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Mixing wall types (CMU and wood framing), comfort and acoustics?

James Howison | Posted in General Questions on

HI all,

I’m considering a renovation/extension on a house built slab on grade with CMU walls in Austin, TX. The house was built around 1950, and the CMU block is pretty dense, much higher fill than modern CMU.

The plan is to use a “perfect wall” approach adding insulation/weather/rainscreen/siding outside the block. Think at least Pretty Good House insulation levels, perhaps PHIUS2015+.

There are two basic options (perhaps three) for the structure in the extension, wood framing, modern CMU (and as the perhaps, AAC block). Whatever was chosen would have the same insulation,weather/air barrier, rainscreen, and siding. FWIW, the inside will likely be american clay (think breathable plaster), which could be directly applied to CMU or AAC block, but if wooden framing would require drywall, so that is one reason to buck the construction trends here and consider CMU or even AAC.

I’d like the house to feel cohesive, so my question is about whether there are any comfort and/or acoustic concerns with the different structure options. For example, does the increased thermal mass of the current walls in one part of the house mean that it will heat/cool/dehumidify differently than the extension. Could having serious thermal mass in one part of the house, but not in the other cause issues with an HVAC design?

Would the house feel really different, acoustically? I get that the insulation on the outside will absorb exterior noise even on a wood framed wall, so the difference might not be dramatic.

If there are substantial differences would it not be avoidable, given that we can’t match the existing CMU (it being so much denser than that available now). So while there would be differences between the old and the new, they are basically unavoidable, whether we were to use modern CMU, AAC, or wood framing for the extension.

I hope that makes sense, happy to clarify anything.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    James,
    Q. "Does the increased thermal mass of the current walls in one part of the house mean that it will heat/cool/dehumidify differently than the extension?"

    A. Not in a significant way -- as long as the addition is built to a high standard (with attention to air sealing and with above-code levels of insulation).

    Q. "Could having serious thermal mass in one part of the house, but not in the other cause issues with an HVAC design?"

    A. No -- again, assuming that the addition is built to a high standard, and that the HVAC system is designed by someone who knows what they are doing.

    Q. "Would the house feel really different, acoustically?"

    A. No. Again, attention to air sealing and the inclusion of above-code levels of insulation will reduce noise transmission compared to conventional construction.

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