Insulating a stone bothy – worth doing the floor or just walls and roof?

Been down this rabbit hole recently when spec'ing out insulation for my garden office build, and the floor question is one I keep going back and forth on.

With stone walls you're dealing with serious thermal mass, which can work for you or against you depending on how you're using the space. If it's occasional use, that mass just sits there robbing heat every time you arrive. Floor insulation becomes really important in that scenario because ground losses are relentless and invisible — you don't feel them the way you feel a cold wall.

From what I've read, uninsulated solid floors can account for 15-20% of total heat loss in a small structure. That's not nothing, especially if you're trying to run a small off-grid heating system efficiently. Every watt matters when you're pulling from a battery bank.

The counterargument I've seen is that retrofitting floor insulation in a bothy is genuinely difficult — you're either losing headroom with an overlay system or you're digging up a flagstone floor that might be original and worth preserving.

My instinct would be:

• Walls and roof first if budget is tight (biggest gains)
• Floor if you can do it without destroying character — even 50mm of rigid insulation under a ply deck makes a difference
• Don't bother with breathable membranes if the stone needs to move moisture

What's the actual usage pattern — weekend visits or extended stays? That changes the calculation quite a bit. Also curious whether anyone's used a Victron setup to monitor heat load before and after insulating — would be interesting data to see.

DebbiePowell76 | 847 posts | Aberdeenshire

@FormerMariner Yes, absolutely do the floor – I learned this the hard way with our bothy conversion. Stone walls get all the attention but cold floors will absolutely kill your heating efficiency. Ground temps in Scotland sit around 10-12°C year-round which sounds fine until you're barefoot in January!

We used 100mm rigid PIR board under a floating timber deck and the difference was remarkable. The floor had previously been acting like a massive heat sink we hadn't accounted for in our calculations.

The other thing worth considering with stone specifically is that you're likely dealing with some ground moisture wicking up. Proper floor insulation with a DPM sorted two problems at once for us.

Don't skimp on it just because it feels like the least glamorous job – it genuinely isn't.

DorsetSolar | 1,203 posts | Dorset

@FormerMariner Floor is non-negotiable in my view. Done three builds now — narrowboat, van, and a mate's shepherd's hut — and the floor is where you lose heat silently. You don't feel it as a draught so it's easy to skip, but cold ground acts like a heat sink all winter long.

For a bothy on soil or stone footings I'd go minimum 100mm PIR (Celotex or Kingspan) under a chipboard deck. If damp is a concern, a DPM under the insulation first.

The maths is pretty simple — floor U-values on an uninsulated slab are often worse than the walls, especially with solid stone. @DebbiePowell76 is right to flag learning it the hard way; I've seen people burn through LPG wondering why the place won't warm up.

Sort the floor first, then argue about wall vs roof priority.

MrBodge73 | 312 posts | Array

Did my garden office floor with 100mm kingspan last year and somehow still managed to route a cable through it six months later when I realised I'd forgotten a circuit — do the floor first, then cry about it second.

CliveKnight | 634 posts | Array

Converted a shepherd's hut a few years back and made the classic mistake of obsessing over the walls while treating the floor as an afterthought — single layer of ply over the steel chassis, basically a radiator pointing downward.

First winter sorted that misconception out sharpish.

What people don't always mention is ground coupling — your floor isn't just losing heat to cold air, it's losing it to thermal mass that sits at a fairly constant low temperature year-round. Stone walls at least benefit from solar gain through the day; the ground never does.

@MrBodge73 I'd be curious what went wrong with your Kingspan install — in my experience it's usually either cold bridging at the edges or inadequate vapour control that causes problems after the fact rather than the insulation itself performing badly.