I live in Los Angeles and have 2 units on my property. I use the front one as my office. I had the floors done years ago and thought I would get tile and have a subfloor of cement put in so that I would have a lot of thermal mass to keep it from getting hot. Sadly, I think it's worse than it was before. The main window that points south got changed too to Low E and it heats up early. Some light comes in from east facing windows but not much. The house has a garage underneath.

Here are next steps I am considering:

  • insulate the floor: there are beams in the garage, so if I filled them with bats then closed them, that would make a total area that would be VERY deep and should be very high R value
  • take other measures to prevent sun in the east-facing windows (like time-controlled electric shades)
  • start keeping the garage closed (though it's open now if you go in it even on a hot day it's not really hot as it has one foot thick walls on all sides and is built into a hill)
  • put in a solar-powered roof deck attic fan (should have done this a long time ago)

BTW, the roof is a composite and has solar panels on it which I thought would also lower heat buildup.

  • 1
    Heat rises, so if there is no insulation between the hot garage and your living area, that could certainly be contributing. Your current floors are likely absorbing heat from the garage and radiating it into the living space--which is likely the opposite of what you wanted.
    – DA01
    Feb 18, 2015 at 0:06
  • But @DAO1, the garage is not hot.
    – Rob
    Apr 24, 2016 at 16:25
  • IIRC, it gets hot in LA. But I admit I don't hang out there a whole lot. :)
    – DA01
    Apr 24, 2016 at 18:02

1 Answer 1


Thermal mass does not block heat; it stores and releases heat, and it slows down the flow of heat. It will absorb heat when it is colder than the surrounding area, and it will release heat when it is warmer than the surrounding area. These characteristics are useful in the following circumstances:

  1. A thermal mass wall in a climate/season where the outdoor temperatures cycle above and below the indoor comfort point. The heavy walls will slow down the flow of heat so much that by the time it reaches the interior, the outside temperature will have dropped, reversing the flow of heat out of the wall. In order for this to work, the wall must be very dense and very thick; 12" thick rammed earth or adobe, for example.

  2. A thermal mass element within a building's insulated envelope. Such a thing can be used to store free heat generated from sunlight, cooling, electricity usage, etc and store them until the sun goes down and everybody is asleep, whereupon it will release it, moderating the interior temperature and reducing its daily fluctuations, especially at night.

Insulating the underside of the floor is a great idea, as it will make your thermal mass floor fall into category 2.

Preventing the sun from hitting your walls or windows is likewise always a good idea; heat gain is driven almost entirely by direct solar gain.

Keeping the garage door closed may or may not help; try it and see.

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