Insulating back yard workshop in a hot climate

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Forum topic by Ian S posted 07-16-2016 01:32 PM 945 views 0 times favorited 18 replies Add to Favorites Watch
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Ian S

19 posts in 120 days

07-16-2016 01:32 PM

Hey folks,
I have a 10×20 shed in the back yard, and as my wife and I get increasingly addicted to woodworking, it is becoming “the workshop” where we spend a lot of time on our shared hobby. We live in Austin [where it will be 101° F today] so we’re focused on making the building cool enough. Heat isn’t much of a concern.

We’re still very new to building stuff—I’d classify us as “advanced novices”—so forgive any silly mistakes below :-D

More specs on the building
Again the size is 10×20. It sits on a mostly-stable concrete slab and we have a bare concrete floor. 2×4 studs (unevenly spaced) which are skinned somewhat loosely with painted plywood and exterior trim. Totally unfinished on the inside. Gable roof with the ridge running along the major axis, probably about 20 – 25 degrees, plywood decking and galvanized steel on top. The “attic” space is not separated from the general interior space—there is no ceiling, only open trusses. The attic is not vented in any way, but we do have two good-sized operable windows (at typical interior height) at either end of the shed, which we usually leave open with the screens. And finally, there’s a well-sealing pre-hung door in the middle of the broad side.

This building is definitely leaky in terms of air flow, because hey it’s a wooden shed right? :-D
As we add insulation here, I don’t imagine we would choose to make it more airtight; that might be a losing battle, and we’re not that concerned about energy efficiency anyway.

What we’re up to so far
The building has a subpanel of its own, and a half dozen outlets and a couple of switches. All of this is (unfortunately) wired on just one breaker from the subpanel, so I know I’m going to need to do some re-wiring (utilize multiple breakers) in order to e.g. separate the lights from the eventual table saw, etc.

Meanwhile we purchased a refurb 6000 BTU window A/C unit, and this morning we are in the process of mounting this in a thru-the-wall fashion. We’re going to get it rigged up and probably test its cooling capabilities even before we’ve done any insulating work.

The plans from here
My thinking is, we’re probably not going to be spending long hours in this shop except on the weekends, so I don’t see us leaving the small AC unit running when we are not in it. This means the temperature and humidity levels will be changing frequently inside. Am I already on the wrong track here?

If not, then here’s what we’re thinking for insulating / closing in the shed:
  • Some kind of moderate insulation (batts or rigid foam) in the stud spaces and roof rafters
  • Close over the insulation (perhaps loosely) with thin plywood sheathing as the interior walls
  • Leave the “attic” space inside the gable open to the interior with the trusses still exposed, so that space can be used as an overhead storage loft, or a place to route wiring and dust collector hoses, etc.
Finally, the questions
  • I know that if I was working on a true interior space, or I was in a colder climate, I would definitely have to worry more about condensation and dew points, etc., think about vapor barriers, and so on. But, given that this is a leaky building that will probably always remain so … do I need to worry about that or am I doing fine with the plans above?
  • The hottest part of the building right now (by far) is the metal roof and the heat coming through it. Will insulating here in the rafters be enough to make a difference? Or do I also need to consider adding some kind of attic vent or fan (passive or forced) to move the hot air out of that space?
  • What have I missed? What would you do here? What are we about to do wrong?

Thanks A BUNCH for reading this far. Appreciate all your help.

18 replies so far

View patcollins's profile


1419 posts in 2287 days

#1 posted 07-16-2016 01:49 PM

I would say go with rigid foam insulation, since you want the attic area open I would try to put some insulation under the metal roof. If you can seal off an area between the insulation and metal roof like a cathedral ceiling type setup you definitely need to vent to the outside some how.

Most of the heat gain in the shed is going to be coming from the sun beating on the roof and maybe two walls.

What you end up with will be determined by how much work/money you wish to put into it.

You might also try this stuff, i had good luck with it and my old cheap fiberglass garage door that allowed the evening sun to beat through it. Got it at Lowes reasonably priced a $50 roll was enough to insulate my garage door, water heater, additional insulation for my lunch box, and a bunch of can koozys.

