Closed/sealed garage vs vented garage

[mastiff0]

I have a 32x34 unfinished detached garage in Houston that I am about to insulate and add a/c in an effort to keep it comfortable. I was doing research on what exactly I needed to do in regards to ventilation and insulation, and came across two different methods of insulating/venting a traditional attic.

The most common method is the vented attic, where the ceiling (not roof) is insulated and the attic is vented to lower moisture levels and prevent attic temps from getting too high. Radiant barriers are recommended for this type of system.

There is a newer method called a closed attic design where an insulation like an expanding foam is sprayed directly to the underside of the decking, and helps insulate, seal, and prevent condensation. No insulation is put on the ceiling. So the concept here is to be more efficient by not letting air inside the house/attic (which has been cooled by your a/c) leak out through your vents or through small leaks in the walls and roof.

My garage doesn’t have an attic ceiling, so I cannot add insulation to the floor of the attic. Its also has a hip roof, so I don’t have a way of doing natural venting unless I put in a power exhaust fan, which uses electricity itself. The way I’m seeing it, my garage would be better off viewed as a closed attic space (where I don’t use venting and apply insulation directly to the decking) rather than a conventional house w/ vented attic structure. This would keep me from having to add a power vent or radiant barrier, and instead I would just need to spend some bucks on a foam insulation on the roof.

All of this comes from internet research, so I have no first hand experience and haven’t talked to any experts about vented vs sealed designs. Does anybody (especially hot weather guys) have any advice or experience?

 

[wbrian63]

You don't need to worry too much about moisture levels in a garage. That's more of a inhabited space issue, where bathrooms (think steamy shower), kitchens (cooking releases lots of moisture into the air) and people (we give off a lot of moisture in our breath) are present.

Sprayed foam is EXPENSIVE. I just got a quote for my shop at nearly $2/sq ft, and I think that would have only given me R12 (I think). With the floorspace you describe, you've probably got 1200 or so sq ft of roof, depending on the pitch, noting that you've got a hip roof.

You might investigate the DIY kits available online, they're cheaper than the professionally installed setups, but not a lot cheaper.

Can you sheathe the ceiling? I assume you've got some sort of cross support - ceiling joists, right? If so, I'd sheathe it - 1/2" sheetrock wold only run about $250 for the entire ceiling - if you upgrade to 5/8", still under $300. Then blow in fiberglass insulation - Home Depot rents an Owens Corning Atticat blower setup for free with the purchase of 8 (or 10, can't remember) bundles of blow-in glass, which run about $25/bundle. I used this setup to insulate the shop, and at my inlaws to reinsulate the attic after some remodeling and it works GREAT. Don't know how this fiberglass is made, but there is ZERO itch factor - at least for me. Still need a mask and eye protection during installation, but I didn't itch after installing 14 bags at the shop, and I usually start to itch just looking at blown-in fiberglass...

Your biggest challenge for cooling a garage is the big doors and the fact that once cooled, the instant you open one of them, most of your cool air will pour out the door. Additionally, heat gain will be your other enemy - over the course of a day, the contents will warm up to ambient temps, and when you start trying to cool the space down to use it, you'll be fighting against that heat load. My garage is only 22 x 24, and (528 sq ft) - I bought an A/C unit big enough to cool 1100 sqft, and it can't hack it at all, even when allowed 1+ hour to cool the space. My attic isn't insulated, and that's part of the problem, but when it's 99f in the garage when I switch the unit on, EVERYTHING in the garage is 99f, not just the air.

ANY money spent on trying to cool a space is wasted if the goal (a comfortable working space) is not obtained. Next summer, I'll probably spring for a central A/C system for the shop, and bite the bullet and leave it on ALL THE TIME. (At 90f when we're not there.) If I allow the contents to heat soak to 100+f during the day (and we've seen 105-110f in the shop recently), I'd never be able to cool the space down to a usable temp in a reasonable time. Therefore, my goal would never be obtained, and the money I spent on electricity to try would have been wasted.

You may want to consider setting the unit to run at 90f or so on a continual basis. If you bring a hot car into the space, you'll need all the headstart you can get to get/keep the space "cool".

Regards

 

[wbrian63]

One more thing - investigate solar powered ventilators. Shop on-line for the best deals. I bought a couple for a storage shed at the house, and figured at 12c/kwh, they'll pay for themselves in 3 years vs 120v powered units.

I got them - same product as were found at Lowes/Home Depot for about 60% of what those places were charging.

 

[mastiff0]

Thanks for the advice. I've been advised against sheathing the ceiling, because my joists are spaced very far apart (up to 4' in some areas) and I've been cautioned that I might not have enough joists to take the weight of a floor of sheetrock or plywood. Already the roof has warped a little bit where the previous owner had stored a lot of plywood in a concentrated location on the joists.

