How to insulate a block house?
repoocyaj
| Posted in General Questions on
i am rebuilding a small block house with a full basement in Northern Ohio. After researching the forum it seems like the consensus is to insulate the outside. My concern is that moisture in the walls from the ground or footings/slab will need to escape to the inside of the house since the rigid insulation XPS will create a vapor barrier on the outside. Any feedback or suggestions will be helpful. Jay
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Jay,
If your house has high interior moisture, the moisture is already in the interior of your house. That's a problem. The way to address this problem is to reduce the rate of moisture entry into your house.
If your basement is wet, fix your basement. This article will tell you what you need to do: Fixing a Wet Basement.
Insulating from the outside is the right way to go, and does not add to the moisture problems. It's likely that most of the moisture drive through the wall is due to rain and dew wetting of the exterior, which goes away if you install exterior foam and do the window & door flashing correctly.
Using XPS is not the greenest way to go, since it is blown with HFC134a, which has a very high lifecycle greenhouse gas potential. EPS or polyisocyanurate are blown with pentane, and are much more benign.
Polyiso has a higher R/inch, but has a severe temperature derating for the outer fraction during the wintertime extremes. In an OH climate with an all-foam insulation solution you would get better performance out of it if you make the outer 25-30% of the total thickness using EPS, which increases in performance at colder temps.
If you're taking it to code-min, in most of OH (US climate zone 5A), on the exterior of a concrete or cinder block wall you would only need R13, which could be 1.5" of polyiso + 1" of EPS for a total foam stackup of 2.5". Of course more is better. 2" of polyiso + 1.5" of EPS would put you at a continuous R18+.
If you're still sweating the moisture issue and want to maximize drying toward the exterior, 3" of rigid rock wool gets you pretty close to code-min. at R12m which is "in the noise" of as-constructed code-min performance and at 30 perms, delivers HUGE drying potential toward the exterior: http://www.roxul.com/files/RX-NA_EN/pdf/tech%20data/ComfortBoard%20IS.pdf
That 30 perms offers 6-10x faster drying than through standard interior latex paint, but if the block is painted on the exterior with multiple layers of exterior paint (likely) it's probably already less than 1 perm, and any foam you put out there is inconsequential to the drying paths & drying rates. But with the insulation on the exterior the it will stay warmer (=drier) year round, and there won't be bulk wetting from rain to find all of the cracks to seep into.
Thanks for the response. This helps a lot.and a better result then sorting through forums. If I leave the block uncovered on the inside and do a stucco finish then it should contribute to thermal mass, no?
Jay,
Q. "If I leave the block uncovered on the inside and do a stucco finish then it should contribute to thermal mass, no?"
A. Yes, but you may be overestimating the advantages of interior thermal mass in your climate. For more information on this topic, see All About Thermal Mass.
Thanks Martin, I read the article and have a better understanding of thermal mass. In discussion of insulating block on the outside of the house, would it be a good idea to excavate and insulate to the bottom of the wall? I plan some excavation on the south side since it partially daylights to that side and I am trying to get some more solar gain.
If you can dig down and insulate to the footings (or at least 2' below grade) that would be the right thing to do when insulating on the exterior.
Having the thermal mass exposed to the interior does add to both comfort and modestly lower energy use both in summer & winter, but the affects of the additional thermal mass of a stucco or plaster applied directly to the CMU would be very difficult even in a lab. Having a modest amount of R value between the thermal mass and conditioned space even helps the energy use figures, since it the thermal mass then goes through larger temperature cycles, storing and releasing more heat. Having a modest amount of R between the conditioned space and mass also allows you to use overnight setback strategies effectively. But if the most of the R is on the interior side of the mass, the thermal mass benefits are slim to none.