Quote:
Originally Posted by Biggd
(Post 305714)
There are pros and cons to the super insulated house. I built one and you end up opening windows to let it air out in the winter. Cook a fish and you will smell it all winter. Just like a living animal a house has to breath. If it can't it's not healthy. I wouldn't build one like that again....[/URL]
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First, the notion that "a house has to breathe" is not right. The people inside have to breathe, while the house's shell has to avoid moisture accumulation problems. Early attempts at building a superinsulated house didn't get it quite right. The arab oil embargo of 1973, which drove up the cost of heating fuels, resulted in attempts to improve insulation and tighten up the shell, recognizing that air leakage can run up the heating bill dramatically and also make the inside cold, drafty, and exceedingly dry. Our grandfathers knew that; remember folded strips of newspaper around the edges of doors and windows?
Air leakage is driven by pressure difference, caused by wind and temperature difference. Air at zero (F) is about 15% more dense than air at 70. Thus a house leaks most when it's bitter cold and windy and practically not at all when the air is mild and windless. You can't design in any rate of leakage that is right for any set of conditions, meaning leakage as a way of providing fresh air for occupants is wrong practically all the time. Further, you can't control it. It's either too much, even in winter, making the place uncomfortable and hard to heat, or it's too little, making for a stuffy house with interior humidity too high for the health of either the house or its occupants.
As exterior air leaks into an "ordinary" house, the walls start to accumulate dust and pollen, adding to dead insects and perhaps rodent droppings. Do you really want your "fresh" air filtered through that?
The only solution that really works for house, occupants, and heating is to make the shell very, very tight and provide mechanical ventilation. In our climate, ventilation is best done with an air-to-air heat exchanger (heat recovery ventilator, or HRV). Incoming fresh air is warmed up by inside air being vented from bathrooms and kitchen (not near the range). Early attempts at superinsulated, very tight shells found out the hard way that active ventilation is needed. Gene Leger, who built the first of these in the northeast back in the late 70s described the first days living in the house as "like living in a baggie." In January he had moisture (from human occupancy) running down the windows. Realizing the mistake right away, they opened a couple of windows a bit, but ultimately cut a hole in the wall and dropped in one of the first HRVs available at the time. Problem solved. All his subsequent houses had the HRV and ductwork built in at the start.
Note that newer versions of the IRC, the code building inspectors go by, have introduced limits on leakage, as measured by blower door at a standard 50 pascal depressurization difference. In some jurisdictions, a measured leakage rate below a certain level requires mechanical ventilation.
A properly built shell, with interior 1-perm vapor retarder layer (not a sheet of polyethylene - a barrier), near zero air leakage of interior air into the shell, and no exterior very-low perm layer, will not suffer from moisture buildup.
I am speaking from experience here. My own house, built 2010-11, has a very tight, superinsulated shell, with triple-pane windows and R-20 foam insulation under the basement slab and on foundation walls. Total gross conditioned area is about 4,000 sq.ft over two levels. Heat is by ground source heat pump; it's only a two-ton unit, and it keeps the house at temp in just first stage. My best estimate of heating cost over a mid-fall to mid-spring winter is less than $600. It has no drafty spots, very uniform temperature, and does not get uncomfortably dry in winter, without humidification. Fresh air is provided by an HRV, running on a low setting 24-7. The concept works. It's a matter of building science, now well understood, but certainly not rocket science.