Roof Ventilation Question

[CapnAl]

My 40x60 garage build is coming along and I need to know what would be the best way to keep the roof/ attic cool in summer: 5/12 pitch, Shingled roof, NO ridge vents, and I was thinking of going with an exhaust fan with louvers/movable slats that I would install in the far gable end. I would like to run a duct from front to rear, have a few pick-up points (registers or? ) along the way, then exit out the exhaust fan at the far end. Wire in a thermostatic switch.
At this point, there would be no heat or A/C on this level, because It is a two-story garage and the lower level will be the work/hobby area. Upper level is storage only.
Options I have considered: 1-Spray foam the roof to keep heat out. 2-Spray foam roof, dry-wall the scissor truss ceiling (which would create an attic of sorts, not enough for any kind of storage, but a finished interior ceiling none the less, OR 3- You guys help me with another method to keep things reasonably cool in the summer heat/sun.
Also, if I did all of the above, spray foam, vent fan, would I still want to have the soffit vented, or should the spray foam completely seal the roof and wall junction?
Thanks for any and all help

 

[matt_i]

I would go with the ridge vent and vented soffits.

I'm not a big fan of spray foaming the underside of the roof unless its metal, and would just plan on the attic area being warmer in summer and cold in the winter unless you have intentions of spending significantly more.

To effectively seal the work envelope of ground floor, its important to have some sort of panelized system on the bottom of the bottom chord, and seal entry to the attic as well, even if a storage attic, otherwise you are trying to trap heat in a chimney with the ridge vent at the top.

 

[CNGsaves]

During the build, absolutely PLAN for the Soffet Vents and whatever insulation that you'll have in ceiling. It's so much easier to install soffet vents . . . AND . . the plastic covers during the build. That way you can use blown insulation later.

For roof ventilation, I doubt you'll need powered gable vent fans. Put in passive turtle vents and/or whirly bird vents up near peak while you are shingling the roof.

Let's SEE some PIC's . . . . GJer's love new builds. Start a new thread in GALLERY section and post up lots of pictures of the build. Good luck.

 

[Falcon67]

I used soffit vents and 5 static vents. If I had a do-over I'd put in a ridge vent. The static works OK but the prevailing winds would make a ridge vent work better. My old shop had gable end vents and a fan - wasn't worth squat. Attic temps 140F+. The new shop uses reflective decking and the static vents plus soffit - 120F max. Those are with outside deck temps in the 150~170F+ range.

 

[csp]

Not sure why you don't want ridge vents. They work so much better than any other type. If snow is an issue there are several that snow will not penetrate.

 

[pjnomad]

I have a 30x60, 10' ceiling shop with insulation above white metal ceiling in Northern Mi. I have a continuous ridge vent with appropriate soffits and added a gable end fan with louvers. Thermostat kicks on around 95 degrees and it runs all day in the summer time. The fan is noisier than I thought it would be (being above the insulated ceiling). Would look for a quieter fan if I was to do it again. PS, don't use one of those new arc fault/ ground fault breakers with the thermostat. Nuisance trips!!

 

[BobRae]

Not sure why you don't want ridge vents. They work so much better than any other type. If snow is an issue there are several that snow will not penetrate.

Ridge vents don't function when they are covered by snow. I know, I have them on my house and when it is covered with a foot of snow, the only venting comes from the soffits to the gable end passive vents.

 

[csp]

How often is a complete ridge vent fully covered with snow? My roof has NEVER been completely covered with enough snow to render the ridge vent useless. Snow here is always accompanied with wind, so the peak stays clear and it collects in the valleys or leeward side of the roof. What's your location BobRae, since in this case it seems applicable.

You've pretty much rendered your ridge vent useless anyhow if you have gable vents also.

Most people in snow country are afraid of ridge vents due to early ones allowing blowing snow into attics. That's why I asked.

