Hello,

I’m planning to build a wood workshop in my garden and would like to hear people’s advice on thermal bridging.

I’m planning to use ground screws as the foundations, with 100x45mm timber walls with the interior walls being exposed birch plywood. The wall build up will be as follows:

Interior:
11mm birch ply
1mm Vapour Control Membrane
100x45mm timber stud (with 90mm PIR insulation within the stud
11mm OSB board sheathing
1mm Tyvec firecurb membrane
25x50mm vertical timber batten
25x50mm horizontal timber batten
20mm timber cladding/ of potentially a different material

My question is, should I be concerned about thermal bridging.

Thanks in advance,

I plan on using it all year round, to make picture frames and store photographs. I don't particularly want it to drop below 10 degrees. But it will be more like 18 when I'm working in it. I'm based on the south coast of the UK, the coldest it normally gets is -3 at night. in the winter it could be around 3-8 degrees outside in the daytime when I plan on using it.

The cold bridging issues will be at lintels, and the junction of wall to floor and roof. Those need some thought or they can end up horrible. Do the same sort of section drawing, but for each of the nasty interfaces. That will also make you think through the construction of those parts, to simplify them.

Cold bridging, has 2 issues - heat loss, and potential to induce condensation. You've low moisture generation in workshop, but cold night followed by mild damp day, could cause a cold spot to condense the mild damp air that comes into the workshop from outside. Keeping the interior warm (at least the 10oC you mentioned) will mitigate that, but be more energy intensive.

Are you going to run services on the surface? Seems a shame to go to the expense of Birch ply, which has increased massively in price, and then fix surface trunking/sockets/lighting. Consider PiH suggestion of a layer of insulation over the studs internally, then VCL, then timber batten (service void), then ply. Actually, that additional layer of insulation could go on the outside (over the OSB), then VPM, then batten layers. Keeps the studs and OSB a bit warmer. Personally I'd use 25mm. That will cover all the unavoidable solid timber parts.

Remember to foam all the edge of the rigid insul around the studs, or where you butt 2 pieces.

Thank you for your suggestions.

Posted By: Peter_in_Hungarythen some sort of heating will be needed on a thermostat for unoccupied times.

I am planning on installing a heat pump with a heater that's also a dehumidifier: https://www.airconcentre.co.uk/products/powrmatic-vision-3-1dw-h-air-conditioning-and-heat-pump-vis3-1dw-h. That should also help maintain a RH.

Posted By: GreenPaddyAre you going to run services on the surface?

Yes, I'm planning on running all the electrics on the surface in galvanised conduit. The birch ply is the desired material, the budget might force me to install a cheaper ply.

The addition of 25mm insulation over the stud work makes sense. The decision is whether it goes on the internal or external. My preference may be external, to gain the extra floor space. Good to know it could go on top of the osb.

Posted By: GreenPaddyDo the same sort of section drawing, but for each of the nasty interfaces

that's a great suggestion. I will do this to consider the best way to mitigate these potential problems.

I would also normally suggest that EWI as a better option unless foot print (due to planning constraints) and maximium internal space are issues

Thanks for all the suggestions so far.

I have drawn up some more plans, taking on board the advice.

Posted By: GreenPaddyActually, that additional layer of insulation could go on the outside

Posted By: Peter_in_HungaryI would also normally suggest that EWI as a better option unless foot print (due to planning constraints) and maximium internal space are issues

I have added 25mm external insulation on top of the OSB boards to help limit any thermal bridging. I have also run this EWI down to the floor joists.

Because of the narrow width of my garden, the studio will be placed within 1 metre of the boundaries, which means i will require planning permission. I will keep the interior floor space to under 14sqm. It will be roughly 3.5x4m.

i want to keep the build under 3m in height. keeping the build under 2.5m would mean I skimp on insulation and the strength of joists. Due to height restrictions, I will build a flat roof with a 1:60 fall. Does anyone have suggestions for a decent material to use? it looks like EPDM is the main type people go for these days.

At the moment I have the floor insulation exposed below the joists, is it advisable to cover this? i plan on cladding down to the floor level.

Here is an image of the roof: