Cohousing-L Mailing List: Re: Common House Ventilation

[Buzz & Denise (72253.2101]

Jay Jacobson wrote (in part):

"the question is how to avoid overheating in summer when there's a lot of window
area.  We plan on using vegetation for cooling but our consultant still predicts
overheating.  Any advice on cooling for the dog days of summer when even opening
windows doesn't help much?  Has anyone designed a ventilation system for
exhausting heat overnight and drawing in cooler air?"

This is a large subject, and there is not enough information to give an informed
reply, but here are a few thoughts:

For those who are building out west, there are two methods that work very well.
1. Thermal mass on the inside - in this case, windows are opened at night and
closed during the day to keep the cool air in.
2. Swamp Coolers -  Officially called Evaporative Coolers, these work very well
.. in areas of low humidity.

For you in Ithaca, the above won't work, as the humidity prevents a large
diurnal range of temperature, but I thought I would mention it for those
westerners listening in or who thought of it for the east.

The question is:  why does your consultant predict overheating?  You didn't say,
but I'm guessing its due to improper glazing design.  This is very important,
but rarely designed into a building.

Windows need overhangs.  Without the proper overhang, good solar gain is
achieved in the winter, but it continues unabated in the warm months when it is
not desired.  Since the angle of insolation (sun's angle as it strikes the
earth) changes from winter to summer, this problem technically is easily
rectified.  In any good design book you can look up the tables and calculate the
amount of overhang you need at your latitude and with your degree heating days
factor.  If you want to really get into it, try to order the "Sun Angle
Calculator" from Libbey-Owens-Ford in Toledo, Ohio.  It calculates the angle of
incidence for any latitude, at any time of day, any day of the year, at any
compass bearing.  It would be nice if your architect did these calculations, but
in my experience, this is not usually done (which is why I do it myself).  The
psuedo drawback of proper overhangs is their appearance has to be visually
incorporated into the look of the building.

Relating to the above, is the placement of the windows.  In calculations by the
National Renewable Energy Laboratory, in many climates *west* windows are just
as much of an energy loss as north windows.  That is because they are difficult
to shade, so while they admit little winter solar gain, they transmit a huge
amount of sun in the summer.  For your climate, no more than about 5% of the
floor square footage should be west windows.  The NREL used to have a killer
pamphlet on this subject, which for $5 would make you more knowledgable than the
majority of the professionals in the field.

Ventilation is neccessary, but like you say, if it's 90 degrees out it will be
pushing 90 degrees inside.  You have to have proper ventilation, but good design
is the starting point.

The other little things are:  
1. Light colored exterior sufaces
2. Good attic or ceiling ventilation
3. Deciduous vegetaion (have to wait about 15 years)
4. Well insulated building
5. Good quality windows (low-e)
6. Totally vent all interior sources of heat
7. Numerous small clever ideas that I can't go into here

In summary, only guessing what your problem might be, I would urge a technical
evaluation of your whole glazing scheme.

Buzz Burrell
Paonia, CO
72253.2101 [at] compuserve.com