I am contemplating building a house in a heating climate (NY state, 4800 HDD), and would like to increase the thermal mass at low cost while minimizing interior design constraints and space limitations. Adding thermal mass material to the inside of interior drywall walls would be appealing. Practically speaking, can one put wallboard scraps, broken brick or cement block pieces, or even local small stones into the interior space between the two wallboards of interior walls? Do any important constraints exist, e.g. is wallboard unable to handle the additional weight stress, will the interior walls bow out and/or split open, do any of these materials attract surface condensation and become a mold medium, etc.?
If practical, this low tech idea could add thermal mass while lowering the amount of construction waste.
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