Dense pack insulation below grade
I have been looking at concrete SIPs. The place I have been looking suggests a a shell of 2 1/2 inch concrete on each side with an interior of 7 inches of XPS.
Okay, this sounds good. You get high thermal mass on the inside and the exterior shell protects the xps from bugs, etc.
But is there a way to make it better(cheaper)? Could it work 2 sheets of xps up against the concrete walls, but then fill the space between the walls with dense pack cellulose (3 inches or maybe even increase it to 5 inches)?
I am nervous because if water gets in the shell is relatively impervious so the water would not be able to get out. Hello mold! I would do the best is can think of to water proof with air entrainment and additives to the concrete. The shell of xps would also provide a moisture barrier, I can see how this probably is a bad idea for the below-grade shell, but maybe not the above grade shell?
Lets make this question more complex (convoluted) by asking if it might be okay if I used rock wool. I understand rockwool does not sustain mildew.
To make this post even more complex, if I made the rockwool dense would this help or hurt the insulation value? I understand (I think) that dense pack insulation has a higher R value, but would this be true of rock wool as well?
Thanks for any advice. I am still a couple years from starting my project but want to learn as much as I can.
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Replies
Robert,
You have a lot of questions. I'll start with the question implied by your title. The answer is, No, you don't want to use dense-packed cellulose insulation below grade.
Robert,
The wall you describe sounds like a variation on the Thermomass system. I would question the integrity of these thin shells of concrete though, and whether they could work together structurally with such a thick insulation layer at the center. Using cellulose in this type of wall would be a big mistake. XPS is a material to avoid because of its very high global warming potential. Rockwool might work though, and if you're interested in the relationship between density and R-value just look to manufacturer specs (see Roxul and Thermafiber). The highest R-value is at roughly 3-6 PCF density.
You could build a double wythe masonry wall using 4" block or brick and rockwool at the center (say 8" thick to keep the module), with ladder type steel reinforcement. That would be thicker than what you describe but more conventional as a construction method. This wall would have incredible sound transmission specs.
Thank you Martin and Thomas for your responses. Martin is getting used to hearing my constant hairbrained questions. Both of you confirmed my thoughts that below grade cellulose might be a bad idea, but I wanted to hear what the experts thought .
Thomas, I have taken my ideas from the following sites but my proposed modifications were my own. The two major sites I looked at were:
http://www.sipcrete.com/index.htm
http://www.solarcrete.com/insulated-concrete-wall.php
Neither of them proposed the thickness or substitutions which I was considering. Solar Crete does propose a 12 inch wall with a 7 inch core of EPS surrounded by 2 1/2 walls on each side. Please note how they use ties to hold both thin walls together and make the whole serve as one structurally strong wall.
I did like you suggestion about a double wall with rockwool in the center. The major problem with that kind of wall is that it probably precludes using surface bonding as advocated by thenaturalhome.com
I liked surface bonding because it seems to take much less skill than the way a traditional mason would do this.
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Thanks again for your good ideas.
Bob
PS: Thomas, are the problems you mentioned with xps also there with polyso (above grade) and eps? I thought they had reformulated the gases used to make them less of a problem.