Building an Energy- and Materials-Efficient Shed
I had asked some initial questions about this project a few months ago and am grateful for the helpful advice I received. I live in the Pacific Northwest (climate similar to Seattle but a little colder,) and I’m building a shed this summer. I’d like to keep it above freezing year-round. Ideally, it will be efficient enough that I can prevent it from getting below freezing with 100-400 watts of incandescent light bulbs.
I’m trying to not go overkill on wall thickness because the maximum size is 100 sq ft as measured along the outside walls. I’m also trying to keep things simple where possible so I don’t end up with too many half-pieces of material at the end of the project.
I am narrowing in on a few things:
1) It will be 8’*12′
2) I will build it on a wooden platform (it will have floor joists) elevated on blocks above the ground. (It may be moved by machine in the future to a different location.)
3) No windows. One 36″*80″ insulated metal door.
4) No vapor barrier but I will be very careful about air sealing.
5) Furring strips/rain screen with horizontal siding. Asphalt shingle roof.
6) It will have a simple roof as shown in this diagram:
I have two questions:
1) For the wall assembly, will there be much of a difference between:
A) 2*6″ studs at 24″ centers filled with Rockwool batts
B) 2*4″ studs at 16″ centers filled with Rockwool batts and then a layer of Comfortboard on the exterior
2) For the roof assembly, I have a few options. Assume 2*6″ rafters.
A) Fill the 2*6″ rafters with Rockwool bats. There will be a sheet of sheathing on the top of the rafters (with tar paper and then shingles above that.) There will be sheets of plywood or OSB on the bottom of the rafters and cross brace, forming an elevated ceiling. The ~6″ thick assembly will be unvented and completely filled with batt insulation (no venting at the soffits).
B) Build a flat ceiling at the height of the walls. I will lose some of the storage volume inside I likely would have struggled to use that space anyway and it will result in a smaller area to heat. Put loose fill or batt insulation on top of the ceiling but leave a gap below the bottom of the roof sheathing. Make it a normal vented attic (ridge vent and soffit vents.)
C) Similar to B, but make it unvented. Cram the entire attic with insulation. Given that it is barely heated and not lived in, I do like the idea of an unvented assembly if I can get away with it. It’s a lot simpler to build, allows me to get some more insulation in there, and I’ll be less likely to have problems with insects or rodents.
Thanks in advance for any ideas and advice.
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It's a very small storage shed so it's a lot more forgiving than it would be with similar constitution on a larger building. However, even small sheds are surprisingly expensive to build, so it's worth doing things right.
- The big difference between the two walls you propose is complexity and cost. You are much better off sticking with the simpler 2"x6" walls.
- You can't typically get away with using batts in a roof with no venting, but with no source of moisture inside the shed you may be alright.
- The rafter tie you show is located too high to be effective. It needs to be in the bottom 1/3 of the span. For that reason, and the problems you may encounter with insulation tight to your rafters, I'd go with a flat ceiling, but include baffles and ventilation.
That is not a shed but a cave. You need windows for light and ventilation. All those things you are keeping from freezing will melt in the summer. It will be dark and gloomy.
Use home built trusses for the roof. You will want to put all kinds of stuff up on the joists.
Or, get a SIP company to but some precut sips together for you and have it done in a day.
Malcolm is right about the rafter ties. If you put them too high, they are more like gusset plates than rafter ties, and that changes how the forces work out in the entire assembly. If you decide to build a flat ceiling, you lose the ability to store stuff up there as Jonathan mentioned, but you gain two things:
1- you can use cheap loose-fill insulation up there, although batts will probably be easier to install in a such a small space
2- the ceiling joists double as "rafter ties", so you don't have that problem to worry about.
The purpose of the rafter tie is to keep the two rafters from wanting to squish down and flatten out by pushing the two walls outward.
You could put in a load bearing ridge BEAM and elimintate the rafter ties altogether, since the ridge beam would now be supporting the load and preventing the rafters from wanting to push the walls outward. This would allow you to tack up rigid foam on the underside of the rafters to act as your roof insulation, with the rafter bays remaining open for ventilation. You'd need to carefully detail this into your walls though.
Batts in the walls will work, or interior rigid foam tied into roof rigid foam like I'd mentioned earlier. The downside with interior rigid foam is it will complicate hanging things like shelves, and it will probably get damaged over time from things you're storing in there bumping against the walls.
If I were building this, I'd probably put batts in the walls, and maybe some exterior rigid foam if I wanted a lot of insulation. Is this going to be a fully conditioned shed, maintained similarly to living space, or is it more of a shop that you'll heat ocassionally while you're working in there? If it's more of a shop, you can get by with less insulation since you won't be heating it all the time, so you won't have as much overall energy loss to worry about. I'd put in a load bearing ridge beam, since that's not difficult with such a small building (just make sure you support the ends of that ridge beam properly!). Once you have a load bearing ridge beam, you have a lot of flexibility with what you do with your ceiling since you don't need rafter ties. This can leave you with a vaulted ceiling for more headroom, or you can put in "shelf joists" to store stuff up there without the roof depending on those "shelf" joists for the building's structure.
