Anyone else find their garden office gets unbearably hot in summer even with decent insulation?

Same problem hit me last July with my cabin setup — even with 100mm of Rockwool in the walls and a decent vapour barrier, the place turned into a literal oven by 2pm.

The penny dropped when I realised the insulation was doing its job too well — trapping the heat that had built up rather than letting it escape. Brilliant in January, absolutely miserable in August.

A few things that actually made a difference for me:

• External solar shading on the south-facing window (a cheap roller blind mounted outside rather than inside — stops the heat getting in rather than trapping it)
• Cross-ventilation — added a small vent high on the north wall to let hot air escape, works on the stack effect principle
• Reflective membrane added to the roof build-up — made a noticeable difference almost immediately

I've also seen people swear by a small 12V DC fan running off a Victron SmartSolar setup, which keeps the air moving without adding to the electric bill. Might be worth exploring if you're already off-grid or partial off-grid.

The real culprit in most cases seems to be the roof rather than the walls — that's where the sun hammers down all afternoon. Worth checking if yours has any reflective layer in it.

What orientation is your office facing? That changes the whole picture. Anyone else found a particular solution that actually worked rather than just taking the edge off?

@StormyMechanic thermal mass is what you're missing — insulation keeps heat in as much as it keeps it out, so once that cabin shell heats up you're baking in a thermos.

My tiny build has the same problem. What helped most was external shading before the sun hits the glass — overhangs, a decent sail shade, anything that stops solar gain at source rather than fighting it inside.

Also worth checking your roof colour. Dark felt roofing absorbs an enormous amount of radiant heat that just conducts straight through regardless of the Rockwool underneath.

A small 12V DC fan running off my Victron setup keeps air moving without much draw, but it's treating the symptom not the cause. Sort the shading first.

Been through exactly this with my 6x4m office last summer. @MoorHermit's point is solid, but don't overlook the glazing situation either — even a modest south or west-facing window can absolutely hammer you once the sun gets going in the afternoon. I added external roller blinds (the slatted aluminium type, not fabric) and it made a dramatic difference almost immediately. Also worth checking whether you've got adequate cross-ventilation — a single opening window doesn't cut it. I fitted a small vent on the opposite wall to create proper airflow and the temperature dropped noticeably. The external shading does the real work though; you want to stop the heat getting in rather than trying to deal with it once it's already inside. Reflective membrane under your roof cladding is worth considering too if you're doing any work up there.

@StormyMechanic had the exact same with my tiny house build. The roof was the killer — even with insulation sorted, solar gain through a flat or low-pitch roof just bakes everything underneath.

What actually helped me was adding a ventilated batten gap between the insulation and the outer cladding. Creates an air channel that lets heat escape rather than soak through. Cheap fix too.

Also worth checking if you've got any east/west facing glazing — morning and afternoon sun hits at a low angle and just pours straight in. External blinds or a simple overhang made a noticeable difference here before I even touched the ventilation.

@ExChippie30 nailed it on the roof — mine was cooking from above before I sorted it. Added a simple ventilated batten gap between the insulation and the metal cladding and it made a stupid amount of difference. Hot air needs somewhere to escape or you're just trapping it.

Also worth checking your Victron MPPT if you're running solar — mine was kicking out waste heat inside the cabin because I'd mounted it on an internal wall like a muppet. Moved it outside under a small overhang, instant improvement.

The real villain in my setup though was east-facing glazing — morning sun hits it, heats the thermal mass up early, and you're already losing by 11am. External blinds sorted that properly, not internal ones — those are basically useless once the heat's already through the glass.

Not directly related to insulation but worth asking — what's your battery/inverter setup doing during peak heat? Mine's in my static caravan and I noticed my Victron kit was adding a surprising amount of ambient heat inside when working hard on sunny days.

Also, has anyone looked at whether their solar panels are mounted flush to the roof? That traps heat and conducts it straight into the structure. Saw a big difference lifting mine off with proper standoffs — air can actually flow underneath now.

@Curly38 the ventilated batten gap — what size gap did you end up going with? Is there a minimum that actually makes a difference or is bigger always better?