Evacuating Detached Garage Attic Heat?

[schalliol]

We are building a detached garage in Central Indiana. We will have heating in there, likely as an electric furnace with recommendations from others here since we don’t have gas available.

In the summer, we can open the garage doors and person doors to help cool the space, but the attic area, while unfinished, will be a place where we can do varying projects as well I have a store items. There will be typical ridge vents in the roof, and the attic will actually be open to the garage level via standard fixed stairs.

I am looking for a good way to replace hot air in the attic with cooler air from the outside. I have been thinking of some sort of active ventilation through the roof when it gets really hot that we can either activate with a thermostat or a switch. I found one vendor of a attic fan add some unique active venting, but they said in order for it to really work, you wouldn’t want to have the conventional venting that we of course would like to have. In the case that we were to use the space, I would want it to be rather quiet. One possibility I looked at and have not completely ruled out is an exchanger from Panasonic Designed to bring in fresh air regardless of season, which may help. It is unclear if it would provide a material cooling effect or just improve the air generally.

Has anyone done anything like this before and/or have recommendations? Thanks!

 

[billconner]

Is there insulation? I'm confused by your "conventional venting". Is that soffit and ridge vents?

 

[schalliol]

There will be installation in the walls and between the garage area ceiling and the attic floor. There will be ridge and soffit vents, which I’m referring to as the conventional venting.

 

[billconner]

I thought the garage below ceiling and insulation was wide open to attic via stair, which would make the insulation not very useful. So I'm confused.

A gable fan on a thermostat will keep the garage cooler and make the attic slightly more tenable would be my guess. The radiant heat from the roof deck on a sunny day can only be mitigated with a thermal barrier.

Perhaps you are past this point, but if you want to use attic, skip ceiling insulation and instead insulate attic. Ridgid foam under rafters maintaining soffit and roof is one option. The "hot roof" with spray foam alone or thinner spray foam and some other insulation is another option.

I just don't think in central Indiana you can cool an attic much if just roof deck and shingles or worse just metal roofing between you and sun.

 

[schalliol]

I thought the garage below ceiling and insulation was wide open to attic via stair, which would make the insulation not very useful. So I'm confused.

A gable fan on a thermostat will keep the garage cooler and make the attic slightly more tenable would be my guess. The radiant heat from the roof deck on a sunny day can only be mitigated with a thermal barrier.

Perhaps you are past this point, but if you want to use attic, skip ceiling insulation and instead insulate attic. Ridgid foam under rafters maintaining soffit and roof is one option. The "hot roof" with spray foam alone or thinner spray foam and some other insulation is another option.

I just don't think in central Indiana you can cool an attic much if just roof deck and shingles or worse just metal roofing between you and sun.

Yeah, we're thinking about putting in a door to the attic but we don't really want to. Our builder highly discouraged spray foam on the ceiling due to humidity being trapped, so I'm not sure what's the best option for the space. Ridgid foam under rafters is something we hadn't considered and will ask our builder about it. I was thinking about a fan on a thermostat as well. Hmm... Thanks!

 

[billconner]

I don't know if the spray foam on metal roof causes problems or not, but based on reading on the internet is Is say slightly more say it's ok than not ok.

But the radiant heat is not addressed well by venting.

 

[schalliol]

We'll have an asphalt/wood underneath roof and concerns about moisture being trapped there by the builder are giving me pause with insulation trapping moisture up there. It does get humid here in the summer.

 

[billconner]

The spray foam should keep inside surface of plywood warm enough that humid air - if it can get to it - won't condense. Honestly, spray foam is not my favorite but many here have used it successfully and praise it.

Just so I'm sure, you have no plans for AC, just want outside air? Simplest is gable fan.

I'm also not a fan, close to being a disbeliever, if "radiant" barriers - silver mylar with or without bubbles - but maybe this a place where it would work to keep that radiant heat off you and stored stuff.

I really think the heater downstairs and being open to attic is your biggest challenge.

 

[schalliol]

Thanks, Bill. Yeah, it's feeling like we should probably put a door in, which would help manage it. I just keep thinking that there's no reason if it's a million degrees in the attic and just warm outside that we shouldn't be able to solve it with exchanging air.

 

[Yankeefarmer]

I wouldn’t even consider standard electric heat due to cost. Take a good look at a cold-weather capable heat pump like the Mr Cool Universal. It’ll heat using 1/3 the electricity (that’s a rough number because it can vary from, say, 1/2 the electricity in very cold weather to 1/5 or less in milder temps), and will function as an air conditioner if you want to make sure that attic space is comfortable to work in. You can still install a vent system for summer days when the space isn’t being occupied if you want.

I have no affiliation with the product or it’s sellers; I just selected one to heat my detached 26x48 workshop because we don’t have natural gas and I wanted to avoid propane. I’ve been delighted with its performance over the winter, maintaining a minimum of 45 deg overnight and heating to the low 60s daily.

 

[schalliol]

Thanks for sharing, and it's a great point! I don't have a great spot to put the heat pump outside, but it's something I thought of. Honestly I don't think we'll normally heat it very warm in the winter, maybe 50° until we need to heat it up for some sort of event.

 

[Relax]

No personal experience, but some thoughts:

Would light colored roofing material help a little?
If you don't want a door up there, how do you feel about a pair of windows/dormers that you could open to create a cross-draft?

 

[billconner]

stallion - I don't know what is comfortable for you. I just think if you go up there on a sunny day you'll feel the heat radiating off the roofing.

 

[jack stand]

How is your "attic" built?
Conventional attic trusses can be finished and insulated and thermostaticaly controlled fan(s) drawing cooler air through your soffits would be the normal approach in residential construction.

 

[PoorUB]

No room for a couple gable windows you can open? One in each end will ventilate fairly well in most situations.