Anyone else find their garden office turns into a freezer once the sun goes down?

Bit of an odd one posting here since I'm more of a van person, but I've been helping my mate sort out his garden office and the thermal drop-off once the sun disappears is genuinely shocking. We're talking 18°C at 3pm and 6°C by 10pm with no heating running.

The building itself is one of those flat-pack timber jobs — looks smart but the insulation is essentially decorative. We've been looking at whether a small Victron system with a decent battery bank could run a low-wattage panel heater overnight without bankrupting him on leccy bills.

A few questions for those who actually use garden offices year-round:

• Is it worth properly insulating before throwing energy solutions at it, or do people just lean on the heating and accept the running costs?
• Has anyone tried phase change materials in the walls/ceiling to hold daytime heat longer? Seen it mentioned elsewhere but no real-world UK feedback
• What's a realistic overnight draw for keeping a small office above, say, 14°C through a British winter night?

The solar intake in winter is obviously going to be minimal, so I imagine it's mostly about battery capacity and keeping the load as low as possible. Probably similar logic to what I've learned with the van build, but the thermal mass situation is completely different.

Interested whether people have found the insulation upgrade alone made a dramatic difference, or whether it's always a combination of factors. Feels like there's no point calculating system size until the building itself is sorted.

SandySparky | 847 posts | ⚡ Verified Installer

@JH_Power Classic problem with garden offices - they're often built like glorified sheds with minimal thermal mass. Unlike a van where you're heating a tiny volume, you've got a larger space that loses heat rapidly once that passive solar gain disappears.

A few things worth checking: what's the insulation spec on the floor? That's where most people overlook heat loss. Also, is there any thermal bridging through the frame itself?

For the electrical side, a small PTC ceramic heater on a thermostat set to a frost-protection temperature overnight keeps the space from dropping to truly Arctic levels without hammering the battery. Much more efficient than trying to reheat from cold each morning.

What's his current setup - is he running off solar storage or purely grid-tied? Makes a big difference to what solutions are practical.

OldSailor | 2,341 posts | 🔋 Battery Obsessive

Garden offices are basically just enthusiastic cardboard boxes with windows — thermal mass of approximately zero.

Key issue: solar gain during the day masks utterly inadequate insulation, then physics sends you the bill after sunset. Check the U-values on those walls; I'd wager they're nowhere near Part L compliance.

Practical fix: a small Victron MultiPlus running a low-wattage infrared panel on a thermostat overnight does wonders — my own setup idles all night for pennies. Pair it with a decent Fogstar Drift LiFePO4 and you've got silent, fume-free heating that won't murder your carbon monoxide detector.

Also worth investigating whether there's any insulation at all between the floor joists — that's where most garden offices haemorrhage heat and nobody ever checks it.

HarbourSoul | 1,156 posts | ⚓ Off-Grid Tinkerer

@JH_Power The thermal mass issue is the killer here - garden offices shed heat so rapidly because there's almost nothing to hold it. On my boat I solved a similar problem with a small phase change material panel tucked behind a false wall. Absorbs heat during the day, releases it slowly overnight. Not cheap but the temperature swing went from about 12°C to maybe 3°C difference.

Worth checking where the heat is actually escaping too - floor is almost always the culprit in these structures and it's the one people ignore. Even a cheap layer of foil-backed insulation board under a ply deck makes a noticeable difference before you touch anything electric.

What's his current setup - is he running any heating at all, or relying purely on passive solar gain?

BordersExplorer | 634 posts | 🔆 Solar Tinkerer

@JH_Power Worth checking the floor specifically — I've seen garden offices lose an alarming amount of heat through an uninsulated timber base. Most builds obsess over wall insulation but a cold floor will defeat everything else.

In my shepherds hut I run a Victron SmartSolar with a small LiFePO4 bank (Fogstar Drift cells) feeding a 300W panel infrared heater on a thermostat. The key thing is pre-heating before sunset — I set mine to ramp up from about 3pm while solar is still contributing, so the thermal load doesn't hit the battery all at once after dark. Dramatically reduces overnight draw.

What's his current insulation spec? Specifically floor U-value and whether there's any thermal break between the timber frame and the base. That'll determine whether heating is fighting physics or just inefficiency.