Adding thermal mass to lightly framed stud walls

A very simple way to add thermal mass is to pour sand or gravel into a hole in the drywall near the top of any interior/uninsulated wall. On a house framed on 16 inch centers with 2 x 4 studs, one stud space producing and 8 foot column of sand mixed with pea gravel will weigh approximately 450 pounds. So it would be very important to only fill walls that are really well supported from beneath. An 8 foot high by 8 foot long section of wall filled with this material will weigh approximately 2700 pounds.  The best candidate walls for this infill would be those which are adjacent to south facing windows or those that are beside the wood stove or other heat source. Walls that are heated with sunlight will absorb more if they are painted black or glazed so that sunlight can heat the thermal mass directly. Thermal curtains hung on these walls would help to store heat which is saved during the late afternoon. The curtain can be removed in the evening when heat is needed.    Don't forget to check for good support!

    Does anyone else have low-tech ways of improving the thermal mass characteristics for stick framed homes?

I dont know where you heard about this "brilliant" idea but i hope you haven't actually tried it. While in theory it would work, there are too many hazards involved. That 450 lbs of sand and gravel, to say nothing of what it might do to electrical wiring/outlets, will not only push down on the sole plate, it will push out on the bottom of the drywall, and eventually blow out, later if not sooner. Thats one heck of a mess to clean up, with numerous dangerous situations.

A better and easier way to add thermal mass would be to place metal drums of water where they can collect solar heat, or add a "raised" platform out of brick or poured adobe or concrete in front of a bank of south windows. Another option would be to build a 3 or 4 foot high wall in front of said windows, or even a cool indoor planter bed with stone or cob or adobe or concrete, similar to the earthship style. How about installing a Trombe wall?

Also it's worth noting sand makes poor thermal mass because it is mostly silicon (i.e. glass... an insulator). It doesn't take in nor give up temperature very well. Plaster, stone, concrete, clay, adobe, and such would be better choices for thermal mass.

      I've seen this done in a situation that lasted for many years until I demolished the building with no blowout situations. It was well supported from beneath and they didn't fill stud spaces containing electrical outlets although there were wires within the wall. The wall in question was filled with sand and pea gravel.

 Cob is usually about 75% sand. Where I live Sand is mostly granite and other glacial till and weighs about 120 pounds per cubic foot. Sand has the same heat capacity as cob which is  .2   

    Glass is an electrical insulator but not a very good thermal insulator. It stores just as much heat per pound as does cob and speed of release is not very important if you have tons of mass. If a mix of sand, pea gravel and fist sized granite or other heavy igneous rock is used the mix weighs approximately 60 pounds more per cubic foot than does cob so you get about 35% more total heat absorption.

     I demolish buildings for a living and have also built many. I'm familiar with how much framing is required to support bathtubs and hot tubs. A big pile of sand is no different.  If I thought I couldn't do something as simple as this without screwing up, I would end it now

Some thoughts.....
I too am concerned about the drywall blowing out at the bottom. That would be ugly! Whether it would do this or not would (I think) depend on a variety of factors like the type of fasteners used (nails or screws), the number used, stud spacing, stud width, thickness of the drywall, and I'm thinking the humidity in your location. Drywall bends very easily under a load.....particularly when moist.
If the other suggestions (drums of water, brick planter, etc.) are not an option, one possibility is to add a small amount of dry Portland cement to the sand mix which would bond the sand particles over time. I live in southern Oregon, which has a very dry climate. Several bags of concrete in my garage began to set up last winter just from the ambient moisture in the air. You'll want to make sure all holes are sealed like around and in electrical boxes.
Or you could add another layer of (5/8") drywall to either side and use screws spaced 6" apart that penetrated at least an inch into the stud. That should do it. The added drywall would add thermal mass as well.
Or you could just add extra fasteners to the existing drywall, particularly in the lower half of the wall and baseboard.....and take a chance!

I have a book called "The Secure Home" and it talks about gravel filled walls for bullet proofing... It says outright that a single layer of drywall won't hold it and recommends plywood for strength and a layer of drywall over it for the finish.

