Adding thermal mass to “sun-tempered” home

We have a 1915 balloon-framed house that includes a south facing three window bay in the living room; right now we have a trapezoidal (14” wide, 42 on the short side, 94 on the long) wood table filling the gap between the back of the couch and the bay windows.  This is our elderly cat’s favorite spot, of course.

It occurred to me that with the light pouring in through those windows it might make sense to add some thermal mass where the table is now.  The two things I thought of were a stone topped table ($$ to cut to a trapezoid, and heavy) or an oval stock tank filled with water and topped with black sheet metal (but the idea of ~80 gallons of water leaking at some point is not a calming one, and the lid would need to be sealed to the tank somehow to prevent evaporation, mold etc.)

Any ideas?

Your idea has merits, but I am concerned about your fears!
Why would a tank leak? Evaporate or grow monsters!
Weight of stone top too much?

Thermal mass by definition is heavy that is how it works.
Pave floor, improve glazing, insulate floor
By the way, what is this?

 balloon-framed house

John C Daley wrote:Your idea has merits, but I am concerned about your fears!
Why would a tank leak? Evaporate or grow monsters!
Weight of stone top too much?

Thermal mass by definition is heavy that is how it works.
Pave floor, improve glazing, insulate floor
By the way, what is this?

 balloon-framed house

Balloon framing was how houses were constructed in much of the US prior to modern stick construction (and after timber framing).  Balloon framed houses have studs that go from the bottom of the first floor, to the top of the top floor, uninterrupted by a fire stop.  Cheap and easy to build (as long as 16’+ 2x4s were cheap) but fires can spread rapidly.

Since posting I have seen some 10 gallon cylinder tanks with inlet and outlet at the top; until I found those I was thinking of tanks with an outlet at the bottom, failure of which could be catastrophic.  So possibly 3 of those with a slab of stone resting on top would work?

The floor and will remain hardwood (the original century+ old oak!).  Walls and attic are as insulated (and air sealed) as we can get them.  The windows are double glazed and we have cellular (honeycomb) shades with side tracks on order, to keep the day’s heat in at night.  

A cement tabletop could be DIYed in any shape, much cheaper than cutting stone. It could be made as thick as you think your floor will support.. much of it could probably be filled with larger stones to save cost and environmental footprint, since it is not very structural..

Or large tiles or pavers or even metal as a tabletop on a bin full of sand.. much easier to move.. no risk of leaks.. I think key here would be having somewhat decent thermal transfer from the tabletop in direct sun, to the rest of the mass..

One could get fancy and put some hollow tubes through this to speed thermal transfer..

Nicole, thanks!  I had not thought of concrete and I do have a spare bag of concrete mix.  My hesitation with masonry is twofold - the weight (my house is definitely not overbuilt…) and the heat transfer rate.  What I have read says that water stores about twice as much heat per pound, and heats up more effectively because convection Carrie’s the heat around faster than conduction through a solid block of masonry (or sand, etc.)

I think I may need to calculate the maximum possible heat gain is based on window size and then see how much thermal mass would be needed - no point in over-sizing it.

The purpose of thermal mass in the living space is to stabilize room temperatures by storing excess heat (and only during the heating season) that is causing uncomfortable overheating, so that the heat is then released later (hours) when the temperature in the space drops to below the temperature of the thermal mass.  It is also not as simple to implement as it may seem.  So.......if the space is not overheating during typical winter/heating season days, then there is no reason to add thermal mass (or even to more simply just circulate the warm room air to heat other rooms) and it would be a waste of time and resources to add thermal mass.  Besides, you are likely to create a "Three Bears" scenario for your cat.  Stone might be too cold (especially in the morning) and can literally suck heat from your cat as well as from sunlight, and black metal would absorb more light instead of reflecting it away to be absorbed by other surfaces in the room, so it could get too hot.  Your cat seem think that right now it is just right, and loves it the way it is.......don't spoil that. ;O)

- Retired designer of passive solar homes -  

Bricks lined along the floor under the table that was mentioned or  against a wall might work for thermal mass for winter and removed during the summer.

Laren Corie wrote: The purpose of thermal mass in the living space is to stabilize room temperatures by storing excess heat ... that is causing uncomfortable overheating, so that the heat is then released later (hours) when the temperature in the space drops to below the temperature of the thermal mass.  

I have found in homes with little or no thermal mass (think stick built), the furnace in winter and the air conditioner in summer, cycles between too cold and too hot, at least partly because of the air flow. So my experience is that adding thermal mass helps to dampen those cycles, rather than it just storing excess heat. Having thermal mass radiating even a little heat, seems more pleasant than vents "blowing" heat.

In my stick-built home the season matters a lot. Around the summer solstice, the sun is so high that even though it's warm outside, the sun doesn't shine into the windows that much. However, come late August and early September, we get a lot of solar gain in the living room with no good way to spread that gain from daytime to evening. In winter, we get too much cloud for solar gain to be useful.

Jay, thought of using a solar air heater?

John C Daley wrote:Jay, thought of using a solar air heater?

Yes, but we have the wrong type of windows to work easily, so with the significant cloud cover in the months we'd want it the most, it hasn't been worth redesigning things.

We heat with wood, which grows like weeds in this area! It will take us years just to go through the trees that Mother Nature has knocked down on our land. I thought of getting some coppicing going for firewood, only to decide that I'll be pushing up daisies before we'd actually need the wood.