 

[Chris705]

Al - If the spray foam is in your budget, do it, it almost seems you built the garage to use it. I have also built my garage with wood sheathing & shingles to receive spray foam so I did not vent the soffits or ridge knowing one day the spray would happen. Matt_i wondering if you can elaborate on why you are not in favor of spray foam unless on metal? Bad spray foam experiences?

 

[Richard Cranium]

If you don't want to do a ridge vent, you also could do a power vent in the gable end.

 

[kbs2244]

If you have a foot of snow on a nearby roof, I would copy it.
It is a perfect example of how it should be done.

 

[ford33]

You don't have a location listed. Hard to give you advice.

We have soffit and ridge vents in Chicago. They work well here.

 

[DC73]

Attic/soffit vents are not needed at all (and not recommended) if you spray foam the roof. By spray foaming the roof, you are bringing the attic into the "conditioned" space. Go to GreenBuildingAdvisor.com and BuildingScience.com to research unvented attics and cathedral ceilings.

There are details to get correct that are based on your climate. For example, around here you could install open cell foam but in colder and/or more humid climates you'll need a certain amount of closed cell foam to avoid condensation issues at the roof deck.

DC

 

[mtbmitch]

Another idea I am trying. Parallel to the roof attached to the underside of the 2x4 trusses I have attached 2 inch foam about 3/4 of the way up. This creates an air gap of the thickness of the 2x4. So with the sofit vents combined with a attic fan this air gap should help reduce the summer time hot attic temp. Yesterday I measured the temp of the underside of foam and right next to it the underside of roof. 12 degree difference. I use drywall screws with fender body washers to hold foam in place. Still plan on insulation on top of drywall to keep winter heat in.

 

[Jlbc212]

Al - If the spray foam is in your budget, do it, it almost seems you built the garage to use it. I have also built my garage with wood sheathing & shingles to receive spray foam so I did not vent the soffits or ridge knowing one day the spray would happen. Matt_i wondering if you can elaborate on why you are not in favor of spray foam unless on metal? Bad spray foam experiences?

i agree with Matt about not using spray foam unless it's a metal roof deck or metal roofing material. If asphalt based roof shingles are used to cover the roof, a flow of air under the roof deck will help keep the shingles cooler. if there's foam insulation directly under the deck, the heat from the sun will cause the shingles to dry out sooner, shortening the life of the shingles. While some asphalt shingle manufacturers offer a "lifetime" warranty, make sure you read the fine print. What "lifetime" are they referring to? Is it a pro-rated warranty? Is the labor for replacement covered?

 

[matt_i]

Two reasons, just from what my gut says. I have no bad experiences.

If you think about thermal gradient from warm to cold, somewhere there's going to be a place where condensation would form if conditions are right. This can't be within the foam thickness, so it will likely be against the plywood/osb.

Also related to roof, if my shingles ever started leaking for any reason, I'd want visible feedback that they were failing, and not have the underdeck "sealed" from view. If water was penetrating slowly, over time, and not fast enough to "burst" the foam, it could rot a heck of a lot of the deck that would have to be replaced later. In general principles, I'm not in favor of "locking" wood between impermeable surfaces, as long as one side can "breathe" it has a good chance.

 

[AMCguy]

Ridge vents don't function when they are covered by snow. I know, I have them on my house and when it is covered with a foot of snow, the only venting comes from the soffits to the gable end passive vents.

I'm not trying to be a smart ***, but why is ventilation a concern in the winter?

I installed continuous ridge vents on both my house and shop to let heat out in the summer.

By the way, our house is a rancher. Before I replaced the roof, It was grey and had gable vents. It was hot as hell in the summer. You could put your hand on the ceiling long after the sun went down and it was still hot. I went with a black roof and installed the Shingle Vent 2 system. It is way cooler in the house now. I'll never do another roof without.

Here's a link.

http://www.airvent.com/index.php/products/exhaust-vents/ridge-vents/shinglevent2