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Old 09-05-13, 09:10 AM   #21
jeff5may
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Quote:
Originally Posted by MN Renovator View Post
What, R10?? How?...You don't gain much here. What really should happen is there should be an insulating layer on either the outside or the inside of the concrete layer to be effective.
OK, let me qualify my statement more clearly.

The OP stated he has hollow block walls with a tiny air gap and 3/8 drywall on the inside. Depending on whether it is skinny, medium, or heavy block, the r-value will fall between 1 and 2. The home is 30 years old, so there will be gaps or cracks in the block that will cause infiltration. He can't get to the top of the walls without tearing off the roof. Money is tight and winter is coming.

Yes, eventually the outer envelope should be improved upon. Wrapping the house with foamboard would take advantage of the thermal mass of the wall and eliminate the need for interior insulation. This involves tearing off siding, inspecting, repairing, insulating, wrapping, sealing, residing, and finishing. When time and money is plentiful, this project would radically reduce the home's heat load and save tons of energy. With great investment comes great gain. Oh, and don't forget the windows and doors.

Insulating the walls from within tackles more than one problem inherent in their design. Block walls are lossy not so much that concrete is a good conductor of heat, but because of the free air inside. Natural convection transfers lots of heat from the inner to the outer surface. Infiltration occurs everywhere it can, aiding this convection from both the inside and outside. Blowing foam into the cavities shuts down the convection cycle and plugs up the infiltration leaks. The worse shape the wall is in (thermally), the better it works.

MN, you are right. This is not a be all, end all solution for insulating the walls. Efficiency levels are not instantly going to become stellar. But it is a good first step that doesn't take long (1-2 days vs. weeks) or cost much ($$$$ vs. $$$$$) to accomplish. And a $400 a month heating bill is better than a $700 a month heating bill.


Last edited by jeff5may; 09-05-13 at 09:40 AM.. Reason: continuity
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