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I'm getting a large new shed built in my garden and I'd like to insulate it to make it a bit more comfortable to work in.

The shed comes with stud framework on the walls, so my plan is to use OSB to board the walls up and create a cavity, and fill those cavities with insulation, similarly to how this article recommends. I'll likely do the same for the sloped roof, but with plasterboard. I figure that OSB will be harder wearing than plasterboard for the walls, and I can paint it for a little aesthetic improvement. The shed only has tiny security windows so there's not a lot of extra work to accommodate those. I'll be installing a pair of roof vents for air blowers (one in, one out), to help circulate air for cooling and damp prevention.

I'm in England, so we're talking temperature extremes of -10°C to +35°C, but generally between 5°C and 25°C for most of the year. The humidity is all over the place but it's generally quite high around here.

My main concern is the heat rather than the cold. A little foam insulation and a small electric heater should do fine in the winter. But I struggle with heat as it is, so the last thing I'm going to want to do is sit in an oven while I work on some DIY job in the summer. I'm struggling to find solid information about options for maximising heat rejection in this context, and I'm concerned about picking a "stackup" of insulation that does the job but runs into damp and condensation problems.

What're my best options here? Which types of insulation (or configurations thereof) have better heat rejection properties? Does it make sense to go for something like a foil-backed panel on the external side and slab behind it? Do I need breather membranes or other damp control layers?

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    Other than paint it white or silver (including the roof) to reduce solar gain if it gets direct sunlight, insulation is insulation, and without cooling, it will be about the average temperature outside.
    – Ecnerwal
    Mar 15 at 21:11
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    insulation doesn't do much for comfort without heat/AC, which it makes far more effective and economical.
    – dandavis
    Mar 15 at 22:41
  • Along with insulation, a radiant barrier can significantly reduce solar heat gain during the summer.
    – bta
    Mar 16 at 18:57
  • I wish I'd used OSB on the inner walls of my shed rather than plasterboard. It would have made fixing shelves and whatever a lot easier. Mar 17 at 11:54
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    I did exactly this last summer. I put 25mm Celotex (foil-backed polyurethane) insulation inside, and OSB on top. It averages around 10C all year round minimum, and never gets too hot. Make sure you do the same on the roof. Mar 17 at 15:55

3 Answers 3

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Insulation works both ways, so it'll reduce the heat coming in from the sunlit outer walls as well. Small windows are also favourable in these conditions.

For warm but not over the top situations, you can mitigate warming up somewhat as @Ecnerwal suggests by making the walls and roof white or reflective. If you're going for reflective (metal), make sure you're not creating a glare problem for adjacent buildings though.

However, the most extreme condition, heat waves, which I assume you're referring to, do not see much cooling down at night (for example 20C down from 35C), so the structure would not sufficiently shed its heat. These periods will simply be difficult to deal with unless you use active cooling (AC, ground sourced heat pump etc). That's probably quite an investment for a shed (not just purchase price, but also electricity consumption), so for that limited number of days per year you may have to accept that working inside will be difficult.

Night ventilation (blowing air in as soon as the outdoor temperature drops below the indoor temperature, a good option for brick/stone/concrete buildings) will not help you much, as your shed does not have much active thermal mass.

Outside of those heat waves, active (day) ventilation, so using a fan in a ventilation opening, will definitely help though. Even the low tech variant of placing a big table fan in a window opening will already have some effect, but a larger built in one will of course work better. You can use a larger one at a lower rpm to get the same airflow without the associated noise. If the shed is well insulated and leak proofed, you need an exhaust somewhere, preferably high up so it pushes out the hottest air.

One other option is the so-called tropical roof, which is essentially a canopy above the existing roof. Because it provides shade but does not directly touch the building (therefore removing heat conduction), the building underneath will warm up less. This is commonly a fixed, second roof over the first one, but it can also be an actual canvas canopy you erect in summer. These only work if they are open on the sides (and at the top in case of a saddle roof), so hot air can escape.

