[GSBN] [nbne] no Timber frame for Alberta house

Sun Oct 2 23:41:06 UTC 2011

Derek,
I assume UNM.edu is New Mexico. You probably have a lot more winter sun than we Nor'easters do. We can go an entire February without a sunny day and a passive solar strategy just ain't gonna keep a house from freezing - in fact, the additional glazing will make the house cool down much faster.
A house in winter loses heat through all surfaces in direct proportion to delta-T and inverse proportion to R-value. Typical equilibrium soil temperatures under and around an insulated slab or crawl space with some snow cover on the ground are somewhere close to the average of indoor setpoint temp and annual average deep ground temp. And typical delta-T to the ground is about half the average winter delta-T to the air in a cold climate.
So a slab or crawlspace can get by with half the R-value of insulation for the same unit heat loss of a wall, ceiling or raised floor deck. The heat loss to the ground in an earth-coupled foundation is, then, no greater than the heat loss through walls or floor on piers. There is then, no greater occupant discomfort due to floor surface temperatures (though the conductivity of the floor will make a difference) and no greater continual heat loss through an earth-coupled system as through a doubly-well insulated raised floor.
However, the ground-coupled floor system will have the advantage of the stored heat in the soil should the indoor temperature drop below ground temp. At some point in a prolonged power outage or unoccupied house, pipes will freeze if the outdoor air temperature is much below freezing, but the ground-coupled house will take longer to get to that point - all other things (such as insolation) being equal.
- Robert

--- On Sat, 10/1/11, Derek Roff <derek at unm.edu> wrote:

From: Derek Roff <derek at unm.edu>
Subject: Re: [GSBN] [nbne] no Timber frame for Alberta house
To: nbne at lists.riseup.net, "Kelly Lerner" <klerner at one-world-design.com>
Cc: "Global Straw Building Network" <GSBN at sustainablesources.com>, "with public archives) Global Straw Building Network (private" <GSBN at greenbuilder.com>, "Alex Wilson" <alex at atwilson.com>
Date: Saturday, October 1, 2011, 9:48 AM

I had a friend who used to describe an unsatisfying alternative with words like, "This is much better than something that isn't as good."  This was usually good for a smile, and a little further reflection.  I'm a big fan of Kelly Lerner and her work, and her words below remind me of the comparison above.  Kelly says that "ground coupling...is clearly a more energy efficient choice...", and I am left wondering, more efficient than what?  Certainly, coupling a house to moderately cold ground is way more efficient than coupling it to very cold, moving air.  
I wanted to thank Robert for his response last week on this topic, and for posting the graphic on thermoclines.  (I apologize for being slow to respond.)  I look at the same data, and come to a contrasting conclusion.  I agree that a building designer can allow energy to flow from the living space into the earth below a house, and create a bubble of warmer soil, that can help prevent freezing, if the house is unheated for some period.  And I agree that passive survivability (non-freezing) of this sort is important.  However, I'm bothered by two aspects of this approach.  In the normal case, heat is continually drained from the house into the soil, which increases energy consumption and decreases comfort.  Not only are cold floors a common homeowner complaint, but they also cause people to turn up the thermostat, thereby magnifying the energy waste.  This energy drain is continuous, and is therefore poorly matched as a remedy to the rare situation
 of trying to keep the pipes from freezing when the home is unoccupied or experiences a prolonged power failure.  
On the other hand, when a building experiences a long period with no power input, its sub-floor thermal battery of warmed soil has a limited capacity.  This kind of earth coupling runs the risk of not offering sufficient protection in the rare cases when passive survivability is really needed.  The pipes in our house may freeze in a severe weather/power event, and the constant energy drain over the years may not provide the anticipated protection from catastrophic events.  I prefer the solution that Rob Tom already mentioned.  We can design our building so that it is not earth coupled, and leaks only a very small amount of energy into the surrounding soil.  Frost protected foundations can be designed, so that the house and pipes will not freeze, even if the house is never occupied, and never has any power input.  Such designs will be more energy efficient, will be more comfortable, and will offer much more reliable passive survivability.  Oh yeah,
 I'm supposed to say more efficient than what.  More efficient than earth coupling, as the term is being used in this discussion.  In most climates, even during severe weather/power events, intelligent solar design (sun coupling?) combined with excellent insulation can provide not only passive survivability for water pipes, but tolerable human habitation temperatures (50˚F or more) with no power input, beyond the suns rays.  
Best wishes,Derelict
On Sep 19, 2011, at 10:08 AM, Kelly Lerner wrote:
HI all,Just chiming in to say that ground coupling (with insulated crawlspace OR slab on grade with excellent perimeter insulation and insulation below based on climate, soil conditions and heating system) is clearly a more energy efficient choice in a cold climate, because the delta-T at the soil is always going to be less in the winter. That means less heat loss at the floor/foundation. Ground coupling also helps with summer cooling if that is an issue in your climate. 

Derek Roffderek at unm.edu

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