Detached Garage Venting

[seanherr]

Hi All,

Just signed up, I have been doing a bunch if searching and having issues calculating what I need for intake vent sizes. I have insulated walls, no insulation on the roof yet, it is a hip roof and it is going to be a project. Anyway, I just installed a 1150 cfm attic fan with a thermostat, I added two 10x10 vents on the same side of the roof as the fan, if I crack a window, air is being pulled in so I know I do not have enough intake. Anyone have a link or calculation to determine how many more vents I need? I get full sun on my black roof, and my 10k A/C just ain't cutting it in the afternoon.

Thanks in advance,

Sean

 

[seanherr]

Finally found a good calculation. Credit The NAHI® Forum


Sizing Power Fans

Power fans are specified in CFM (cubic feet of air moved per minute)
instead of Net Free Area (NFA). To determine the proper size power
fan for a particular attic use the following formula based on the Home
Ventilating Institute’s recommendation of 10-12 air exchanges per hour.
Attic sq. ft. x 0.7 = CFM needed

For example:
• 2,000 sq. ft. attic x 0.7 = 1400 CFM power fan is needed.
To calculate proper intake ventilation for the power vent, divide the
CFM capacity of the power fan by a factor of 300 and then convert to
square inches.

For example:
• 1400 CFM power fan ÷ 300 = 4.6 sq. ft. of intake NFA needed.
Now convert to square inches by multiplying by 144 (the number of sq.
in. per square ft.).

For example:
• 4.6 sq. ft. of intake x 144 = 663 sq. in. of intake NFA needed.

 

[yeldogt]

all you are doing is pulling the AC out of the building ..

You need to insulate and seal up the building

 

[JRC3]

Does the garage have soffits/eaves? If so that is where the intakes need to be, all the way around.

If you're saying you added 2) 10x10 box vents on the same side as the fan, then all that's happening s the fresh air from the vents is going directly to the fan.

 

[seanherr]

I am pulling hot air out with the vents where they are, but I do like the soffit vent idea, I think that is the best way to pull the air across the whole roof then out, I can only do this on the front and one side though, no soffit in back, and I don't need my neighbors being nosey and listening in.

 

[seanherr]

all you are doing is pulling the AC out of the building ..

You need to insulate and seal up the building

Yes, insulating is the best option, I am trying to do that before winter. Just cannot do that right now.

 

[zmotorsports]

Personally I am not a fan (get it...fan) of powered roof vents. I didn't have them in my last 1150 square foot shop and don't have them in my new 3k square foot shop now. Never any issues with shingles or excessive heat. I would rather seal up the building, have enough static roof vents and vented soffits allow the air to flow through naturally.

We just had a local guy here in our local Model A club lose his 50x50 detached shop to a fire because his attic vent overheated and caught fire. Another reason I'm not a fan. My theory is keep things as simple as possible and don't design in future potential issues.

 

[yeldogt]

It all comes back to ..what are you venting and why ?

This idea of venting grew out of hot attics and poor insulation .. cheap heating .. and no AC.

Venting pulls conditioned air from buildings and creates negative pressure --- it also introduces moisture at many times of the year.

 

[Innovate1]

I am pulling hot air out with the vents where they are, but I do like the soffit vent idea, I think that is the best way to pull the air across the whole roof then out, I can only do this on the front and one side though, no soffit in back, and I don't need my neighbors being nosey and listening in.

Apparently you don't have a ceiling?

If you have a ceiling with insulation that should be a tight air seal between the usable space and the attic. Then soffit and ridge vents keep the attic from getting quite so hot. But with limited soffit and ridge it won't be as good as a gable roof but definitely still worth doing. Use insulation chutes to allow airflow from the soffits.

There are a couple different setups for "attic fans". Some just blow air out of the attic. Some push inside air into the attic and then out the soffit/ridge vents although those are generally called whole house fans and require open windows when operating. Whole house fans work well when the outside air is cooler and the humidity is fairly low. I use mine frequently on cool nights in the spring and fall. But otherwise they need to be covered or they are a huge leak in the envelope.

Cooling that with no ceiling insulation is going to be a losing battle.

 

[yeldogt]

Yes, insulating is the best option, I am trying to do that before winter. Just cannot do that right now.

You are mixing things up.

A ventilated attic -- with ridge and soffit vents assumes a ceiling. You are just pulling air out of the main space ... that's a whole house fan.

I don't do ventilated attic's -- I insulate the roof rafters and either do cathedral (normally) ... or ceilings with some insulation (not much)

I like fully contained -- conditioned attic and crawl spaces

 

[JRC3]

Then soffit and ridge vents keep the attic from getting quite so hot. But with limited soffit and ridge it won't be as good as a gable roof but definitely still worth doing. Use insulation chutes to allow airflow from the soffits.

Pretty sure it a square hip roof, so no ridge or very little ridge. Probable the only setup where a roof fan is the best option.

 

[Innovate1]

Pretty sure it a square hip roof, so no ridge or very little ridge. Probable the only setup where a roof fan is the best option.

Still could put some vents up high. Soffit and ridge vents (or high vents near the ridge) work well without a fan. With a ceiling of course...

Best without a ceiling is a big fan pointed at you. That's the situation I have right now but just because I haven't finished up the build yet.

 

[JRC3]

I have a 24x24 with full sun and no insulation in the attic. It has a plywood floor but it has a 20" gap along the sidewalls. My overhead door also doesn't have a seal so there's a good 1/2-3/8' gap all the way around. My 15K window unit through the wall keeps it ice cold out there. Right now my garage is a frosty 65, it's cloudy today. Last week it was in the 90s with full sun my garage stayed at 69-71 every day. The motion sensor that shows the temp is the farthest from the A/C unit. My garage has no windows and a newer man door with my leaky overhead door. OP, maybe it's time to go a little bigger on the A/C unit.

Sadly I have no eaves to put vents in or I'd had them put in a ridge vent when the roof was just done. I'm considering a roof fan too because the eave vents keep it pretty damn hot up there. If I do a fan, then the attic floor needs completely sealed.

I'll ask here, what are my best options to vent the attic? I'm debating on a roof fan in the middle/ridge with the gable vents. I know it won't pull air across the whole roof surface, but maybe it will pull enough fresh air to help it out. I only have the A/C on when I'm working out there, 99% of the time it's off.