# Shed/workshop build.



## Peter601926 (29 Jun 2020)

Hi i am looking to build a 4.5m x 2.5m shed/workshop frame will be 75mm x 47mm treated timber. I want to put 12mm ply on the outside and fix cladding straight to it. Is there a breathable barrier i can put between them. Once all the outside is done i want to insulate it and board it too. What would be a good insulation to use.


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## DBT85 (30 Jun 2020)

You'll have to leave a gap between the boarding and the cladding using battens. There's loads of membrane available.

Have a look though some of the active builds (mine, sheptonphils) or other builds (Mike's, Steve Maskery etc) and see what's been done.

My immediate suggestion would that at that size use 100*45 instead.

Insulation you have 2 real choices. Rockwool or similar, it's cheaper, easy if itchy to fit jut has lower insulation properties. In your house roof you are supposed to have around 300mm unless it's gone up again. The other choice is PIR rigid foam boards. Typically a fair bit more expensive but you need a lot less thickness for the same insulation value as the wool type. You can get it as seconds or cutoffs for less than usual. 

I think most people just use the wool type for workshops. I used PIR boards because I got them pathetically cheap.


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## MikeG. (30 Jun 2020)

Peter601926":34zb4t8o said:


> Hi i am looking to build a..... shed/workshop frame......... I want to put 12mm ply on the outside and fix cladding straight to it............



Please don't do that. That would be a huge mistake, placing a vapour barrier (the ply) on the wrong side of the insulation, and not leaving a gap behind the cladding. Please read the top thread linked to in my signature before you make any decisions, and/or read my ownbuild thread.


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## MikeG. (30 Jun 2020)

DBT85":2ewfzmzi said:


> ....My immediate suggestion would that at that size use 100*45 instead......



It's a small building. Structurally, 3x2s would be fine, so long as the roof were properly designed. The issue really is whether 70mm is enough insulation)


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## El Barto (30 Jun 2020)

MikeG.":vgd5cfk4 said:


> Peter601926":vgd5cfk4 said:
> 
> 
> > Hi i am looking to build a..... shed/workshop frame......... I want to put 12mm ply on the outside and fix cladding straight to it............
> ...



Never read your build thread before Mike, really interesting!


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## Peter601926 (30 Jun 2020)

Thanks for the replies. I wanted to use 75 x 47mm for the framing i was going to put a double top plate on it for strength. i was going for a pent roof using 150 x 47mm boards to put it on. Been looking at the insulation boards can only see 50mm and 100mm thickness online but it is expensive at £384 for 50mm and £564 for the 100mm. I was told there was a breathable paper i could use to put between ply and cladding without leaving a gap. If not i have seen people who put up frame then wrap it in some sort of barrier material and then put cladding over it. Then they insulate on inside of paper then board the inside. Not sure what material to use on roof but i am going to cover it with EDPM rubber.


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## MikeG. (30 Jun 2020)

You've ignored what I wrote then. OK.


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## El Barto (30 Jun 2020)

MikeG.":1irv8va9 said:


> You've ignored what I wrote then. OK.





Mike in your build thread you said that what you were doing was specifically for outbuildings and not for extensions. Could you elaborate on that? What are the differences you’ve made?


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## MikeG. (30 Jun 2020)

Outbuildings which don't have to comply with building regs can have, for instance, cold bridging issues and discontinuous DPC/ DPM, which are not permissable in extensions. Also, the thickness of insulation is different. The major difference though is that you can't build an extension directly off a slab. A raft is an altogether different beast from a slab, and requires structural engineering input. Bar the amount of insulation, however, the rooves are directly comparable.

It's my constant fear when giving professional advice on the internet: that people take it out of context. Hence the caveat you refer to.


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## Peter601926 (30 Jun 2020)

MikeG.":1wqix02v said:


> You've ignored what I wrote then. OK.



Sorry i am confused now what i said in my second post is completely different to my first post. So what is the right way to do it , most people i see building sheds don't put ply on the outside. If i put ply on the outside of frame and a vapour barrier and battens then cladding plus the thickness of insulation people suggest i use plus ply on the inside the walls will 150mm thick at least. Pleas explain the right way please layer by layer and do i need ply on the outside.


