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

Same problem hit me hard last July with my shepherd's hut setup. Decent 50mm PIR throughout the walls and roof, yet by 2pm it was genuinely unusable — thermometer was reading 34°C inside whilst it was only 26°C outside. The insulation that keeps you warm in winter becomes a heat trap in summer if you haven't thought through the thermal mass and ventilation side of things.

A few things that actually made a difference for me:

• External solar film on the glazing — the cheap internal stuff does very little; you want it blocking heat before it enters
• Ridge vent plus low-level inlet — stack effect ventilation only works if you give it a proper differential in vent positions
• 12V DC ceiling fan running off my Victron/Fogstar system — minimal draw, massive difference to perceived temperature

The real problem with most garden buildings is they're essentially glazed boxes with no thermal mass whatsoever. A stone or brick structure buffers temperature swings naturally; a timber clad cabin with 2mm of plasterboard does not. If you can add any internal thermal mass — even dense shelving, stone flooring, anything — it helps flatten out those afternoon peaks.

Also worth checking whether your insulation install left any significant cold bridges, because those become heat bridges in summer and can cause localised hotspots near roof junctions.

What orientation is your building? South-facing glazing with no overhang is essentially a solar collector. An external blind or simple timber louvre above the window made more difference than anything else I tried.

Curious whether anyone's looked seriously at a small vapour compression cooling unit running off solar — the COP numbers are getting more reasonable.

@LiamFrost70 insulation is brilliant at keeping heat in once it's built up — which is exactly your problem. 50mm PIR in a shepherd's hut is great for winter but in summer it becomes a thermos flask by midday.

A few things that actually work:

• Ventilation ridge — cross-flow is everything, hot air needs somewhere to escape upward
• External shading before the sun hits the glazing, not internal blinds (those are useless, heat's already in)
• Reflective roof membrane or even just a light-coloured roof finish makes a surprising difference

I run a small 12V fan from my Victron setup in my static caravan and it's night and day compared to passive ventilation alone. Doesn't cool it per se but stops the air stagnating.

What orientation is the hut facing? South-facing with west glazing is the worst combination.

@LiamFrost70 your insulation isn't broken, it's just doing its job too well — like a thermos flask that went to the wrong party.

Have you looked at a small phase change material (PCM) panel on the ceiling? Absorbs heat during the day, releases it overnight — basically thermal mass without the actual mass. Couple that with a Victron SmartSolar running a 12v ceiling fan off a small LiFePO4 (Fogstar do decent cells) and you've got passive cooling that costs pennies to run off-grid.

The real villain though is solar gain through any glazing — reflective film on south-facing windows made a night-and-day difference in my setup. Literally cut peak temps by about 6°C for about £15 of film off Amazon.

Had the exact same revelation on my narrowboat last summer — the PIR that keeps you toasty in February becomes your enemy in July.

What actually shifted things for me was a two-pronged attack on the roof specifically: external reflective membrane underneath the outer cladding layer, plus a small 12v fan running on solar to purge the hot air that accumulates up near the ceiling before it radiates back down.

The ceiling is where you're losing the battle, @LiamFrost70. Walls are fine, but heat from that roof mass just dumps itself downward all afternoon.

I run a cheap Victron SmartSolar keeping a small battery topped up purely for that purge fan — draws almost nothing but makes a measurable difference. Roof temperature reads noticeably cooler by mid-afternoon now versus before.

@LiamFrost70 ran into this exact nightmare in my shepherd's hut — solution was a Dometic RTX 2000 12V compressor aircon unit pulling off a Victron 200Ah lithium setup, but honestly the cheapest fix first was just cutting cross-ventilation properly: one low vent on the shaded north face, one high vent opposite, stack effect does the heavy lifting before noon and costs nowt.

Also worth checking whether your PIR has a reflective foil face — if it's installed foil-side facing a dead air gap it'll bounce radiant heat back out rather than absorbing it like a glorified greenhouse pane.

@ExFarmer79 the Dometic RTX 2000 is a solid shout — had mine running off a Victron SmartSolar setup last August and it genuinely saved the hut from becoming a sauna.

One thing worth adding though: phase change materials tucked into the roof void can shave a surprising amount off peak temp before the compressor even kicks in. Bought some PCM panels off a UK supplier last year, fairly niche but effective.

Also worth checking your roof colour if you haven't already — my hut had dark felt and swapping to a lighter membrane knocked maybe 3-4°C off before I'd done anything else. Dead simple win.

The 12V cooling route does hammer your battery though, so size your bank accordingly 🔋