More HVAC Questions

[Suedehead]

Does anyone here have a whole house fan for their bungalow? Does it work well? Does it really keep you from having to use your a/c a lot? Do any of the hvac companies in Berwyn install them? Ballpark price?

[T-Stan RPCV]

Be very carefull with whole house fans.  They may be effective but can cause pretty bad structural damage if not installed right or if the attic ventilation system is not sufficient.

Assuming you have a cold roof deck (uninsulated/cold in winter).  During the winter, warm air from your home can diffuse at a faster rate than normal into your attic via incomplete seals on the house fans louvers or around the sides of the fan.  This causes the roof deck to heat up and melt snow when outside temps are still below freezing.  The melted snow travels down above the eaves which are still cold and freezes in the gutter creating ice dams.  As additional snow melts from the top, travels down, it hits the ice dam, has no where to go and backs up under the roof shingles entering the homes envelop at or around exterior walls.  

I've seen homes litterally rotting out from the wall cavities because of this.  The idea is to make sure your fan (and other ceiling penetrations) are sealed and your attic ventilation is sufficient to diffuse the heat that enters the attic before it can warm the roof deck above freezing.

Also, they could cause condensation to form on the roof deck by providing a pathway for warmer moist air to enter your attic during the winter.  Again a function of propper sealing and sufficient attic ventilation.

[Suedehead]

excellent post. thanks for the reply.

[T-Stan RPCV]

I forgot to mention - they do make covers for whole house fans that can help seal them for the winter, I've heard this helps.

[Suedehead]

This is a shot of my attic, i'm looking to place it over the stairwell. There is still about 3 feet above the ceiling. The insulation is stuffed between the rafters? or would they be considered joists? either way, the insulation is there, it's not sitting on top of the ceiling. I'm pretty sure i'm going to have to add more vents. I found a site the other day that has a formula to calculate how much ventilation you need. This will be a weekend project once it cools down a bit outside.

[T-Stan RPCV]

Are you sure the insulation is stuffed between the rafters and against the roofdeck?  Often they will put insulation barriers (soffit dams) between the rafters to prevent direct contact between the insulation and the roof deck.  This is a curved piece of plastic, an inch or two deep, that holds the insulation back to allow for air movement between the insulation and the roof deck.  With this type of system there are ussually outside vents under the eaves and at the top ridge of the roof.  Outside air flows in under the eaves, travels in the pathway created by the insulation barrier and out through the ridge vent or can vents at the top of the roof.

If your insulation is in fact, directly against the roof deck then you have a warm roof deck. Your picture shows a knee wall, I'm not sure how a warm deck/knee wall combination should be vented.  Actually, it probably should not have much ventilation, if any, but I do not know for sure.  I work with a bunch of architects and could check if needed. I ussually only get involved with buildings that have severe problems, so if I havent seen this very often, that may be a good thing.  

[Suedehead]

we have someone coming out to quote out the project to install the fan and vent it straight up and through the roof with a duct running from the fan to the resulting mushroom looking thing on my roof. I've talked to a few other people who have used this setup and they say it works great. thanks for the tips, i'll post how it goes when we get it done.