A slab in the dark- Cooling in a humid climate

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A slab in the dark- Cooling in a humid climate

Postby guzzihead » Wed Aug 04, 2010 7:29 pm


A quick question about using thermal mass in hot humid climates. We are building a house in Japan so we are faced with very cool winters (compared to Canada) with peak low temperatures of -5 C (peak highs of 15 C) for about a month, and hot humid weather 25- 33 peak highs ( 18- 24 peak lows) relative humidity will be constant around 80% for about 2 months. The rest of the year it is fairly comfortable. The house will be small (180 square meters) with a walk out basement facing south, with a balcony above for summer shade. There are no obstructions to the south for solar heating in winter and the windows will be shaded with an sun sail / trellis with plants for the summer. The house will be well insulated with 300 mm of Cellulose insulation in wall and ceiling. Windows will be double glazed argon filled. East and West windows will be very minimal 15% of total. North Windows 3 %. All South facing windows will be shaded during the hot season.

My questions are:

The slab and bermed walls are concrete, but should the main floor be a concrete slab or wood? Is there too much thermal mass in this house for the summer? The house will often be empty during the day for the summer (with windows closed and shaded) Air conditioning will often be used to control humidity.

The house is on sandy soil, but Japan experiences heavy rain in the spring and fall. Should the slab be insulated, or the slab perimeter, or both? with the added question of how to protect any insulation that is in contact with the ground from termite attack?

Thanks in advance for any suggestions you may have.

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Joined: Wed Aug 04, 2010 6:55 pm

Re: A slab in the dark- Cooling in a humid climate

Postby Albrecht Stoecklein » Sat Aug 07, 2010 7:25 pm

Hi guzzihead,

Seems you are doing everything right. Very good insulation, good window type and size, good orientation. So you are certainly on the right track.

Now to answer your questions:

I don't think there will be too much thermal mass if you have concrete in the main floor. I say this because you have very good insulation levels. If insulation would be any less the house may loose the stored heat from the thermal mass and you would have to heat this mass with your heating system. So the question is less whether there is too much mass in the summer rather than whether there is too much mass in the winter and your heating system would have to heat the cooled-down mass rather than the room air.

For these reasons it is quite important to insulate the slab. If you practically can do it you should also insulate the edges (As you have probably read on this Message Board, that it still a much debated topic...). The wetter the soil is the lower is its insulation value. Although sand would probably dry out quite quickly compared to other soil types if there is a lot of rain in fall and spring it is worth insulating.

I am not familiar with termites at all (lucky New Zealand! :P ). As you probably know they have big termite issues in Australia. There may be some Australian website which give an indication of termite resistant materials. Maybe you find something in the Australian Building Code regarding that. http://www.abcb.gov.au/

I would also insulate the underside of the main floor above the basement. That allows you to target your heating, in particular in the winter, and the basement has sufficient mass anyway not to require the mass of the main floor. So if the basement is unoccupied for long periods you could heat upstairs only.

Maybe you want to increase your East facing windows a bit to capture early morning sun which penetrates deep into the building. That depends of course on the surroundings.

I hope that helps.

Sunny regards

Albrecht Stoecklein
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Joined: Sat Jun 13, 2009 5:30 pm

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