Thanks all. This has been very helpful as usual and has given me a new idea. Basically, I will make an insulated ceiling (very similar to the walls - insulation in the ceiling cavity with sheathing on the top and bottom) and then have a vented attic above.
Would that qualify as a vented and insulated roof assembly?
To answer the questions:
- I agree this will be a cave but I'm OK with no window. I need to be able to rummage around in the shed in the dark so it will have good artificial lighting.
- This won't be fully conditioned. I just want to keep it from dropping below 33F in the winter so that I can store some things in there that shouldn't be subject to freezing temperatures. Given my climate (similar to Seattle,) that will require occasional heat from October to April.
I'll also be building most of this myself. I can get occasional help but using lighter lumber will make the process more practical.
I'm (possibly unreasonably) opposed to loose fill or batt fill insulation in a tiny vented acted. I've just had some bad experiences in this area with rodents, etc. Given that this attic is so small, it won't be practical to ever get in there to clean it up.
What if I build this in two stages:
- I build a fully enclosed box.
- The floor will be 2*6s or 2*8s for the floor joists, filled with batt insulation. Sheathed on the top and bottom with plywood. (I'll also put linoleum or a rubber mat on the floor of the shed.)
- The walls will be 8' high and made of 2*4s. I will fill in the wall cavities with batt insulation and then place Rockwool Comfortboard on the INTERIOR side of the walls. The outside and inside will be sheathed with plywood or OSB (there will then be furring strips and siding on the outside wall.)
- I will then build a flat ceiling. I will use 2*4s to span the distance between the walls. I will then place a layer of Rockwool Comfortboard below the 2*4s (on the interior side, similar to the walls) and then a piece of plywood or OSB as the interior ceiling sheathing. On the top side, I will fill the 2*4 spaces with batts and then place a piece of plywood or OSB as the top sheathing.
- The "box" can be easily air-sealed (and rodent sealed) on the inside and the outside by caulking/taping. The only hole will be for the insulated metal door.
At this stage, I will now have a fully enclosed insulated box sheathed with plywood/OSB on all 8 sides on the outside and inside.
Stage 2: Put a vented roof on it.
- I'll use 2*4s to build home-built trusses.
- The attic will be vented.
- The attic will have a no loose-fill or batt insulation. The insulation is inside the ceiling cavity and the sheathing on top of the ceiling cavity will be the floor of the attic.
- I'll put a little door on the attic from the outside on the gable (accessed on a ladder). I often have long light things to store that can be frozen (sections of pipe, long pieces of lumber, etc.). I'll use the unconditioned vented attic space to store those.
You plan is sound. Probably the easiest way to vent the attic is on the gables at each end.
You might consider strapping the top of the rafters with 2"x4"s at 24"oc to form generous overhangs.
Consider using drywall on the inside. Blow-in insulation from the inside for the walls and ceiling.
Thanks Malcolm and Jonathan. Re the blown-in, that is probably not a bad idea. I don't have experience using a blower and I'm a big fan of Rockwool but I will take a look at the potential of renting a blower for the pink stuff.
Here is one interesting thing that goes back to one of my earlier questions.
Which is better?
A) 2*4 on 16" centers with 3.5" batts and a layer of 1.5" Comfortboard as a thermal break?
B) 2*6 on 24" centers with 5.5" batts and no thermal break?
I expected A to be better but I found this tool on the Rockwool site which actually shows that B is better.
I may skip the complexity of using the Comfortboard product entirely and just go with 2*6 floor, walls, and ceiling, all filled with Rockwool Batts.
I would skip the comfortboard, mostly just for cost savings. If you want some continous insulation, put up some polyiso, ideally reclaimed polyiso, or EPS. I think you'd be better off putting any continous insulation on the EXTERIOR, so that you would have more durable interior walls for your storage building, but it will work either way.
Remember that you're requirements to only keep it above freezing means you'll only be heating to maybe 50*F or so (to give a little margin over freezing, don't try to just use 33*F as your target here), so you'll have minimal energy consumption compared to a home that may be heated 20*F warmer than your shed will be. The lower setpoint will mean less energy used, which will also mean less energy lost through the walls, which makes the payoffs for any of the nifty insulation tricks we like on GBA take even longer. While I'm a normally a big fan of rigid exterior foam, for your shed project it adds a lot of extra cost and complexity for minimal extra benefit in terms of cost savings. You'd probably be better off building 2x6 exterior walls instead, and using thicker batts if you wanted some extra R value.