I have a semi-finished garage - the 3rd car portion was framed, insulated, drywalled plus electrical years ago before I purchased the home. There is a door separating the main garage from the 3rd car. This 3rd car garage is now our Media room. I live in coastal San Diego area, and don't yet require AC in my house. I have a whole house fan that I'm pretty happy with. However, this 3rd car garage/media room does get pretty stuffy since there is no ventilation.

I'm thinking of adding some passive thru-the-wall ventilation to allow my whole house fan to suck cooler air from my main garage (which is already ventilated). I was planning do something like 12"x12" Air Return and Air Register. Any recommendations on the right way to DIY this one?

 

Fire code requires that attached garages be isolated from living space that is connected to the house by a firewall. So what you are proposing is likely a code violation. If the media room is still technically garage (not open to the house in any way) then what you propose is probably okay.

 

Thanks for the tips on fire-codes. The wall separating main garage and 3rd car wasn't built-to-code as a firewall. It's just framed more like an interior wall. The entire garage still has the original firewall completely intact to separate it from the main house. So this entire room isn't built to code and isn't officially counted as part of the home size/sq ft.

Any tips on materials? I did see a Thru the wall adaptor that seemed interesting - https://www.homedepot.com/p/ThruWall...W408/302038347 - and may actually help suck air into the room to help out the whole house fan.. Though I am not sure I really need a fan.. If I were to just do the Air Return/Register option (no fan), do you recommend 'sealing' the register-to-return space in the wall so that it's not just open to the insulation and dust in the wall?

 

I'd probably install something like this low on the wall. Then up high cut a hole and put on a cold air return grill over each side.

Blow cool garage air in, and it will passively vent warmer air out.

Close off the space between studs if you want to with a short stud cut to the right length... tap it down and put a screw in the drywall to secure it.

 

Got it - thanks for the tips @XSleeper