Opinions needed on insulation vs. whole house fan

[krogers]

I need some help on a project and I am looking for some wisdom from the forum. I am debating which would be a better solution, attic insulation versus a whole house fan. Let me give you some background on the structure. It is a summer cottage in Maine constructed around 1900. There is no insulation in the walls or attic. Roof is asphalt shingles on 1x sheathing and 2x6 rafters. The cottage is only used for 4 months a year. Memorial day through September. There are some cold days at the beginning and end, but mainly we are dealing with high heat in mid summer. Currently there is no ac, no central fan, and no attic vents at all. One recommendation was to add sprayfoam to the underside of the roof sheathing. My thought is that since the house is not mechanically cooled, that this wouldn't provide the biggest bang for the buck. There was this thread last year about whole house fans and the superfan.

http://www.bt3central.com/showthread...ight=house+fan

I think I would get a better cooling affect from adding gable vents and or the superfan, than I would get from attic insulation. What do you think?

[Thalermade]

I think you are correct to go with some type of venting first. the attic does need to breathe. Keep in mind, as you make your decision, the amount of vent space needed to vent any powered fan. As I learned a few years back, the square footage is reduced in accordance with the screen size you may use to keep insects from crawling through the vents. A gable fan on one end, a screened vent on the other and a eaves vents if possible.

Russ

[pelligrini]

I think the fan would be a better choice too.

Insulation just slows down heat tranfer. Adding the insulation to the underside of the roof would slow it from heating up, but once that attic warms up it will stay that way.

[JimD]

Fan is cheaper and probably more effective. If you only insulate the ceiling you will still heat up through the walls. Unless you have good air circulation without a fan (like cool breezes consistently through open windows) I would do the fan. I would put either a timer or thermostat on the fan. You don't always need it all night.

Jim

[Bill in Buena Park]

Combination

You may want to consider the combination of both an electric, roof-mounted attic exhaust fan, along with a whole-house fan. The attic fan, with proper attic intake (vents) will help keep the attic cooler; the whole house fan will draw outside air in through the windows when the living space warms up (venting into the attic, and out through the attic fan/vents). Be careful to provide enough intake for either fan, or they'll be constantly overheating and shutting down.

If you're not worried about a little more expense, you might then also consider adding batting or roll insulation in the attic over the ceiling, not on sheathing itself - this creates the additional barrier between the living space and the attic climate.

I did this to one of my former homes that had a similar situation. Unvented attic reached +140 f in the summer, and kept the house close to 100 f throughout the night. The 2 fans & insulation worked perfectly.

[gjat]

You really need to do both. As you know, Florida is hot. My shop is in my uninsulated garage. The first thing I did was power vent the attic, which helped some. I then tried to A/C it, but the hottest part of the day would overcome the unit, so I added insulation. That helped tremendously and the garage stays cooler.

What you're dealing with is transfer of heat (radiant heat). If the air in the attic heats up, it radiates outward and downward. Venting moves the hot air out and replaces it with cooler air, lessening it's ability to radiate heat. The roof deck, beams and ceiling will build up heat over a period of time, and radiate heat to the living area. Insulation slows the heat transfer into the living area.

Since we don't get snow or freezing weather, I don't know what the effects of venting are during the winter when heat is radiated from the living area and meets the frigid outside air of the roof deck. Can that cause ice to form at the eaves?

[jackellis]

Whole house fan first, to bring cool air into the house and push hot air out of the attic. Insulation second, and only of the attic continues to radiate heat into the house.

If you can keep the attic and the house cool in summer with a whole house fan, you can probably get away without insulation. Roof ventilators, or roof fans, help but are not adequate.

Remember that like any dark flat plate, a roof is an excellent solar collector, especially when it covers a largely enclosed space. Keep lots of air circulating through the space underneath the roof and you avoid transferring heat (via conduction and radiation) from the roof to what lies beneath space under the roof. Gable vents are good. Soffit vents may be better. Have the fan pull air through the house.

[krogers]

Thanks for all the replies.

Doing both vents and insulation are an option. I was just concerned with the first opinion I received from a contractor. I was not there, so I heard this after the fact. But the contractor recommended spray foam against the sheathing. I guess I shouldn't be surprised, but there was no mention of vents and air flow. I was asked for my advice after this contractor took a look at the space. A whole house fan and vents would be cheaper than the insulation to start with. Even if we use the expensive option of the Superfan, the cost should come out about the same. Eave vents aren't possible, but gable vents are. Since the house has three bedrooms, the direct vent option of the superfan looks like a leading contender.