Solar panels can provide this function, as long as they're mounted over the roof (therefore having an air gap underneath) rather than integrated into it. The added advantage is they perform better (air cooling on both sides), and they provide the most electricity when your active cooling device (see above) would need it the most.

And finally, if the shed is still in the design stage, you could go with fancier options like making two sides of it open up like barn doors. You will need to make sure they properly seal in winter, but this will keep things pleasant inside under most summer conditions.

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    Regarding the tropical roof, I presume solar panels would have a similar effect? I was only half-considering putting some on the roof, but if I'm getting two benefits for the price of one then that really tips the scales in their favour.
    – Polynomial
    Mar 15 at 21:42
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    Would maybe add the use of a fan for the warm times. Hot/humid oh my god temps probably need A/C, but a fan is nice for less hot times.
    – crip659
    Mar 15 at 22:06
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    @Polynomial Solar panels work, as long as they're mounted with an air gap between them and the roofing. Because they're air cooled on both sides this way, they will also perform a bit better than integrated ones (the higher the temperature, the higher the electrical resistance, which would reduce maximum output). I'll add it to the answer.
    – MiG
    Mar 16 at 7:38
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    We had the entire south side of our roof covered with solar panels. The difference in summer is amazing. The rooms underneath stay so much cooler.
    – RedSonja
    Mar 16 at 14:19
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    Concur on the paint... I had a galvinised garage that got very hot. Roughly slapping some stale old waterbased white paint on the roof and sun-facing walls made it a lot more bearable, and cost nothing but time and effort. Solar panels can go on top some time later.
    – Criggie
    Mar 16 at 23:32
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My shed must be similar to what you plan. Mine is 8' X 14' with a "barn", half octagon roof. The roof comes down to 4' above the ground on the sides. The roof is about half of the total surface. It is ordinary asphalt shingles, tan color. Walls have 3.5" fiberglas with OSB walls. the roof has 3.5" fiberglas with foil backed ridged foam panels. E.TX location so 30 C most of the summer with about half shade through the day. I open a 3' door in one end and a 30" X 16" window in the other end; the temperature is no hotter than outside. So, you don't need extra ordinary features to keep it reasonably comfortable. There are 3 aquariums, each with 100 W thermostat heater and that holds a reasonable temperature in our 0 C winter nights. It is very humid , the only condensation is on the 3' aluminum door(uninsulated) which is no problem.

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  • I bought a room AC for it but never installed it as I do not spend much time in the shed. Mar 16 at 0:15
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It sounds like you're only planning on short-term heating while you're actually using the shed during the winter, so insulation to keep the heat in long-term isn't that vital. For winter heating purposes, make sure you can close off your vent window to keep the heat from just blowing straight out, and rely on the OSB to do a "good enough" job of "insulating" for the few hours a day you'll be out there in winter.

For working in the summer, completely skip the insulation and plant deciduous trees on the South & West sides of the shed. They will provide natural shade in the summer keeping the shed from getting too hot in the first place, while letting sunlight through for some heat in the winter.

I'd make both roof fans exhaust and rely on an open door (possibly augmented by a box fan as necessary) for intake air. This will suck the hottest air at the roof out while drawing the cooler (but maybe not cool during the summer) air in at ground level where you're working. The moving air won't cool you much, but it will make you feel cooler.

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  • Thanks for the answer. Unfortunately the trees aren't an option because of a tree height restriction at the property. The dual exhaust is a good idea, though, and I might actually see if I can find a reversible blower for one of them so I can have automatic flow for preventing air stagnation while I'm not in there, and dual exhaust while I am.
    – Polynomial
    Mar 16 at 17:46
  • Unless the doors seal really well, @Polynomial, I doubt you'll have any issues with pulling air through, even when they're closed. You might consider a passive vent, too.
    – FreeMan
    Mar 16 at 17:56

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