Anyone else had issues with damp creeping in over winter on a timber frame build?

Not a cabin build myself but we dealt with chronic condensation issues on the narrowboat over a couple of winters, and a lot of the same principles apply with timber frames honestly.

The thing that made the biggest difference for us was accepting that you can't just seal everything and hope for the wind — you need controlled ventilation rather than zero ventilation. Counterintuitive but trying to make everything airtight without a proper vapour barrier strategy just moved the problem somewhere worse (inside the walls, lovely).

We ended up fitting a small MVHR unit and it's been genuinely transformative. Running costs are peanuts and the air quality is noticeably better too.

A few things worth checking if you're struggling:

• Cold bridging at sole plates and corners — this is where I see most people go wrong on timber builds
• Roofline junctions, especially if you've got a warm roof spec that wasn't fully detailed
• Whether your vapour control layer has any unsealed penetrations (cables, pipes etc.)

The van conversion community has some decent resources on this too weirdly — they deal with exactly the same condensation-in-a-timber-structure problem in miniature, so sometimes their solutions scale up nicely.

What's your wall build-up? And are you seeing the damp more on north-facing elevations or is it uniform? That might point to whether it's driving rain ingress or pure condensation. Would help narrow it down a bit.

RussWilson | 847 posts

@CornishCruiser curious what you were going to say there, looks like your post got cut off mate!

But yes, absolutely relevant crossover between boats and timber frames in my experience. The cold bridging through the frame members themselves is something people consistently underestimate - you can have brilliant insulation between the studs but the studs themselves are wicking moisture and conducting cold.

On our build we ended up running a continuous layer of rigid insulation across the outside of the frame specifically to break that thermal bridge, and it made a noticeable difference to where the dew point was sitting within the wall assembly.

The other thing worth mentioning is ventilation strategy - so many people treat it as an afterthought then wonder why they've got black mould in the corners by February.

LindaClark90 | 203 posts

@CornishCruiser yes your post got cut off mid-sentence, would love to hear the rest! We've had this exact problem on our narrowboat — the gap between the steel hull getting cold and the warm interior air is brutal for condensation. Currently trying to figure out whether the same moisture management issues would apply to my garden office build (timber frame, going up in spring). Is it primarily a vapour barrier question or more about ventilation strategy? I've read conflicting things about whether you actually want a fully sealed vapour barrier or whether the timber needs to breathe a bit. Genuinely confused about where to draw the line with insulation spec versus moisture risk on a small timber structure.

Squib82 | 1,204 posts

Relevant to this — on my narrowboat the single biggest game-changer wasn't insulation thickness but the vapour control layer continuity. Every pinhole matters. I used a proprietary VCL tape (Wraptite-SA) on all overlaps and penetrations and the difference going into the second winter was measurable on a hygro-thermometer.

Timber frames are arguably worse than steel hulls because the thermal bridging through the studs themselves creates localised cold spots right where the structure is. Worth modelling the dew point position for your specific wall buildup — there are free calculators online.

@CornishCruiser — yes, curious what you were building to. The principle that timber frames and boats share is that you're dealing with an occupied, heated envelope in a cold external environment. That's exactly where interstitial condensation wants to form.

LK_Solar | 312 posts

@Squib82 another post cut off 😂 what is going on today, did the forum eat everyone's replies??

Anyway — on the garden office front (timber frame, 50mm PIR throughout) I battled damp for a whole winter before realising my vapour barrier had gaps around every single cable entry point. Classic. Sealed those up with acoustic mastic and the difference was immediate.

The other thing nobody mentions: thermal bridging at the sole plate. All that lovely insulation in the walls means nothing if you've got a continuous timber sitting on cold concrete wicking moisture upward. Bit of DPC foam tape sorted mine.