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Thermal mass working against me?  RSS feed

 
Matthew Tillman
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Hey yall. I've been lurking around for awhile.

Within the next year I am planning on buying 15-20 acres and homesteading/building for retirement.

I live in South Texas, where it gets HOT, but doesn't always cool off at night like more arid deserts. We can reach 105-110 during the day and only cool down to upper 90s at night sometimes. Typical is more like 98/76 though.

Will the thermal mass of a Cob house work against me? Is the night time temperature too high to effectively allow the mass to cool?

We will absolutely need some type of heating\cooling. Will the hot thermal mass counter my attempts of cooling the house down?

Many thanks!
Matthew
 
John Elliott
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Lots of thermal mass is what they use in the Sahara desert to keep (relatively) cool. By the time the middle of the wall has fully heated up from the day's relentless sun, that sun has set and the wall begins to reradiate the heat. If the wall is thick enough, the heat on the surface of it doesn't have time to make it all the way through. So yes, thermal mass will work for you.

Would you be interested in a solar A/C system? The more the sun beats down on the solar panels, the more the coolant cycles, and the more heat that the coolant loop (inside the house) will pick up and reject to the outside. No compressor, no moving parts (well, except for a fan to blow over the coolant loop), sealed system that doesn't need annual maintenance and will last decades? If so, send me a P/M.
 
Brian Knight
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Location: Asheville NC
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Will the hot thermal mass counter my attempts of cooling the house down?

Yes!

There is some good research that looks into all this so there is no need to reinvent the wheel unless you just dont believe its true. The only type of climate where a purely thermal mass wall (no insulation) works are high elevation deserts with wide diurnal temp swings yet fairly consistent temps throughout the year. Does such a climate even exist anymore? ORNL, who I think has done the main body of research suggests Phoenix AZ and Bakersfield CA as examples but even there I have my doubts.

Outside of those climates, thermal mass should be completely inside the building envelope (air and thermal barriers) to have any benefit. Its very debatable if adding thermal mass beyond what is needed anyway is cost-effective.

In South Texas (US building climate zone 2) the temps wont cool off enough at night to swing the temps back to your favor for cooling. Even your coolest listed nights of 76 wont cut it. Your house will be much more comfortable, cheaper to condition, and more durable if you follow the IECC 2012 table 402.2 for prescriptive R values. This is the MINIMUM for most concerned with building performance. More R= more comfort, less energy costs.

This current energy code calls for a blower door test of at least 5 ACH50 in your climate zone. This requirement is sure to be lowered in the upcoming 2015 version and I would shoot for an ACH50 of 1.5 or below. All other things being average, this is the cheapest and most effective path to staying comfortable and having low energy/environmental costs. Less air infiltration= more comfort, less energy costs. The grand caveat of this paragraph is that below 5 ACH50 most experts think you should have a mechanical system for fresh air introduction. I think if youre concerned about Indoor Air Quality, you should have that anyway.

Along with this "modern building science" advice, take some tips from the old timers that didnt have AC: Mass the shape longer from East to West to minimize surface area in those directions and avoid glazing (windows) on those elevations if they arent shaded by a big porch (still avoid any west windows). Landscape to shade these walls and roof if possible. Build big overhangs with highly reflective roofing. Roof insulation is your friend

 
Matthew Tillman
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Many thanks, I figured that was the case. Looks like strawbale will be the direction I head.
 
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