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irishthump

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Hi guys,

Will be starting to build a new workshop in a week or two (14x12ft, all timber construction) and would love some feedback in what I have planned...

The back garden will be fully slabbed by then which should provide a solid and level base to start with.

Subframe will be 4x2 treated timber raised on concrete blocks to minimize the chance of damp from the ground. Floor will be 18mm OSB. Toying witht he idea of insulating thre floor with Kingspan or something similar.

Walls using 3x2 stud frames. skinned with 10mm OSB then wrapped with a plastic DPM. Finished outside with Treated T&G cladding.

Roof will be flat design with 4x2 roof joists covered with 18mm OSB. Most liely uses roofing felt to keep the costs down, possibly sheet metal is affordable.

Couple of things I'm not sure off.
Should the outer T&G cladding be fixed to battens attached to the OSB or is it ok to fix then directly to it?
Also not sure 4x2 will be strong enough for a span of 12ft!

Like I said any advice or comments welcome!

Thanks in advance...
 

Jameshow

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4x2 Defo will not span 12ft adequately. I'd go 6x2 or even 8x2 as you would in house joists.

Re battens yes use battens to keep the cladding from rotting.

Cheers James
 

Fitzroy

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Epdm for the roof. Tough, long lasting and easy to apply.
You state walls, stud work, osb, wrapped with DPM. I assume you mean a breather membrane. This is fine so long as you don’t plan to insulate the walls
 

irishthump

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Epdm for the roof. Tough, long lasting and easy to apply.
You state walls, stud work, osb, wrapped with DPM. I assume you mean a breather membrane. This is fine so long as you don’t plan to insulate the walls

Hi Fitzroy,

I had'nt planned to insulate the walls just now but may in the future. How does this affect the decision on installing a membrane?
I've also seen videos on Youtube where people have used 250 Micron DPM to sheath the OSB then placed the cladding over that.
 
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Just remember the vapour barrier goes on the warm side of the insulation and you can't go wrong. If you intend to insulate in future you will have to rip off all the vapour barrier. For the cost of insulation on the walls and roof, I'd do it now.
I used expanded polystyrene on the walls (got some for free off a demolition site and Kingspan xtherm 50mm on roof.
 

Jameshow

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Most people think that using ply or OSB inside means you don't need need a vapour barrier.

Cheers James
 

irishthump

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Just remember the vapour barrier goes on the warm side of the insulation and you can't go wrong. If you intend to insulate in future you will have to rip off all the vapour barrier. For the cost of insulation on the walls and roof, I'd do it now.
I used expanded polystyrene on the walls (got some for free off a demolition site and Kingspan xtherm 50mm on roof.

So from what I gather I need breather membrane under the cladding whether I decide to insulate or not. Then a vapour barrier would be needed if I was to insulate the walls and ceiling and this would go over the insulation. OSB would fulfil this role.
That correct?
 
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I've seen it mentioned here and elsewhere that due to the amount of glue in OSB, it can act as a vapour barrier as long as the gaps between boards isn't too big.
Given the fractional cost and effort of a vapour barrier, vs the cost and effort of having to fix any potential condensation problems in the walls, I am not sure I'd take that risk. To each his own.
 

Fitzroy

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Damp walls, leading to rot etc, can occur due to moisture from the outside (rain) or inside (moist air /condensation). From the outside you ideally have a rain screen (cladding), a gap to allow drying, then a secondary barrier (building wrap/membrane). You dont want dpm as it is impermeable from both directions, if any moisture gets past it it will be trapped on your osb and rot it. A building wrap is breathable and will allow the building to dry if moisture gets past it. From the inside you need to avoid warm moist air getting to a part of the building that is cold (so the moisture condenses) and unvented (so the moisture cannot evaporated and dry out). So (ideally) you either have:
- osb (or plasterboard and vapour barrier), stud work and insulation, then building wrap, gap, cladding
- stud work no insulation, osb, building wrap, gap, cladding.

if you have osb/plasterboard, stud work and insulation, osb, building wrap, cladding. You risk moist air getting past you insulation, contacting the external osb ( which is cold), condensation of water on the osb. There is no route for the damp osb to dry and you get rot.

