New Workshop - Saved around £12-15k

[robdr1]

That looks great and you've got to love a spreadsheet....

I'd appreciate your thoughts on using 50mm PIR over anything deeper in the walls given you've used 4x2's for the uprights and the electrical cabling seems to be run through surface-mounted trunking so doesn't appear to be within the airgap behind the OSB, suggesting that there's space.

I ask this as I'm currently helping our eldest with a smaller (than yours) garden room where he can occasionally work and set up his road bike training frame but where we've considered 75mm PIR too.

I believe the company he's looking to use for the PIR are called Pinks where the difference in cost between each sheet is nearly £9 (£20 vs.£28.74) and I frankly don't feel I've enough experience to persuade him that the extra costs will be more beneficial in the long run. Perhaps it's just illogical on my part but I want to persuade him to get the 75mm and just fund it for him but if there's no real difference and he's happy with 50mm, I don't want to interfere.

I'm no expert, I followed the advice of Liam at https://www.youtube.com/@thegardenroomguru who has built many, many garden rooms. Plus plenty of others on YouTube all seem to be using 50mm PIR in the walls.

All the T&E runs in front of the PIR, which is pushed hard against the outer OSB.
So yes, you get an air gap between the PIR and the inner OSB. See attached photo.

I was told not to waste my time with a plastic sheet vapor barrier, as is often used when plasterboarding the walls instead of OSB in this type of construction. It is a low-occupancy building, not a kitchen, and the inner OSB is glued together with waterproofing adhesive, so it is a sufficient vapor barrier. Time will tell

Also attached is an image of the Excel I used to make sure I was under the 2.5M height for permitted development.
As this was my first construction project, it came in handy!

 

[Fitzroy]

That looks great and you've got to love a spreadsheet....

I'd appreciate your thoughts on using 50mm PIR over anything deeper in the walls given you've used 4x2's for the uprights and the electrical cabling seems to be run through surface-mounted trunking so doesn't appear to be within the airgap behind the OSB, suggesting that there's space.

I ask this as I'm currently helping our eldest with a smaller (than yours) garden room where he can occasionally work and set up his road bike training frame but where we've considered 75mm PIR too.

I believe the company he's looking to use for the PIR are called Pinks where the difference in cost between each sheet is nearly £9 (£20 vs.£28.74) and I frankly don't feel I've enough experience to persuade him that the extra costs will be more beneficial in the long run. Perhaps it's just illogical on my part but I want to persuade him to get the 75mm and just fund it for him but if there's no real difference and he's happy with 50mm, I don't want to interfere.

A training bike will develop lots of moisture, if the space is well sealed this moisture will be trapped in the room and condense on surfaces as the temperature in the room drops after use. After a heavy hand planing session the inside of my workshop windows will gather condensation as the room cools. I tend to ventilate the room before I leave it. If I were using the space for regular exercise I would get a dehumidifier with humidity control.

In my shed/shop I have 4x2 (45x95) framing and i put 100mm RWA45 insulation in the walls and ceiling. Roughly roughly PIR has twice the insulation value of rockwool so equivalent to 50mm PIR. On a cold morning close to freezing I need to run the fan heater for 30-45mins then if I am in the workshop it stays warm just from me being in there, 3mx6m space. For an irregularly used space I'd say 50mm is plenty, if you plan to keep it warm 24x7 then I'd go for the thickest possible. For a permanently warm space I'd for 100mm in 4x2 framing. You construct the wall (inside to outside) Internal boarding (PLY/OSB), vapour barrier membrane, studwork with insulation, breathable membrane, 25mm counter battens creating air gap, outer cladding.

Fitz

 

[kizzi]

@thegardenroomguru

I thought the process looked familiar. I watched @thegardenroomguru a couple months ago building a garden room, very interesting. I bet you are pleased with the finished workshop and rightly so, it looks good.

What things would you do differently, now that you've finished?

 

[robdr1]

I thought the process looked familiar. I watched @thegardenroomguru a couple months ago building a garden room, very interesting. I bet you are pleased with the finished workshop and rightly so, it looks good.

What things would you do differently, now that you've finished?

Good question. To be honest, nothing. Maybe remind myself more often it's just a shed to save some money
I'm sure if I went around a few other people's workshops I would find plenty of ideas I wish I had incorporated.

I thoroughly enjoyed building it and it was a great learning experience.

 

[Noel]

Same spec, but fully insulated. 3.8x7.2 ext, 3.6x7 internal. £4600 inc insulation and electrics. Fully clad in decent 19mm finished Shiplap. Took five weeks single handdd to construct. Quote for an off the shelf was £12k installed, no electric or insulation or membrane.

Looks very nice. No issues with water ingress on right wall below the paved area?

 

[robdr1]

Looks very nice. No issues with water ingress on right wall below the paved area?