View GR8HUNTER's profile


997 posts in 135 days

#2 posted 07-16-2016 03:30 PM

This means the temperature and humidity levels will be changing frequently inside

keep your machines waxed well


View clin's profile


485 posts in 418 days

#3 posted 07-16-2016 05:26 PM

I think you are on the right track. However, I would stress sealing the building. It’s not just air infiltration, but moisture and insects you want to keep out. Without seeing some photos I can’t say what you would need to do, but if the exterior siding is that bad, perhaps just add some new siding over what is already there.

The uneven stud spacing complicates insulating. But fiberglass batts are going to be the least expensive. You may have to buy 24” wide batts and cut them down as needed, but you’ll most likely have plenty of odd sized places to use the scraps.

For the roof, I think foil sided foam board might be a good way to go. Just attach it to the underside of the roof framing with the foil side up, facing the metal roof. The foil helps to reflect heat back. But it must not touch the metal roof. For a radiant barrier to work, there needs to be an air space. Which you would have by attaching the underside of the framing. Doing this, you don’t have to worry about fitting insulation between odd spaced framing.

If possible, create a ridge vent and have the lower edge of the roof open. This creates an open air channel between the foam board and metal roof. This will eliminate any possibility of condensation on the roof and will help remove heat.

You could just leave it at that, but the foam board will get damaged as you use the overhead space for storage. So I would still cover that with something to protect it.

For the walls, after adding the insulation, I’d add thick plywood on the walls. That allows you to attach things to the walls wherever you want. That might be real helpful in a small shop space where putting a lot of cabinets for storage is not practical.

I’d consider painting all the interior walls and ceiling a light color. The paint will help seal the plywood, that is on the walls, and of course improve lighting.

Given that you plan to be weekend warriors, I agree with using a window AC unit.

I don’t think you have much to worry about as far as condensation and vapor barriers. As you mentioned, you’re not in a cold climate. And you won’t be generating much moisture in the space (no kitchen or bath). Moisture will mostly be limited to what comes from you. Though I think some might come up through the slab. So painting the slab might be a good idea.

Sealing and insulating will reduce temperature and humidity extremes, and will slow temperature changes. That should help mitigate the tendency for moisture to condense on tools. Of course running the AC unit will pull moisture from the air.

-- Clin

View Blindhog's profile


7 posts in 471 days

#4 posted 07-16-2016 08:17 PM

As with most things, money will be the determining factor. Ideally, if you want to leave the trusses open, you could spray foam insulation directly onto the rafters/roof deck. This would provide a good insulation/sound barrier and some air seal as well. That product is quit expensive but very effective.
I live in Austin also and I have been converting my 3-car garage into a shop for the past year. Definitely insulate the walls with batt insulation. Covering with s/r or some type of osb is fine. But the most heat gain will be coming from your roof/attic space. You can install cheap roof vents that will help relieve heat build up but it will not be enough to overcome with window unit. To keep the space cooled you’ll have to insulate the attic or resign yourself to using large fans to move air and suffer through the summer.
If you want talk further, you can PM me with contact info and I’ll be happy to share what I know.


View tmasondarnell's profile


17 posts in 1212 days

#5 posted 07-16-2016 10:24 PM

+1 on Blindhog.

I am in Manor, so I know what you are up again. You need to seal the building better to give that AC unit a chance. If it is in the budget, I highly recommend doing closed cell spray foam. Even 1or 2 inch layer will make a big difference.

View rwe2156's profile


2122 posts in 903 days

#6 posted 07-16-2016 11:45 PM

You definitely will gain an advantage insulating the roof, probably more than the walls because this is where most of your heat gain is coming from, especially if the building is not shaded.

If you use standard fiberglass insulation against a metal roof you really need a vapor barrier (plastic or housewrap), which means either lining each joist bay.

I put a couple layers of foam board.

