Passive cooling is every bit as important as passive heating!
South facing Low-E windows really beg for passive cooling even in the winter. Clear winter days can bring the house temperature up to more than 80F without any form of passive cooling to assist. Active floor cooling with the heat pump helps a lot, but passive cooling is still even necessary on some days. These windows perform almost too well, so I will describe methods we used to incorporate passive cooling.
Passive cooling assistance from a Heat Recovery Ventilation system.
We purchased a heat recovery ventilation system (HRV) that replaces the air in our house about 14 times daily. We only run the HRV during the night to reduce outside noise from barking dogs. Also, it helps both my wife and I with our dry eye conditions from low home humidity conditions. We still get about seven complete air exchanges per day for only the two of us occupants. The HRV exhaust air does heat the intake air through a heat exchanger, but it is not 100% efficient. Thus, in the winter, the fresh intake air can still be 10-20 degrees cooler than the room temperature. So the HRV is one contributor to our passive cooling system.
Using shades to assist passive cooling efforts.
We still do not have our window shades installed. The ones we purchased are motorized and programmable to lower at night for heat retention. In the mornings the shades will automatically rise to let us take in the views. This should also let in as much passive solar gain as possible to reduce the heat pump run times. On sunny days, we will lower the shades to block a portion of sunlight to help in our passive cooling efforts. We do not know how effective the shades will be, but we plan to report in our blogs later.
Passive cooling with an internal re-circulation fan for most of the winter .
An attached shop/garage spans the entire north side of the house. That was done in an effort to minimize heat loss through north facing windows. The separating wall reduces heat loss significantly because it is another run of R30 Structural Insulated Panels (SIPs). We keep the garage temperature at about 50 to 60F all winter which assists in our passive cooling plans.
With shades installed, we don’t expect to lose as much heat through the windows at night. That will hopefully reduce the heating requirements from the heat pump. In the morning we should not require passive cooling, but possibly some active heat. As the sun rises it will start heating the house in every room (they all have south facing windows). We will lower the shades once the temperature rises to about 72 if it is sunny outside. A fan placed in the east end of the house/garage wall will start blowing hot house air into the cooler garage. This is done manually now, but a thermostat will control it in the future. A filtered cold air return at the other end of the building supports house air replacement.
I know that many reader would consider an electric fan as not being passive and I understand why. But those thinkers should also consider the fact that even an efficient heat pump requires tremendously more electric energy than a fan. Besides the fan only has to run for a short time compared to a heat pump to move the same number of BTUs from one venue to another.
Exhaust fan to garage is key to passive cooling the house.
Passive cooling of the house also supports passive warming of the shop!
The garage seldom calls for heat from the heat pump during the winter unless the outside weather is extreme. During the late fall I go around and install R8 foam boards (picture here) over all the garage windows. Over the years that has made a difference in how much heat the garage requires. The whole house fan blows in warm passive solar air into the garage on all sunny winter days. As a result, the heat pump seldom operates from about noon to about midnight all winter.
We sometimes have to open windows in the winter at each end of the house. The shades should add an extra bit of passive cooling to eliminate that need altogether.
Passive cooling in the summer months with larger whole house attic fan.
Last year we purchased a large capacity 5500 CFM whole house fan for the garage attic. With no help, I installed the AirScape fan in the attic per the directions in the manual. I can recommend installing it to any DIYer who is in reasonable physical condition and not afraid of heights or attics. The AirScape big fan grabbed me with the 5500 CFM capacity and the very energy efficient fan motor.
I typically rise about six in the morning and in our part of Montana it might be 55 to 60 degrees F outside. During the afternoon and evening hours the inside house heat without active floor cooling might rise to 76F. Hence, I would open a large shop window upon rising and then turn on the big attic fan. The fresh cool air traveled from the west end of the shop and up through the attic fan hatch. The hatch opening is in the center of the high shop ceiling. That means that much of the shop is cooled as the air travels toward the fan. The air flowing out of the fan is dispersed in the attic so that it discharges through the peak roof and soffit vents.
The exhaust intake focuses shop air into the damper box (left). R40 insulated damper box kills heat loss when the fan is off! (below)
Conclusion for Passive Cooling
I have enjoyed putting this page together trying to figure out how to assemble all the pictures! Please let me know if it is improving the readability or understand as compared to previous pages. Thank you! Mike