cavity insulation the larger the percentage of heat that is getting out through the voids. For superinsulation levels you have to deal effectively with this issue.

http://www.bibs.com/

When BIBS is used with spray foam as a air sealing/high-R insulation barrier at the framing stage it is an unbeatable combination. In Jan's double wall configuration this should result in an astoundingly low heat transfer rate.

We are looking into our options for closed cell spray foam insulation. To minimize costs, we are considering doing one inch of spray foam and then completing the walls with cellulose. I have gotten some mixed reviews of this idea but wanted to get your opinion. What do you think? Anybody else have any experience with this?

[MountainDon]

By cellulose you mean dense pack blown in cellulose?



http://www.100khouse.com/2010/08/16/blown-in-cellulose-the-ultimate-green-insulation/

Installed by a qualified pro it does an excellent job of air sealing itself.

[Chuck Adze]

I have used cellulose, PolyIso foam, and fiberglass in my present home.
I did not use blown cellulose in wall cavities, primarily because my brother in law used to blow (standard loose dry)cellulose for a living and said they sometimes have settling problems in standard walls (maybe our climate?), this was not the sticky stuff, it was loose fill.
However I know of improperly installed fiberglass in manufactured housing walls that had done the same thing while in transit.
The sticky stuff (cellulose) is different from what I have seen on TV.

I did use blown cellulose in the knee walls (behind the fiberglass) of the house I am in now, and I like it.
I would do attic spaces, even on top of fiberglass.

However, nothing beats foam (IMO).
You can get it used from recyclers much cheaper (like foam insulation depot), and I am a really cheap person (they now call this being green; we used to call it not much money).
Acrylic caulk works well on the joints (buck or less a tube), and you can tape over with the clear wide packaging tape.

I have not used spray foam, because of cost (I have heard some people have had problems with spray foam pulling back from framing and causing air leakage....might be do to PAD lumber or some other factor?).

I have used foam on both interior and exterior walls.
The prefered method in my region is now exterior walls.

The comfort level of the home increases dramtically.

[Chuck Adze]

Also I forgot to mention Roccio Romero uses "double walls" in her LV series home kits.

[John Raabe]

I would think that a 1" thick closed cell spray foam properly applied to dry framing will get a good start on the insulation and - more importantly - airseal the wall. The BIBS system that I mention earlier uses a latex binder to coat a fluffy white fiberglass that fills voids around wiring and plumbing. It is not as good at airsealing as foam.

Filling every void is not as critical once you have airsealed and insulated the walls with a layer of foam. There can still be voids using standard batts but they will not have a great impact on the overall efficiency since the foam has "covered your back". That said, it is best to get the most complete coverage you can out of whatever insulation you use.

Exactly which combination is the most cost effective - overall effective R-value per $$$ - is a bit of a mysterious art.

In general I think mountainmomma is on the right track.