Shed build, many many questions...

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Lazurus

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EPDM over 19mm osb for the roof, easy to do, very long lasting and competitive pricing against other materials. I did 40m2 myself in a day, all in including tools and adhesives £1200
DPM.jpg
 

Torx

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Hi Torx. In researching my own upcoming build, I've stumbled on this thread, and would like to share some of what I've put together. Please take it with a heap of salt, as I've likely forgotten more than I've learnt over this past year. And do ignore my advice where it conflicts with that of people more credentialed (I have practically none, so the bar is low).

OSB3 on the exterior as sheathing should be fine, provided you take steps elsewhere to reduce condensation risk; most OSB3 is at least a little vapour permeable, albeit much less than OSB2, etc. The main cause of condensation risk will be airtightness and vapour permeability of the building envelope, including the fitting of the insulation. Most often, insulation isn't fit - or foamed - tight to the frame. The gaps left, big or small, allow air and water vapour to exit the enclosure, and can cause condensation on the interior faces of external sheathing, which can cause damp problems you won't find until it's too late. It's especially a problem in roofs, I think due to mistakes or compromises in the buildup.

As noted by others here, a vapour barrier (VCL or vapour control layer) should go on the warm side of the insulation. Something to consider for your posh shed, and particularly for your garden office: You won't need to increase your buildings ability to transmit vapour to the outside (such as by sheathing the interior only), if you put adequate controls in place to reduce the transmission of vapour from the interior through the envelope. A plastic vapour barrier can be fitted to the floor, wall, and ceiling, and taped at the joints and openings using an appropriate tape. Provided your door and window openings are weather sealed, this can greatly decrease the transmission of vapour/ condensation from the interior to the exterior. Something people often neglect is that any openings made in the vapour barrier - such as for electrics - need to be sealed properly. I don't recall how much moisture can be exchanged through holes made for downlighters, sockets, etc. but I remember it was alarming.

Another thing to consider is the cold bridging that could occur on the interior faces, where the timber frame meets the interior sheathing. This is when heat from the interior escapes quicker at points where the frame is in contact with the interior surfaces, and creates a cold spot where condensation can form on the interior. It happens most where many timbers are sandwiched together, or in corners that aren't insulated. One common way to overcome this is to use insulated plasterboard to sheathe the interior, or thin PIR insulation underneath sheets of plasterboard, where cheaper.

The purpose of some of the advice in this thread seems to be to reduce the risk of interstitial condensation, which is important for the longevity of your build. But, I'd say it's also important to look at vapour control more broadly, and find ways to stop it getting into the building fabric in the first place. Unfortunately, most of the stuff I've mentioned will inevitably add to cost, but should also improve the performance of your building.

Hopefully I've not bludgeoned you with my essay. Good luck with your build!

PS Yes to the weed membrane. Pin it down with plastic pegs.

Thanks, good info. It does seem a little like the more thermally efficient a building is the more there is to go wrong! I’ve hear of problems with SIPs simply because the building end up too well sealed. Actually I’m curious what sort of OSB SIPs are made from.
 

Torx

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Been on holiday so not much time for this but nearly there with the ‘groundwork’s’.

Is there any advantage to having a block set 50mm deep in hardcore with a second block on top (mortar between) or just sit one block straight on top of the hardcore with sharp sand?

Thanks
 

Jameshow

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Been on holiday so not much time for this but nearly there with the ‘groundwork’s’.

Is there any advantage to having a block set 50mm deep in hardcore with a second block on top (mortar between) or just sit one block straight on top of the hardcore with sharp sand?

Thanks
What about blocks set in mortar on top of the hardcore the block is then stuck the top layer of hardcore and isn't going anywhere.

Make it a strong but fairly runny mix so it runs though much of the hardcore??
 

PDW125

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Is there any advantage to having a block set 50mm deep in hardcore with a second block on top (mortar between) or just sit one block straight on top of the hardcore with sharp sand?

6:1 sharp sand and cement dry mix will absorb moisture from the ground and set nicely to stop them moving. Easy to level too and will allow you to move things about
 

fiveforty

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Been on holiday so not much time for this but nearly there with the ‘groundwork’s’.

Is there any advantage to having a block set 50mm deep in hardcore with a second block on top (mortar between) or just sit one block straight on top of the hardcore with sharp sand?

Thanks
Sharp sand on its own will eventually wash through the hardcore, as PDW125 says a 6:1 sand/cement mix
 

Torx

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6:1 sharp sand and cement dry mix will absorb moisture from the ground and set nicely to stop them moving. Easy to level too and will allow you to move things about
I like this plan, time to level everything before it goes off!
 

mancmc

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For the shed bearers or floor framework, I would use composite timbers say 100 x 50mm section.
Therefore, no timber, tantalised or otherwise in contact with the ground block.
More expensive at the moment but you will get the longevity. Tantalised timbers are not as good as say 20 years ago!
 

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