As for the expense of the foam insulation, your telling the truth- stuff isn't cheap. However, from my understanding of the closed attic system, you wouldn't need a radiant barrier, as it wouldn't help you. Radiant barriers are very expensive themselves, and I think the cost of radiant barrier + conventional batting and the cost of foam insulation is about the same. These are all installed prices, maybe DIY is better.



For the a/c, my plan is to keep it at 85-90 most of the time to dehumidify the place and allow me to cool it quickly when I want to use it. Your right, when its 105 outside and the garage has been baking all day, you can't expect it to be able to cool it down quickly in the evening.

 

[Falcon67]

One more thing - investigate solar powered ventilators. Shop on-line for the best deals. I bought a couple for a storage shed at the house, and figured at 12c/kwh, they'll pay for themselves in 3 years vs 120v powered units.

I got them - same product as were found at Lowes/Home Depot for about 60% of what those places were charging.

Saw one in Sams yesterday for $149. Thinking about one or two of those for the house or shop.

The spray foam is big $ and the idea is to seal the building. Your garage would work sorta the same. It makes the building like a giant thermos bottle. If the building (talking new construction, not retrofit) is done right, you will need an air exchanger to get fresh air in it. I don't think you can do that with an older building. Maybe.

I would investigate what it would take to add ceiling joists to your building. If it's free span, that might be a challenge also. Somebody could chime in on what size 2 x ? would span that distance. If you could reduce the volume of space to cool, you could use a smaller AC unit.

 

[mastiff0]

Good point- hadn't thought about the need for the air exchanger. That might make the decision easier.

I could add ceiling joists, but once I do that, the costs needed to go the traditional vented attic route get pretty high. I would have to add the joints, then the sheetrock, then the insulation, then a power/solar vent fan, and probably still a radiant barrier because its Texas and its hot.
All of that is not cheaper than foam insulation, IF I didn't have to get an air exchanger. Will have to look into that - thanks guys.

 

[digdug18]

I'm planning on completely sealing my in the coming future, air to air heat exchanger included. Another benefit being that if your air conditioning equipment is in the cooled/heated attic space, it'll run more efficiently. The only problem, is that there isn't much written on it online, and how it is over time.

 

[tcianci]

If you have read any of my posts, you will know I'm in the camp of not sealing anything. Maybe it's because my business consists of remodeling buildings. The only buildings that tend to last are the ones that have some air moving through them.
There seems to be this constant quest for the holy grail of HVAC... the building that you can bring up to temperature with one good warm fart and it'll stay that way all winter.
While it is certainly possible to slant everything in your construction to wards saving "energy", even energy savings comes with a cost. The materials and methods used may never yield a break even point depending on how you use the building and how far overboard you go. Sealed buildings are not without their own set of unique problems... the air to air heat exchanger was mentioned, a device with an initial cost and an energy usage of its own. The foam insulation: spendy initially, if you use it in your walls as well, the conventional methods of running a new wire or cable go out the window, a roof leak may take months to pinpoint once it is discovered and you may not discover it for quite some time after it originated.
The experts have a very narrow view of what makes a truly functional building. I always like to use my "hot rod" analogy. We have probably all done this either with a car (or if your name is Tim Taylor, a weed wacker, garbage disposer or floor sander!) We take the device and modify it, hot rodding it, if you will, so that it goes like a bat out of hell. Ok, you say, what's the problem with that? Well, the perceived increase in performance comes at the expense of other qualities or capabilities of the machine. Maybe it will swill through gasoline at an outrageous rate, maybe it's noisy as hell, maybe the fat meats you put on it will launch it great in a straight line but it couldn't corner worth a ****.

Maybe you seal the daylights out of it with pricey insulation, add radiant barriers, air to air heat exchangers, extremely high efficiency (read big bucks) HVAC equipment.
All of this stuff is more than likely all part of your monthly nut. Figure the difference between the whiz bang stuff and a conventional building and add that to your mortgage and figure out what all the latest greatest stuff actually ends up costing over the term of the note. You may never break even on some of it. After all, isn't annual energy cost only a part of the cost of the building?
There will be plenty of posts refuting just about everything I have mentioned, that's OK, to each his own. Maybe 20 years from now, some of this stuff will seem fine to me too but right now my business is taking apart buildings, and it has been my observation that the newer the construction technology, the fussier the building and the more maintenance problems you will encounter with the structure over its service life.
I'm on the east coast, some of the oldest buildings in the country are on the east coast. I routinely work on buildings that are in excess of 100 years old with much of that being the original structure in most cases. I also do a lot of work on newer buildings, these newer buildings are without exception the ones with the rot and mold problems and insect infestations. Those problems are directly attributable to buildings that do not breathe.