Window AC units and Gable vent

[Energyblue98]

I just bought a house because of the garage. It's a 24x24.5 detached garage. It has one large door in the front (maybe 18 feet wide?), and one side door, and two windows. It is not insulated. It gets quite hot in there, like a greenhouse.

So I was going to put a gable vent in the roof (or wall near the top). But before I do that, I wanted to ask you all... I plan on insulated it in the next month or two. I was going to do R-13 in the walls, and around R-25 blow in insulation for the ceiling. It's currently open with rafters to hold the roof, I was going to drywall the ceiling and put blow in insulation on top of that. (unless there's a better way to insulate the roof and keep the openness feeling of the rafters). Then I was going to put in window AC units to cool the plan down in the summer, and a few electric heaters (maybe kerosene heater) for the winter.

My questions...
1. should i still install an electric gable vent if I plan on insulating the place, if so does the gable vent need to be in the insulated area, or above it?

2. Should I get 2 smaller window ac units, 8000 BTU. Or 1 larger ac unit 14,000 BTU. Which would be more energy efficient? the upfront cost is about the same if i'm buying them on craigslist.

 

[Energyblue98]

Anyone?

 

[rlitman]

You've opened a can of worms.
1) Those "rafters" may be collar ties or the bottom chord of a roof truss. Neither is designed to hold a ceiling
2) How much insulation you need depends on your location, which you have not stated
3) An "attic" space MUST be vented. The most energy efficient method is with soffit and ridge venting, but since this is a retro-fit, that may not be practical, so a gable vent is a next best solution.
4) Your plan on window AC cooling and electric heat may or may not be feasible, again, based on your location.
5) The efficiency of a window AC is based on the EER, not how many windows you have filled.

 

[Falcon67]

Here, I can cool a 24x24 building with an 8' ceiling R-13 insulation walls/ceiling with a 12K BTU window unit. Provided the walls/ceiling are sealed to prevent air leaks. SO yes, more info needed. Gable vents are not bad - better than nothing. What kind of eves/soffits? If it has soffits you may can add vents there and put in a ridge vent. I use soffitt vents and 9" passive vents on mine near the peak, 5 near the ridge on a 40 long building. I can also heat it adequately with a 5 kW electric heater.

 

[Energyblue98]

Okay, thank you for the responses, I'll get a picture of it when I get home from work today. I don't believe it has soffits but i could be wrong. Location is central virginia. It can get up to 105 degrees and down to -10 degrees at the extremes, and it runs between 90 degrees and 20 degrees on average.

Hadn't thought about the ceiling not being able to support the weight of drywall.. I'll take a few pictures when I get home and hopefully you all will be able to help out. Thanks!

 

[CNGsaves]

OP . . . . you've also "opened the can of worms" of PLANNING (in the correct order) all the things you'll want to do to garage. All your rough electrical and lighting improvements should be done BEFORE you proceed with insulation. Do you have 240v circuits added so you can have large compressor or welder?? Is an electrical subpanel planned for garage for the electrical & lighting improvements??

Ventilation, along with your Cooling and Heating ideas, all GO TOGETHER so you need to solve your overall plan in one fell swoop, and in proper order. Thus, install NG supply line for a hanging heater while walls are all open studs, etc. Good luck.

 

[Jagmandave]

We have similar weather to you and I heat and cool my 600 sq ft shop with a window unit, mine is 18,000 cool and 16,000 heat IIRC - it does a great job and doesn't add much to my bill tho I don't leave it on all the time - only when I need it. However, do you have enough electrical service to the shop to support a big unit or 2 small ones? Anything bigger than a 5K A/C will require 240 volt service - mine required 30 amp.

Insulation and stopping air infiltration are your keys.

Pics will help....

 

[rlitman]

nything bigger than a 5K A/C will require 240 volt service - mine required 30 amp.

I've got 8kBTU windows ACs that plug into a standard outlet. I know of 12kBTU and larger units that run on a standard 20A 120V circuit, though they should really be on a dedicated outlet.

My 18kBTU mini split is on 240V but I'm pretty sure it is on a 20A breaker (might be 30 though, I'd have to look)...

 

[LS6 Tommy]

Anything bigger than a 5K A/C will require 240 volt service

There are lots of 15k Btu 115VAC units. I had a 12.5/115 window unit 20 years ago that was at least 15 years old.

Tommy

 

[Energyblue98]

wiring in a 240 volt plug wouldn't be that big of a deal, just more work for me. but since there's no insulation or drywall it wouldn't be too hard.

It has a breaker box installed with 100amps coming from the house's 200amp breaker. Looks like it was properly installed with an underground conduit. However, it looks like the previous owner wired in everything in the garage and did a pretty horendous job.. I've cut the breaker going to the garage and I'm going to redo all of the wiring.. he has house ceiling fans hard wired into the breaker box and such... I'm going to remove all the random stuff he has hanging from the ceiling and have 3 large flouresent light fixtures flush in the ceiling in a row, right where my car's engine bays will be. Then I'll want to have led lighting around the edge of the ceiling all the way around the garage to reduce any shadows and for a nice looking effect.

anyway, I intend on doing all the electrical outlets and light fixture wiring before insulating. and before drywall gets put up. still not sure what to do about the ceiling though... here are some pictures of it's current state (just bought the house, don't judge me based on the previous owners hack job).

