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cost/benefit analysis

by pk last modified Aug 07, 2005 04:35 PM
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cost/benefit analysis

Posted by pk at August 07. 2005

In health care, lots of time and money are spent on what's called cost/benefit analysis. You weigh costs of a particular treatment modailty against possible outcomes to choose the most cost effective approach to achieve the desired outcome.

So, anything like that available for materials used in building? For example an ICF system by a local distributer American Polysteel Austin ( vs. AAC by out of San Antionio vs. lets say, some locally available SIP system. Has the greenbuilder program come up with anything like that?

AND has anyone decided once and for all whether it is best to have adequate cross-ventilation and a thermal chimney vs. an air tight envelope with a whole house fan for dehumidifying? These two methods seem directly counter to one another. So which is best for our area? I'll bet it's option 2.

Then there's the dilema of thermal mass/passive solar design. Seems the only thermal mass that would be any benefit in our climate is the (shaded) concrete slab that can draw some coolness from the earth? Or a slab on a cistern with cooling coming from the water stored beneath.

Vertical walls and roofs that are exposed to direct sunlight would need to be super-insulated on the interior, highly reflective on the exterior, and have the least amount of mass possible...correct? Rammed earth/concrete would work with walls that were interior or well shaded, but not if they are exposed to sunlight.

Re: cost/benefit analysis

Posted by Mark Meyer at August 07. 2005

The cost benefit analysis would be a great thing to have at hand, but sadly when it comes to materials, a lot of the cost comes from what you ask the materials to do. AAC and contec aren't going to be the best way to make a wall that calls for large unbroken expanses of glass, as they are massive materials, and massive walls usually want to have smaller windows and openings. SIPs on the otehr hand can do a lot of differnt things, but also give an entirely differnt effect (which is just as important in architecture as how much it costs) If the idea of thick walls with built-in window seats floats your boat, then SIPs might not be the way to go (at least for that particular wall, but don't be afraid to mix and match and do AAC on some special interior walls, and a SIP shell to encase the rest of the building.)

As far as passive ventilation in Austin is concerned, keep in mind that the yearly average temp is 68 degrees. This is well within the comfort range of most mortal humans, and coupled with passive venting/thermal chimney/capturing of breezes, is perfect for 9 months out of the year. The other three terribly hot and humid months you need to rely on mechanical AC and dehumidification (a part of the HVAC system). In reality the best solution for the Austin area is a hybrid, with a tight and well insulated shell (for when it is hot and humid or those few weeks when we need heat) that has adequately oriented and shaded fenestration (and a thermal chimney/operable clerestories). This is all doable and within a reasonable budget.

Thermal mass in Autsin is a bit tricky. There are a few weeks during the winter that you might want low angle winter sunlight to be admitted into the depths of your living space, but we also have 80 degree days in the middle of the winter, so in my mind thermal mass is best used as thermal inertia on the interior of a space, used to temper the diurnal temperature swings of our climate. You do NOT want a bunch of thermal mass exposed to the exterior temps at any point during the year, that is for sure.

Mark Meyer

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