My hot attic

[Steve Kulpa]

our house in the Nashville area has a huge (cubic feet) attic that is hotter then hell. There are 8 of what I call 'passive' vents mounted near the peak, and plenty of soffet vents. Even so, there is absolutely NO air flow. During the hot summer days, the few metal shelves I have in there are HOT to the touch. So the attic is absolutely useless as storage space for anything but the heartiest items.

My plan is to replace 2 of the vents w/ those mushroom-shaped powered vents, one on each end of the roof line, and block off the remaining passive vents from the underside w/ a thin piece of plywood.I figure this would force air to come in through the soffet vents and move all that hot air out.

does that sound reasonable, and do any of you experts see a problem w/ that? I mean blocking off the remaining vents should suffice, right, or do I have to remove them and re-roof?

and can you believe that the dumb assed builder actually mounted some of the vents so they cut across a rafter??? Lucky for me there are a couple where the hole is completely between two rafters, or I'd have a heck of a time mounting new vents.

thanks, steve

[Steve Kulpa]

oops - I forgot to mention - our 'bonus room' is upstairs too and is cooled by a small heat pump mounted in the attic. Even w/ that, it's still quite warm in the bonus room during the summer and I assume every degree of temperature I can drop in the attic will improve the effenciency of the heat pump, seeing how all it's ductwork in in the 'oven'.

steve again

[Phisherman]

You will increase your energy consumption with powered fans, but it will work. Don't block the existing vents, you can't have too many vents. It will take time, but consider planting some shade trees on the south or south-west side of the house.

[SQLit]

I agree with the increased electrical usage. I disagree that fans will provide any great difference in temp. Unless your moving > 6000 cfm through the attic. Huge area says to me large roof area. Really no way stop the sun from shining.

Spend your money on more insulation and a pro to check out the units.

[Carpenter]

I had a similar problem with the attic above my detached 2-car garage and installing an attic fan with thermostat kept the temperature down.

It's easy enough to do

but the difficult part is to figure out how much air flow you need.

I wouldn't block any vent, but you will have to do some calculations to find the right sized fan. Google for "attic fan" for more...

[CJT]

In my experience those are worthless.

one on each end of the roof line, and block off the remaining

I live in Texas. It's hot here. The only thing I've found that actually works is a powered ventilator (in my case in one of the gables).

[CJT]

I'm a firm believer that gable fans are effective. Eventually I hope to be able to prove that quantitatively -- I've been adding instrumentation to my house to log temperatures, energy consumption, etc. It's a work in progress at this point, though.

[CJT]

The complication I encountered was the need for a safety interlock to keep the furnace from running at the same time as the fan (my furnace is in the attic). That one detail slowed my progress dramatically.

[tbasc]

I suggest looking at the Building Science Corporation web site. The item "Venting on Venting" would be worth consideration before cutting more holes in the roof. TB

[Joseph Meehan]

Don't bother. Make sure the vents you have meet local codes. Don't expect it to every be cool up there. That is why you have insulation under that area. You ventilate to keep the temperatures down a little and really to keep the moisture down.

My experience is that most all attempts at using power vents end up without merit, except to those people selling them.

[Carpenter]

Glad you made it work in the end...

[Bob]

Existing vents near the exhaust fan will "short circuit" the air flow. He wants the air to be drawn from the lowest, furthest vents, not from the closest vent at the top. Of course, it would be easy to experiment to find the best result.

Bob

[Robert Gammon]

I fail to see any conditions where a thermostatically controlled fan would EVER be operational in a 95F attic at the SAME time that HEATING was needed in the living space.

Ok, an IDIOT of a building inspector would INSIST on such a safety interlock as his/her thinking is that one day the thermostat will fail and the fan will run when the attic temp is 35F, outside temp is 20F and the furnace needs to run, but STARVES for air flow and the flame goes out, gas builds in the attic and on the next spark, BOOM!!!!

[Alec]

Steve,

You may also want to look at these Solar Roof Vents:

They move about 800 CFM under full sun light. So depending on the size of your attic you may need more than one.

Below is some data from the Florida Solar Energy Center : Comparing periods with similar weather conditions, the test revealed that the PV vent fans have the potential to reduce measured peak summer attic air temperatures by over 20oF. However, the impact over the cooling season is fairly modest with well insulated attics. Measured space cooling reduction was approximately 6% - worth about 460 kWh annually at the test home.

So, you might gain a 20 Temp decrease and a 6% savings depending on the particulars of your home.

Steve Kulpa wrote:

[Norminn]

Do you have any idea what the AREA is for soffit vents, peak vents, and roof?

[mm]

Huge is relative of course

Any chance your insulation is covering the soffitt vents inside?

I never measured airflow but I know it was incredibly hot until I got my roof fan (I call them. A lot of people call them powered fans, I guess to exclude wind powered turbines, but to me unless it's a hand fan that's not moving, it's powered. :-)

I'm a big believer in roof fans. Read all my old posts in google with author meirman. Recently I've not had a sig and I don't know if google will search on my from address.

As to blocking off passive vents, it seems to make sense. I didn't do it and I have a ridge vent the whole width of my townhouse. Yet,

15 years after I installed the fan, I was up on a ladder by the soffitts, and there was a layer of "lint"** across the entire soffit screen, which also runs th ewhole width of the house, front and back. My neighbors without fans didn't have any of this. **Like from a lint screen in a dryer, mostly white or grey with black parts, but not as thick and made up of plant seeds etc.

You should be able to plug a hole. Even a layer of carboard stapled over the hole will stop 95 to 100% of the convection. If you're happy, when you reroof anyhow, you can remove them.

[CJT]

Operator error and malfunction are both possible and call for a failsafe control system.

[Steve Kulpa]

sorry, I didn't measure - but due to the high peaked roof, there are many many cubic feet of air space.

those flat plastic 'vent extensions' are in place which I assume allow an unrestricted path to the soffit - but I haven't actually checked them.

steve

[Steve Kulpa]

Well, shoot - I posted a long response to one of the replies above and it never made it. anyway, here's a picture of the roof I'm talking about:

steve

[Norminn]

The stuff I have read refers to area of attic. I have no idea of how or when the volume of the attic applies. It seems a larger roof area exposed to sun would take on more heat, but perhaps the higher peak (more volume) offsets the rise in the interior temp. So, area of attic floor .. a 50' by 30' attic floor is 1500 sq. feet of floor. The 1:300 formula gives 5 sq. ft. vent area for 1500 sq. feet of attic floor.

The "bonus room" is an add-on? Heat pump in attic? Does it ventilate to outside? If not, it seems it would heat the attic .. our heat pump/AC puts out an awful lot of heat in summer, to the outdoors.