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

Same issue here, though mine's a shepherd's hut rather than a garden office — similar problem though. Even with 100mm of insulation in the walls and roof, once that metal skin heats up in direct sun it's like working inside a slow cooker by early afternoon.

A few things that have actually helped me:

• External solar shading — a simple canvas awning on the south-facing side made a bigger difference than anything else. Stopping the sun hitting the structure in the first place beats trying to deal with the heat once it's in
• Ventilation positioning — I added a vent low on the shaded north wall and one high on the south. Stack effect pulls air through surprisingly well on still days
• Thermal mass — I know this sounds counterintuitive for a small structure but even adding some slate tiles to the floor helped buffer temperature swings

What I haven't cracked is the roof. Metal roofs just absorb so much radiant heat. Considering a "cool roof" paint coating — anyone actually tried one of these on a garden office or similar? There are a few products on the market but I've seen very mixed reviews.

Also curious whether anyone's combined passive measures with a small 12V fan setup running off solar — something low-draw that could run all day without eating into the battery. My Victron system has plenty of headroom in summer so power isn't really the constraint, more about finding a decent quiet fan that actually moves enough air.

What's your roof construction like? Flat roofs seem to suffer worst in my experience.

@EwanChapman have you looked at the U-value of your glazing? I found my garden office was cooking despite decent wall insulation — turned out the single-glazed skylights were the main culprit, essentially acting as a greenhouse effect multiplier.

Also worth considering external shading rather than relying purely on insulation. A simple sail shade or even a well-placed pergola with climbing plants can dramatically reduce the solar gain hitting the structure in the first place. Insulation works both ways but it can't fix excessive heat entering through unshaded glass or a dark roof absorbing radiation all day.

What orientation is your shepherd's hut facing? South-facing setups with no overhang are notoriously brutal in June/July even up here in the north.

Not directly relevant to a shepherd's hut but my static caravan has the same fundamental problem — the outer skin becomes a massive thermal mass that just radiates heat inward all afternoon long.

What actually made the biggest difference for me wasn't more insulation but reflective foil sarking fitted between the outer skin and insulation layer. Combined with a decent ridge vent to allow convective airflow through the roof void, peak internal temps dropped noticeably.

Also worth considering: external solar shading is dramatically more effective than internal blinds. Once the IR radiation has passed through glazing it's already become heat inside the space. A simple external timber louvre or retractable awning stops it at source.

@T6Project raises a fair point on glazing U-values, but even triple glazing won't help much if the opaque surfaces are absorbing heat — solar gain coefficient (g-value) matters more than U-value in summer.

@EwanChapman the metal skin issue is the crux of it — insulation value becomes almost irrelevant once you've got that thermal mass radiating inward. On my narrowboat (aluminium hull, similar problem) the thing that actually moved the needle was external shading before the sun hits the skin. A simple sail shade or even a well-positioned deciduous tree does more than doubling your insulation thickness.

Worth also looking at ventilation strategy — cross-flow is king. A small 12v fan on a Victron MPPT-controlled circuit running directly off a panel means it ramps up exactly when you need it most. Dead simple to retrofit.

What orientation is the hut sitting at? South-facing with no overhang is the worst-case scenario, but even a modest reflective coating on the roof can drop skin temperature noticeably.

@EwanChapman had this exact problem on the boat before I went off-grid properly. Metal shell, decent insulation, still a furnace by midday.

Two things that actually helped:

• External shade first — a simple awning or even fast-growing climbers on a trellis kept the skin from heating up in the first place. Stops the problem before it starts.
• Ventilation strategy — cross-flow, not just one opening. Get air moving through rather than just venting hot air upward.

The insulation is doing its job once the skin is already hot, it's just keeping that heat trapped inside. You need to stop the skin absorbing it in the first place.

On the boat I've got a 12v fan running off the Victron setup which helps shift air overnight to pre-cool the space before morning sun hits. Worth considering if you've got any off-grid power available.

@EwanChapman my narrowboat is essentially a tin can with delusions of grandeur — reflective foil on the exterior roof made a genuinely shocking difference before I even touched the insulation.