Insulating a block building from the inside
I have read all i could find on insulating a block building. It seems the best method would be to add foam to the outside. Unfortunately this is not in the budget for this project. The only mention of interior insulation seemed to be adding spray foam. The main building for this project is a two story from the 1930s with two later single story additions. Currently the inside has been completely gutted, while we hope to keep the outside intact. It has 3/4″ furring nailed to the block, on 16″ centers, with sheets of foil faced foam on top .The foam is untaped and not done particularly well in the areas that can be seen. Vinyl siding finishes off the assembly.
The insulation approach is further complicated by two other details. The second floor of the original building is supported by essentially rim joists that are bolted directly to the block. I am concerned that spray foaming over them may lead to rot. Secondly, the additions create block wall surfaces that are interior on both sides, on the first floor, but have one exterior and one interior faces on the second floor.
From everything I have read and heard the following seems like the best approach. Frame new walls in front of the interior block 1/2″ back. Spray foam closed cell to a depth of 1 1/2″ in the stud bay. Fill the remaining space in the stud bays with an unfaced batt, either roxul or fiberglass. Do not spray foam over the band joists, just batt over them. I am not sure if it would be advisable to spray foam both sides of the block in those areas of the building where the first floor block is interior on both sides but has an exterior component on the second floor. If one side is foamed and batted could the other side just be batted?
One insulation contractor i spoke with suggested tar papering the interior face of the block, build stud walls, and then use roxul batts only. That certainty seems like a more cost effective approach.
Finally, is the foam insulation on the outside a problem? I was hoping the 3/4″ airspace would provide drying potential if the inside is spray foamed. It seems like there are no great “one size fits all” solutions here. any advice would be greatly appreciated.
Thanks.
noah woodruff
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Replies
Noah,
It's always a good idea to tell us your geographical location or climate zone. That helps us give you good advice.
The concrete blocks are not susceptible to rot, so it's OK if the blocks have foam insulation on both sides.
If you have good water management details on the exterior -- window flashing, for example, and adequate roof overhangs -- there is no reason to believe that your concrete blocks will get wet. So I wouldn't worry too much about your rim joists. They should be fine, even if they are insulated on the interior with spray foam.
It's unclear from your description whether the exterior rigid foam is continuous, or whether it is cut into narrow rectangles that are inserted between the furring strips. Continuous rigid foam would be better, of course.
Two inches of closed-cell spray foam will give you about R-12 or R-13 -- not much for a cold climate, but adequate for the warmer areas of the U.S. More insulation would be better.
If you are insulating on the interior of the concrete walls, the two best options are spray foam or rigid foam. Either will work. You can use fluffy insulation on the interior side of the foam insulation layer if you want. Pay attention to transitions -- you want a continuous thermal envelope if possible. Tricky areas are intersections with partitions and the intersection of the walls and the insulated ceilings.
You probably also want to investigate whether the floor slab is insulated. Ideally, you will install vertical insulation at the perimeter of the slab if there isn't any insulation there.
Mr. Holladay thanks for the help. To clarify:
1. The building is in climate zone 5
2. The foam insulation on the exterior is on top of the furring strips.
If spray foam turns out not to be in the budget, for the interior, is one type of rigid foam better then another? Also, one insulation contractor suggested tar paper on the block and then roxul batts. Is that a viable alternative to foam? Thanks again for the help.
Noah,
Q. "If spray foam turns out not to be in the budget, for the interior, is one type of rigid foam better then another?"
A. Any one of the three main types of rigid foam can be used, although most green builders avoid the use of XPS, which is manufactured with a blowing agent that has a high global warming potential. For more information, see Choosing Rigid Foam.
Q. "Also, one insulation contractor suggested tar paper on the block and then Roxul batts. Is that a viable alternative to foam?"
A. That suggestion would probably work, especially since you have a layer of continuous exterior foam to reduce inward solar vapor drive.