I seriously doubut 6000 BTU is not going to be enough for that space in the kind of heat you’re talking about unless it is extremely well insulated and sealed. I think it will take too long to cool it down and my guess it will never turn off so you’ll need to keep it running when you’re not using the shop.

I think you need at least and 8 and probably a 10K (which may put you to 240V).

I have an 8000 BTU unit in my studio which is 14×16 (well insulated) and shaded and its just about right.

Years ago the big thing where I’m from (FL) was putting soaker hoses along the ridge until everyone figured out it accelerates rusting.

-- Everything is a prototype thats why its one of a kind!!

View patcollins's profile


1419 posts in 2287 days

#7 posted 07-17-2016 12:39 AM

Years ago the big thing where I m from (FL) was putting soaker hoses along the ridge until everyone figured out it accelerates rusting.

- rwe2156

What was the idea behind the soaker hoses?

View splatman's profile


546 posts in 821 days

#8 posted 07-17-2016 02:37 AM

+1 on spray foam insulation, if you can afford it. Especially because of the (I presume) “right about here looks good” stud spacing, and the air-untightness issue you describe.

And +1 on using strong plywood on the walls after insulating and wiring. Lets you hang cabinets and tools anywhere you wish.

View WhoMe's profile


1441 posts in 2666 days

#9 posted 07-17-2016 05:16 AM

The op also mentioned something about a storage area in the attic.
If they do the spray foam and then create that storage area, if sealed from the main shop, that will also add an additional heat barrier for part of the shop which should reduce the cooling needs for the overall shop. And at that point would a ridgeline vent with soffit vents help evacuate the sealed attic of more hot air?

-- I'm not clumsy.. It's just the floor hates me, the tables and chairs are bullies, the wall gets in the way AAANNNDDD table saws BITE my fingers!!!.. - Mike -

View splatman's profile


546 posts in 821 days

#10 posted 07-17-2016 05:31 AM

Put something below the roof deck to provide the needed airspace. Maybe the foil-faced foam board clin mentioned. Spray the insulation over that.

View jwmalone's profile


769 posts in 125 days

#11 posted 07-19-2016 04:36 AM

cool seal on the roof helps a lot, if you do have problems with moister you can get a dehumidifier for less than a hundred bucks that solve that problem. it will run constantly but doesn’t pull much juice.

-- "Boy you could get more work done it you quit flapping your pie hole" Grandpa

View rwe2156's profile


2122 posts in 903 days

#12 posted 07-19-2016 03:06 PM

Years ago the big thing where I m from (FL) was putting soaker hoses along the ridge until everyone figured out it accelerates rusting.

- rwe2156

What was the idea behind the soaker hoses?

- patcollins

Keeps the metal cool.

-- Everything is a prototype thats why its one of a kind!!

View patcollins's profile


1419 posts in 2287 days

#13 posted 07-19-2016 10:41 PM

Years ago the big thing where I m from (FL) was putting soaker hoses along the ridge until everyone figured out it accelerates rusting.

- rwe2156

What was the idea behind the soaker hoses?

- patcollins
Keeps the metal cool.

- rwe2156

lol, I was thinking they were trying to make a swamp cooler some place with 100% humidity. Personally I would have just put some of that building camoflage stuff over it like the buildings in MASH had.

View Jim Finn's profile

Jim Finn

2390 posts in 2344 days

#14 posted 07-23-2016 01:20 AM

I have a little larger shop than yours. Mine is 23×13 insulated with a drop ceiling. I am in Lubbock where it was 101° today. I have two window AC units. an 8000 and a 6000 and they just barely keep me cool. I think you will be putting another unit in one of your windows. IF I was you I would insulate the walls and roof rafters and put pre painted white Masonite on the walls over the insulation and wiring in conduit on the surface of the wall.

-- "You may have your PHD but I have my GED and my DD 214"

View BurlyBob's profile


3484 posts in 1688 days

#15 posted 07-23-2016 02:08 AM

Put a good epoxy floor coating down. I found mine helped reduce humidity quite noticeably. Do it in a light color to help with brightening the space.

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