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## MikeG. (30 Jun 2020)

Peter601926":pbwj0hdf said:


> ........ Pleas explain the right way please layer by layer and do i need ply on the outside.



Have you clicked on the link in my signature? If you had, you would have already seen the answer.


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## El Barto (30 Jun 2020)

MikeG.":a5yxx6v0 said:


> Outbuildings which don't have to comply with building regs can have, for instance, cold bridging issues and discontinuous DPC/ DPM, which are not permissable in extensions. Also, the thickness of insulation is different. The major difference though is that you can't build an extension directly off a slab. A raft is an altogether different beast from a slab, and requires structural engineering input. Bar the amount of insulation, however, the rooves are directly comparable.
> 
> It's my constant fear when giving professional advice on the internet: that people take it out of context. Hence the caveat you refer to.



Thanks for clarifying!


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## Peter601926 (30 Jun 2020)

MikeG.":2i0qe77d said:


> Peter601926":2i0qe77d said:
> 
> 
> > ........ Pleas explain the right way please layer by layer and do i need ply on the outside.
> ...


Sorry i did not see that link. I can't put polythene dpm under the base as the base is already there. The wall is almost as i said in my second post covered in a breather membrane then cladding with a gap imbetween then insulation and boarded inside but you used OSB which i would find ugly inside. can i cover the inside any other way. I guess mineral wool is a lot cheaper than the solid insulation.


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## MikeG. (30 Jun 2020)

You could use ply or foil backed plasterboard internally. 

The two fundamentals you absolutely cannot get away from with a proper timber framed building are, as it says in that linked thread, to raise the wooden structure off the ground on a plinth, and to have a void behind the cladding. Working from the inside out, with measurements in mm in brackets .....ply or p/bd (10 or 15mm), timber studwork fully filled with insulation (70mm), breather membrane, counterbattened void (25mm), feather-edge boarding (c. 30mm)........ gives a total thickness of 135 or 140mm, so something went wrong with your maths somewhere.

As for insulation, you can get Celotex in most thicknesses between 10mm and 200mm in 10 or even 5mm increments. It is cheaper from a seconds supplier than new, obviously, but still pricier than mineral wool (fibreglass). It performs about twice as well as the same thickness of fibreglass.


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## Peter601926 (30 Jun 2020)

Thanks you are making it easier for me to understand. So i assume i could use any cladding on the outside. is there anything i should be looking for when buying the breather membrane. Also would the plywood be as good as a vapour barrier as OSB or would i need something else to add to it. One other question the OSB i had in my shed rotted due to moisture. Sorry to be a pain with all the questions but id like to get it right.


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## MikeG. (30 Jun 2020)

The cladding on the outside can be anything, but the differences in its performance then lead to different approaches to the details. For instance, vapour-impermeable finishes such as sheet metal or render, for instance, would require additional vents.

The breather membrane can be a roof one, often available as off cuts on Ebay. You can use a roofing membrane in walls, but not a wall membrane in a roof. Or you can use a specialist wall membrane (Tyvek Housewrap, for instance). Or you could use a building paper. Although that's what I used for my workshop I don't recommend it as it isn't very robust.

OSB takes an awful long time in really poor conditions to break down, so I can't imagine what the inside of your old shed was like. If you are heating the shed intermittently and doing normal workshop activities, then ply is perfectly adequate as a vapour barrier. If you steam wood in there regularly, or do other high moisture activities, then you might consider putting some polythene up first before the ply.


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## Peter601926 (30 Jun 2020)

Thanks for all the advice MikeG that all makes perfect sense it would most likely be wood cladding of some sort but have considered composite too. At least i have some idea of what to do now as i was getting so many different ways to do it. I shall make a list of materials and get ordering soon. 

Thanks again.


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## defsdoor (30 Jun 2020)

I have an unopened roll of Tyvek breathable membrane and a 3/4 remaining one if its of any use to you (or anyone else that wants to collect it).


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