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.