Okay, just spent an hour sweating my *ss of in the attic. I finished early today so before I put the tools away I decided to install some powered gable venting in my own little house. It was 85 and sunny all day, and I started at about five in the afternoon, which should give you an idea of how much that little cardboard box full o’ fan & tin has been nagging me as I pass by it every morning. Anywho, the job’s done, its pumping a good deal of heat out the gable, and I’m wondering how to improve it.
The house has inadequate soffit venting, an ample amount of roof vents (new), and three gable vents. Its about 4/12 over 1100 sq ft. Here’s my thought. Another vent in the opposing gable providing positive pressure to compliment the negative pressure the newly installed vent provides. Either I’d set the thermostat higher on the inflow side, or wire them to start simultainiously. That way no inadvertant positive pressure and the blown in insultation stays put. The roof vents and the third gable, and I suppose the birdblock vents, would ensure nothing got too out of hand pressure wise.
But, alas I’m a cabinet guy, so lets run my idea past the HVAC and Insulation folks. Tell me if its good bad or just plain ugly.
-duke
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That's an interesting thought. Let us know how it works.
Hot there, eh?
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My first thought on venting is to maximize natural convection and, if using fans, add to convection rather than fight it.
So if the soffit vents are minimal, depressurizing the attic would help get more air coming up through them. And that would be the fan you installed and/or another one blowing OUT.
Better yet, can you upsize the soffit vents and eliminate that bottleneck? Pipes or ducts or vents - a particular size yields a typical flow rate. Putting more pressure across it increases the flow, but not by a lot. Diminishing returns. Flow increases with the square root of pressure drop so bigger fans aren't nearly as helpful as bigger holes.
I'm not an HVAC guy, but I agree with what David Thomas posted--enlarge or increase the number of the soffit vents. That way, cooler air enters low and hot air exits high, like it naturally wants to.
Yeah, more soffit vents. If you have a hollow soffit they're easy to install, and cheap.
I suppose everyone makes a good point about fixing the inadequate soffit vents, excepting that it'll be easy. The attic has about 6 1/2" of nasty grey cellulose/rockwool/rathair/asbestos/leadpaintdust/airborneebolavirus blown-in insulation, and I really don't like spending too much quality time with it. To soffit vent better, I'd have to belly crawl past the wall plate and into the soffit to box out the insulation. As you know on a 4/12 hip roof with 24" eaves, there's about four millimeters of working clearance out there.
But, it could be that the hard way is the only way, so I appreciate your posts. They add to what that little voice says in my head that always makes the voluntary callbacks. As for the fan in there now, its set to go on between 95-100 F, and if I leave it powered up it'll run all day, so we know its not doing its job yet.
-duke
Just a thought: could you go at it from the outside of the soffit--cut holes and shove the insulation aside and stuff the baffles in to kepp it back? Again, don't know your situation except that you said there's not much clearance. At least you wouldn't have to crawl through the stuff! Of course working over your head from a ladder with it pouring down your neck can be fun too.
In my house the vent fan in bath (second floor, natch) didn't seem to be doing much and I'd traced it to the soffit, so I climbed a ladder and removed the aluminum soffit--turns out previous HO had squashed the last 8" of the four inch dia. duct and put it between the top plate and the roof sheathing (must have had at least an inch and a half space to fit it through). That explained the lack of ventilation. So I stand up there with my saber saw and chisels and what not trying to remove part of the top plate so I can unsquish the duct and fit a one way damper on and try to direct the flow down the the little holes in the soffit. After I got that done I tried to replace the fan itself with one that would move more air--none of the new models would fit the old space or the old configuration. Finally made my own fan and stuck it in the window. It moved an additional 32 cu. ft./min and helped a lot. Now the paint sticks on the walls and ceilings almost a whole year before it peels off! Now if I can just evict the bird or squirrel family that has made its home in our wall through another soffit botch job the former owner did.
With a 24" eave you could put in vents large enough to be able to cut and remove insulation from the exterior.
MES
Cut the holes, shove in a coupla short pieces of corrugated plastic drain "tile" or some such to hold the insulation at bay, then install the grill.
Dunno where your house is, but we had a house in a moderate part of N. Cal... I put in a thermostatically controlled mushroom fan and it ran about 6 months of the year. That will jack your electric bill for you, so I'd suggest doing something passive if possible. Things you can do: get an insulator to come and vacuum out the nasty insulation so you can work up there, and then cut in the correct soffit vents, basically one in every rafter bay. Make 'em big, not the little 2" round aluminum ones. Then baffle them and have new cellulose blown in. From above cut in a continuous ridge vent or have a roofer do it. This can be done as a retro and not interfere with the roofing much at all. You may need to block the sheathing from below. Next time you reroof go with a light color and it'll reflect more heat.
You can add more powered ventilation but it needs proper intake air to really work. If you go under the soffit on a ladder and light up a stogie, it should suck the smoke in the soffit vents. A fan running without an intake source just churns the air.
passive is better
add insulation it works in winter
florida power and light company web site says power roof vents DO NOT SAVE ENERGY
cost to run them
+ they suck air out of the house thru small holes in ceiling lights plumbing