Drywall to Increase Thermal Mass for Passive House Construction
In a passive house, thermal mass is your friend. The more of it that you have, provided it is thermally broken, the better you are off. Thermal mass acts as a battery, or at least a capacitor, of temperature.
The interior and surface temperatures of everything inside the conditioned space will work to stabilizing the air interior temperature inside the building. This works in both heating and cooling seasons.
During the heating season, free solar energy (heat) that is harvested through high-heat gain (SGHC) windows will "heat-up" the temperatures all the thermal mass that is directly in contact with the sun's rays and indirectly with the other thermal mass that benefits from the increased interior air temperatures that also occur.
In a properly constructed passive house, there will be very little temperature differences between the interior thermal mass of the building and the building's conditioned air, with the exception of the windows and doors that "essentially act as thermal bridges" to the outside of the building envelope, since their U-values (U-factors) are substantially less.
We selected 5/8 inch drywall to increase the thermal mass in the house, not the typical 1/2 inch stuff. Yes the drywall was more expensive than the thinner sheets as well as the drywall installation costs (it's a bear to work with), but it offers you some distinct advantages:
- 5/8" Drywall providers superior thermal mass
- 5/8" Drywall provides superior fire-resistance (as 5/8" drywall is fire-rated)
- 5/8" Drywall provides superior sound-deadening between rooms
- 5/8" Drywall provides superior strength
As part of our air-sealing efforts, we had the installation of our drywall broken into two phases. The first phase was the ceilings into the unconditioned attic spaces immediately below our airtight vapor barrier membrane and one side-only of the interior studded walls. No drywall was installed on any exterior wall of the building. The second phase of drywall installation occurred after all air-sealing, insulation and sound-deadening from sprayed cellulose was completed. We choose to insulate all interior walls as well, primarily for sound insulating purposes, increase fire-resistance, and a modest increase in additional thermal mass.
Fortunately for us, we discovered that the drywall crew damaged the membrane by missing trusses, with drywall screwing, and tearing the membrane at certain points. This we knew because we could feel the air leaks occurring at the edges of the drywall and other "penetrations," such as the recessed light cans, that up until that time were not there. The only way to seal those failures was to address the problems at their source, above the membrane and drywall from the attic. We located the obvious failures ourselves, but ultimately had to rely on a professional spray foam crew to "flash" small amounts of spray foam along all areas of truss membrane contact. We believe it is much more cost effective to use any foam in this manner, because using a lot of the material becomes very expensive quickly.
While we were at it, the recessed cans, that were IC (insulation contact) AT ("air-tight"), were skinned with foam as well even though they were already taped completely.
Note: More recently, some, including PHIUS, have raised concerns about the use of certain types of spray foam because of their potential for contribution to global warming (GWP) and subsequent off-gassing. Others have suggested that improper application may lead to problems after the fact. In light of these developments, I would suggest if you choose to use spray foam in some manner, carefully select a company that knows what it is doing and, as we ultimately did, use it sparingly. It will also save you a lot of money and there are other ways to achieve a vapor barrier where you need it, as we did with a special membrane.