 

[Energyblue98]

sorry the pictures are dark, there's no lights in the garage that are hooked up yet. (i don't trust the previous owners wiring job).

 

[noisenormajean]

I'm in Houston with a 21x13 detached, uninsulated, open-attic, single-car garage with soffits. Garage door faces north and 2 small windows on south and east walls with an entry door on east wall.

The heat became unbearable. Bought a Frigidaire 15,100 BTU, 115V (energy star) window unit and couldn't be happier. On hottest days, the unit cycles regularly when thermostat is set to 70°F...Perhaps too much, indicative of overkill BTU unit size. I understood that this might happen, however, it wouldn't run 24/7 and I wanted it to cool down very rapidly when needed. This, at the risk of increased dampness/humidity. Again, this is Houston where increased air-water saturation is a universal constant anyway. When it does overcycle, I switch it to stay on or eco mode.

It's great working in a/c. I have never had the privilege. My girlfriend wears my FR coveralls, safety glasses, and earmuffs (she calls them headphones) when in the garage to be comfortable because she's too cold while I've barely broken a sweat.

For me, getting an oversized, bigger-in-Texas window unit was a much better/less costly alternative than finishing wiring, insulating, framing, hanging drywall, tape/float, and blowing insulation. Then still buying a slightly smaller window unit or $1200 Mitsubishi mini-split unit and sealing all the doors, windows, and garage door.

 

[Falcon67]

Hard to tell from photos, but some trusses are designed for a 5 lb dead load (ceiling). Drywall and OSB run about 2.5~3 lbs sq/ft (5/8 and 7/16" respectively). You could reduce that by using 1/4 OSB but you'll get some sag with that light stuff, similar with 1/2 drywall.

Just thinking out loud, what I might do is run 2x8 or 2x10 (probably 2x10 for that span) across the gaps in between the trusses. Or run them next to the trusses. That would pick up the entire load of the ceiling easy without having to worry about any extra load on the truss next to it. 7/16 OSB spans 24" OC without an issue - mine have been up since 2011 with no sag. I had to use 2x12s for joists here because I'd would have had to buy a full pallet of 2x10s since they were not stock.

I did a double up in an older building at our last house because the 2x6x16 joists in that shed were breaking up. I did not put a ceiling in that building, but it would not have been an issue. Cutting and placing the additional joists was small effort.

 

[Zeke]

Tell you what I'd do if you want to keep the 'openness'. Rip 8" widths of cheap plywood and nail them along side the rafters. Then staple in R-30 foil faced insulation. It will be exposed. Of you could use kraft faced insulation. That could be painted by spray.

The other idea is to put in a suspended ceiling just below the truss cords. The would be great for your lights and will hold up insulation just fine.

 

[CNGsaves]

OP (. . . . ie EnergyBlue) sounds like you've got proper gameplan to "Start Over" with wiring as previous owner did a hack job. Since you've got open studs, this is perfect time to "Do It Once, and Do It Right" . . . .

Looks like your roof trusses have virtually no overhang on outer edge so there's really no way to have soffit vents. You may want to consider spray foam insulation for last couple feet of roof that's right next to walls (. . . of course AFTER you've got all your rough electrical and lighting all done). Also look into the lightweight sheetrock for your ceiling if you don't end up beefing it up with any sister joists.

For your A/C unit, I'd recommend a 240v unit cut into wall somewhere that will cool area most likely where you'll be working. Those little 120v units do work but they are energy hogs, where the 240v units are more efficient. I used to interact with some PoCo guys and their running joke was 120v window units were the "Cash Cow" for PoCo as they send the meter spinning like a rocket !!

 

[Energyblue98]

I believe there are 12 trusses and it's 24 feet deep, so that's one truss every 24" and they're a 2x4 truss with two "v" beams in the middle. Is there a formula out there to see how much weight that supports? Perhaps I could used recycled press board (like what is on the roof) and use that to hold some blown in insulation and then paint it black, with the walls painted light grey, that would still give it the feel of a taller ceiling. But how would that stuff hold up as a ceiling? I would assume it's lighter than drywall, although it's still pretty heavy.

Or I could leave the ceiling without any drywall or wood boards, and just put up faced insulation on the ceiling portion, but I would only be able to get r-13 to fit up there since it's 2x4 joints.

 

[rlitman]

Is there a formula out there to see how much weight that supports?

Engineered trusses are just that, engineered. They're all a little different, so there's no specific formula (like a wood span table). You really need to find the manufacturer's specs and recommendations, though specs from similar setups may be close enough (so someone here may be able to help).

 

[jwvess00]

Hi there!

If you really want to hang a ceiling in there, and you really aren't sure if it will hold the weight of what you want to install, call a structural engineer. I did just that when I bought my house, and I didn't know the specs. of the trusses in the pole barn that the previous owner built. I spent $450 for the inspection, and he told me what I needed to do to support the weight of the ceiling, as well as other things I should do to strengthen the building. For me it was money well spent.