Thoughts on wall to floor detail for timber frame/clad garden office?

[Zenith63]

Hey all,

We're having a 30sqm garden office built in January which will be used 6-8 hours 4-5 days a week. It's going in very near an existing A2W heatpump with plenty of extra capacity, so the plan is to put in UFH.

I'd like it to be as comfortable as possible without going totally overboard. In starting my research I came across the merits of warm flat roofs, so we will build it that way (EPDM on top of 100mm PIR then another 100mm PIR between the joists with various OSB and membranes sprinkled in there appropriately). That journey lead me to the concept of warm walls, again with the purpose of removing the cold bridges caused by the stud timbers.

So I wonder does anybody have input on the below drawing I put together? The areas I had concerns as I drew this were -
1. I need to add OSB to the outside of the studwork to strengthen it, but should I then be adding another layer on the outside of the external layer of insulation to give the cladding battens something firm to fix into as shown in the drawing?
2. Does the breathable membrane need to wrap in and under the external insulation and effectively meet the DPC or is there no need given the cladding will protect everything down there from rain?
3. I haven't drawn in how the sole plate will be fixed down to the foundations as I've read lots of the options and wonder what is the most practical to build. Can the builder just drill down through sole plate and put in something like 300mm concrete/masonry screws or expansion anchors or is that likely to split those blocks?
4. Does there need to be another 150x44 timber on top of the sole plate for the studs to be built on?

A builder + carpenter will be doing the build not me, I just enjoy nerding out on this stuff and I'm conscious they may not be too familiar with the warm wall concept so it would be safer to have a drawing I can hand them.

Any and all thoughts appreciated!

 

[23vc]

Potential cold bridge on the sole plate?
Not sure if it’s diminishing returns on going from 100mm to 200mm PIR. These kind of buildings thermal mass means they’re always going to be more sensitive to outside temp than a larger, more solid building.

 

[Zenith63]

Thanks @23vc,

I wasn't sure how to easily remove that cold bridge, it's a tricky one! I guess if the sole/stud was shifted outward 50mm you could run the 50mm PIR upstand higher then batten out the plasterboard 50mm, but I felt that was getting even deeper into overkill territory .

And yes I did wonder if 200mm PIR and the complexity of the double layer is overkill. Comfort is definitely a more important factor than cost, but I also don't want to be literally wasting money.

Any thoughts on the two layers of OSB, required?

Thanks!

 

[23vc]

even if you double up the wall insulation, I don’t see the point in doing it like a warm roof, you may aswell frame the whole lot, then you haven’t got to mess about with 150mm+ screws through the PIR. Cold bridging would be negligible through 200mm of timber. Then you could do away with the outer osb as the battens can fix to the studs with breather membrane in between

 

[mrrusty]

Have a look at my build here https://community.screwfix.com/threads/garden-room.256842/ You're using timber frame on top of block footings. Ours uses 122mm SIPs which is 11mm OSB either side of 100mm EPS. Our suspended floor has 50mm PIR and the roof is the same 122mm SIPs.

We use it nearly every day, and currently with outside temperatures in the low single digits if we use the main area we run a 1.5kW wall fan heater which is more than adequate. It has a small divided office, and most of the time the waste heat from the PC is enough to keep the little office warm. I have a 2kW creda wall heater in the office part but almost never turn it on.

Ours is about 16sqm internal, so your's obviously somewhat bigger and will need a bit more heat, but my opinion is that 200mm PIR is overkill - it'll be fairly expensive and not sure you need it.

The good think with SIPs is there is no framing inside or between the main panels - an insulated jointing spline is used. I weighed up the difference in cost between framing and insulating and using SIPs and it was insignificant. You can of course get SIPs much thicker than 122mm (which wouldn't satisfy regs for a house)

 

[Zenith63]

even if you double up the wall insulation, I don’t see the point in doing it like a warm roof, you may aswell frame the whole lot, then you haven’t got to mess about with 150mm+ screws through the PIR. Cold bridging would be negligible through 200mm of timber. Then you could do away with the outer osb as the battens can fix to the studs with breather membrane in between

Thanks for the input and that answers my question really on whether I'm overkilling it - yes .

Have a look at my build here https://community.screwfix.com/threads/garden-room.256842/ You're using timber frame on top of block footings. Ours uses 122mm SIPs which is 11mm OSB either side of 100mm EPS. Our suspended floor has 50mm PIR and the roof is the same 122mm SIPs.

We use it nearly every day, and currently with outside temperatures in the low single digits if we use the main area we run a 1.5kW wall fan heater which is more than adequate. It has a small divided office, and most of the time the waste heat from the PC is enough to keep the little office warm. I have a 2kW creda wall heater in the office part but almost never turn it on.

Ours is about 16sqm internal, so your's obviously somewhat bigger and will need a bit more heat, but my opinion is that 200mm PIR is overkill - it'll be fairly expensive and not sure you need it.

The good think with SIPs is there is no framing inside or between the main panels - an insulated jointing spline is used. I weighed up the difference in cost between framing and insulating and using SIPs and it was insignificant. You can of course get SIPs much thicker than 122mm (which wouldn't satisfy regs for a house)

That's a good thread thanks! I had initially looks at SIPs but ruled them out because once you start looking at custom designed kits the numbers go a bit nuts. But for such a simple structure I'm sure I would get away with standard 8x4 panels and cut the few windows into them as you did.

 

[mrrusty]

But for such a simple structure I'm sure I would get away with standard 8x4 panels and cut the few windows into them as you did.

They are easy to cut to size - circ saw down both sides. The little challenge is cutting the recess in the cut end for the jointing timber/spline. The thread shows how I did it with a multitool, but I also found you can do it by cutting several grooves with a circ saw and then just breaking out the slivers in between. I wouldn't hesitate to use SIPs again.