I second the notion that you should check your load-bearing capabilities before you do something like this, even on an exterior wall.

I've become enamored with turning stud walls into gabion walls since starting this 2.5 years ago. Wire attached to the studs with fence staples holds the rocks. This allows heat storage and transfer along with some air movement. With the right sized aggregate, light and sound penetration are not a problem.

It still needs good support. Check out the weights in the opening comment.

Dale Hodgins wrote:I've become enamored with turning stud walls into gabion walls since starting this 2.5 years ago. Wire attached to the studs with fence staples holds the rocks. This allows heat storage and transfer along with some air movement. With the right sized aggregate, light and sound penetration are not a problem.

It still needs good support. Check out the weights in the opening comment.

With the right wire and rocks, that would be downright trendy.

Thanks R. This could be done to walls that are near a stove. After the stud spaces are filled, 4 inch slices of sandstone or slate could be wired over the studs. They could be bedded in fire clay and joints could be filled. Heat shielding and storage in a fraction of the time needed to build a masonry wall.

Dale, et al sorry old thread, but can't find much info)

I have good diurnal temp swings (warmer days and cooler nights in Northern California) and I'm putting in a high efficiency wood burning fireplace.
I was interested in stapling chickenwire to the 2x4 framing 16" OC to the interior bedroom walls; fill with 3/4" drain rock and then cover with 5/8" inch drywall.
My theory is the blowers off the fireplace would push the heat into bedrooms and be soaked up by the walls and given off at night.  So I would run the wood burning stove/fireplace at the higher (more efficient and cleaner) temp for a little longer, but then I would not need to run it all night at the (less efficient and dirtier) low air setting.

I will talk to the contractor about the weight load on the TJIs.

So a bunch of questions: Is there a right amount of mass?  Would 4' high of drain rock work as well as 8' high?  Any issues with condensation leading to mold?  Does drywall become an insulator and not make this very effective?
It seems weird that no one talks about this which then worries me that it is a bad idea.

Thanks,
Patrick

I didn't know what a gabion wall was, so I googled it.

WOW, there are $THOUSANDS$ of dollars of rock in some of those!

like the idea of gabion walls, double these, a bit of mortar and plastering on them... and insulated between them would be an excellent thermal mass south wall.

for what you are talking about in the OP i think the best solution would be light clay straw

Adding thermal mass to building walls to provide a dampening effect on temperature swings within the building can be both effective and sustainable if adequately designed.  One of the most vexing design challenges is maintaining a high performance thermal envelope while incorporating a functional dampening system via thermal mass.  Because most thermal mass systems must function as a "black body" to achieve their desired dampening effect, they cannot be insulated; they must be designed to emit as much as they absorb.  They can therefore not be constructed as an exterior wall and function as a dampening system with any appreciable effect.  Traditional wall sections for functional dampening systems generally include a winter sun-facing exterior wall glazed with low iron, uncoated lights, an interior thermal mass system, and an environmentally controlled interior space between the exterior glazed wall and interior thermal mass system.  The exterior glazing is generally 100% of the non-structural portion of the exterior wall, shaded by an overhang to decrease summertime heat gain.  The environmentally controlled interior space is generally continuously insulated to the lowest practical U-value on all non-glazed and non-thermally absorptive surfaces and finished, and temperature sensitive dampers are used to transfer heat from the controlled interior space through the thermal mass system to well insulated living areas.

I have designed 37 of these types of systems, and have been mostly successful using the above design parameters as a basis.  One of the most successful was an unheated greenhouse retrofit where we used water filled 3" X 72" re-purposed schedule 40 PVC pipes to provide thermal mass.  Water is by far the best material for use in these types of systems in my opinion.  The greenhouse maintains a reliable night time temperature gradient 12F above exterior temperatures, even when exterior temperatures fall below 32F.

Simply filling stud bays with loose granular material as a stand-alone system will provide negligible dampening results, if anything, if located in an interior wall.  If stud bays on an exterior wall are filled in this way, it will likely produce an effect opposite of what is desired, especially in winter, and will probably provide the added nuisance of organic growth within the cavity.   Not recommended.