Not to date. I have a 50mm air gap from the cement to the base of the shed. This helps reduce the splash up and allows airflow under the shed.

I attached a photo so you can see it better. Also you can see the stainless rodent mesh, that is a good idea to add.

 

[Noel]

Makes sense. Thanks for the photo, good planning.

 

[Puggers]

@robdr1 , thanks for the additional photo of the cabling - I'd wrongly believed from the earlier pictures that there was nothing behind the OSB. I too have the mesh beneath my own place although clocked the cat 'guarding' an area to the back last year which I hadn't noticed had become torn away and on investigation found a family of rabbits underneath. No harm done and they moved on soon enough and I could replace the mesh.

I've watched some of Liam's videos in the past and like his forthright style of presentation but hadn't picked up anything specifically about the size of the PIR used. My lad is bound to have watched far more than I have and perhaps that's why he's gone with 50mm too.

@Fitzroy , appreciate the comment and given the exercise element of use, we're already following the structure you summarise, to include the vapour barrier. Like @robdr1 , I don't have the barrier in my own (just the breather membrane) but have also used rockwool. Your simple reminder about the double insulation value of PIR over this makes complete sense and I should have had the wits to reach that conclusion myself

 

[robdr1]

Makes sense. Thanks for the photo, good planning.

Thanks.

One thing is for sure, planning is 50% of the work. So many great resources and communities like this one, enable a newbie like me to tackle a project like this and still keep it fun and enjoyable.

 

[Sheptonphil]

Looks very nice. No issues with water ingress on right wall below the paved area?

The bit below paved (and against fence) is steel sheet clad, not wood. The construction from inside is OSB, frame, RW45 insulation (accoustic as much as thermal), breathable membrane, battened, cladding. So it’s effectively an insulated building inside a waterproof shell. This one been in use two years nearly and no damp at all. Even in winter no condensation on cast iron lathe bed or band saw table. 2kw heater takes 10-15 mins to warm the place then only kicks in every half hour for 5 mins to maintain cozy work space.

 

[flying haggis]

there appears to be gaps under your rodent mesh ....

 

[shedk]

Can I ask where that cable entry duct is from? Looks smart.

 

[robdr1]

Can I ask where that cable entry duct is from? Looks smart.

Tank connectors. Cheap and very good for large cable pass-throughs.

https://www.eco-filtration.co.uk/product/63mm-threaded-tank-connector/
https://www.eco-filtration.co.uk/product/63mm-flanged-tank-connector/

 

[robdr1]

there appears to be gaps under your rodent mesh ....

The area around the base is now covered in small decorative stones, so if they can get in now, good luck to them

 

[Molynoox]

I mostly see guys push the cable hard to the ceiling so as not to interfere too much with the PIR installation.
Is that a code requirement in the UK, I have no idea, I just followed the herd

I have to be honest I haven't read the electrical regs, but I think the 50mm rule applies to distance from plasterboard internally on ceiling but not the OSB or roof boards on the upper roof surface. It's super unlikely somebody is going to be nailing down through your waterproof roof surface.

I would use a 50mm guideline on all the following however:

-Distance from external wall surface (I can imagine somebody drilling into external wall)
-Distance from internal wall surface (I can imagine somebody drilling into this surface too)
-When routing cables and drilling into wall studs and especially when drilling into ceiling joists (the bending stresses are maximized on the outer edges of the timber and reduce down to zero in the middle so this is the best place to compromise the timbers if drilling into them)

Having said that I always keep everything 50mm from any surface, you can't predict what will happen in future, but on diy jobs a bit of common sense and risk analysis is probably a reasonable way to proceed.

Martin

 

[Inspector]

" It's super unlikely somebody is going to be nailing down through your waterproof roof surface."

Asphalt roofing (shingles) here is nailed on and proper installation has the nails coming through the sheathing a 1/4" or a bit more. When that roof has reached the end of its life, 15 to 40 years, it is usually ripped off and replaced. No roofer is going to know what is close to the sheathing under the shingles and can easily penetrate whatever is there. That's why our codes call for plumbing, wiring, etc to be run in the middle of rafters, studs and joists. I'm surprised yours isn't too.

Pete

 

[Molynoox]

" It's super unlikely somebody is going to be nailing down through your waterproof roof surface."

Asphalt roofing (shingles) here is nailed on and proper installation has the nails coming through the sheathing a 1/4" or a bit more. When that roof has reached the end of its life, 15 to 40 years, it is usually ripped off and replaced. No roofer is going to know what is close to the sheathing under the shingles and can easily penetrate whatever is there. That's why our codes call for plumbing, wiring, etc to be run in the middle of rafters, studs and joists. I'm surprised yours isn't too.

Pete

Our regs may well be the same as yours, it would make sense based on what you say.