Of course all of this is idealised and if you permanently occupied the building then you’d likely have a problem. Lots of people all the time results in lots of warm moist air. If you spend a few hours a week in the workshop then you have little moisture and lots of time between uses to dry out. However, with a non standard construction no one can tell you it will be ok. History will judge ;)

fitz.
 

Glitch

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Edit: What Fitz said.

If you follow Mike G’s way you can’t go wrong.
I’d argue the theory that OSB acts as a vapour barrier (it isn’t) but the key is to get warm damp air to the outside and not condense in the building structure.

I only found his method after I’d followed YouTubers who did it a different way which could lead to condensation problems.

From the inside out Mike has OSB, insulated framework, breathable waterproof membrane, battens then cladding.

No vapour barrier and no OSB (or ply) on the outside of the framework which is what I ended up with.

With a cold roof you must make sure there is unobstructed airflow between rafters on the underside of the roof boards.
 

Skydivermel

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Mines a 16 X 8 loglap on a 3 X 2 frame. What I did was glued 1" squares inside the frame, floor and roof to give a 1" air gap. Insulated the floor and the whole inside with 2" kingspan TP10 50mm insulation board. Sealed everything with 2" wide silver tape. Lined the internals with 12mm OSB and painted it white. No condensation problems and very warm in the winter & cool in the summer. Roof is black shingles which look very smart. As a side note I made a base from easyfit plastic grid squares layed on a base of sharp sand and a weed membrane then filled with shingle. Drains very well and keeps the floor bearers dry.
 

OldWood

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I would avoid felting if you can. I don't know anything about epdm, but used Onduline over osb on 3 of my sheds over 20 years.

The relative cost of doing felt properly - 3 layers - plus adhesive is little different from Onduline but the installation time is relatively zero in comparison and particularly this - clean!!
Rob
 

irishthump

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Damp walls, leading to rot etc, can occur due to moisture from the outside (rain) or inside (moist air /condensation). From the outside you ideally have a rain screen (cladding), a gap to allow drying, then a secondary barrier (building wrap/membrane). You dont want dpm as it is impermeable from both directions, if any moisture gets past it it will be trapped on your osb and rot it. A building wrap is breathable and will allow the building to dry if moisture gets past it. From the inside you need to avoid warm moist air getting to a part of the building that is cold (so the moisture condenses) and unvented (so the moisture cannot evaporated and dry out). So (ideally) you either have:
- osb (or plasterboard and vapour barrier), stud work and insulation, then building wrap, gap, cladding
- stud work no insulation, osb, building wrap, gap, cladding.

if you have osb/plasterboard, stud work and insulation, osb, building wrap, cladding. You risk moist air getting past you insulation, contacting the external osb ( which is cold), condensation of water on the osb. There is no route for the damp osb to dry and you get rot.

Of course all of this is idealised and if you permanently occupied the building then you’d likely have a problem. Lots of people all the time results in lots of warm moist air. If you spend a few hours a week in the workshop then you have little moisture and lots of time between uses to dry out. However, with a non standard construction no one can tell you it will be ok. History will judge ;)

fitz.

Thanks for the in depth reply, Fitz.

I reckon I'll go with - stud work no insulation, osb, building wrap, gap, cladding.
This will save me a few quid and allow for the option of adding insulation at a later date. My workshop will get only casual use and won't be heated so I think this will work fine.
 

irishthump

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I would avoid felting if you can. I don't know anything about epdm, but used Onduline over osb on 3 of my sheds over 20 years.

The relative cost of doing felt properly - 3 layers - plus adhesive is little different from Onduline but the installation time is relatively zero in comparison and particularly this - clean!!
Rob
.

I've been looking into corrugated sheet for the roof, but it may be cost prohibitive. I'm also not sure of the process for installing it on a flat roof, particularly the finish on the ends and sides. Any advice here would be appreciated....
 
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