I'm building a timber framed workshop on a concrete base. I'll use a single course of blocks to keep the timber off the ground but that means that the blocks will be inside the dpm, unless I do a two stage concrete process. Is that a good idea?
Walls will be 4 x 2 clad with waney edge, insulated and lined with ply or similar.
Answers Vapour barrier (check is a better word) on the warm side of the insulation always. I have failed to understand your question on the concrete but ideally the concrete should be isolated in its entirety from what ever is below it ( I assume hardcore, hogging, dpm then concrete) with the whole lot "surrounded" by say Tanalised shuttering to stay or if you want something nicer a low wall of bricks. The dpm can then be turned in on top of the concrete and glued down The stud can then go on a row of bricks on top of the original external ones with a normal brick dpm (dont point it in) and the timber on top of that Being a belt and braces man I would also put another dpm on the bottom of the timber but not real need the cladding then goes down the outside covering the brick as far down or not as you like to stop the weather getting in at the base I would recommend the whole brick If no brick but using shuttering put the new layer of bricks on the concrete ON the turned in layer of dpc and the timber on top and bring the cladding down to the shuttering consider a lead or other cover flashing over this join from inside to out dressed over the top of the timber
Your bit about the blocks / dpm / concrete is a bit garbled, what do you mean 'a two stage concrete process'? - do you mean sandwich the dpm in concrete above and below? - I can't see any need for this, you simply need a layer of dpm underneath your concrete, and it's irrelevant whether it comes up the sides or not, the object is to stop damp from underneath.
Once your slab is laid, you can lay a course of blocks on rolled out dpc and build your timberwork off that, I wouldn't put dpc between timber and blockwork, otherwise you are creating a trap for water to hold.
Oh dear, and I did try to make my self clear. What I meant was that if I make a concrete pad with some dpm underneath and going up the sides then the blocks will have to go inside the dpm. To get the blocks outside the dpm like a house I'd have to make a small wall with it's own foundation then fill in the floor with concrete in a separate process.
However between your reply and chris's I think I can work out what to do
each case, the dpm and dpc are denoted in red, concrete in grey and timber in blue, ground level is a green/brown line. Depending upon your existing ground levels, and how much of a step up you want into your workshop, bearing in mind potential flooding etc, any of these will be suitable, although D has the dpm visible outside, which, apart from being ugly as sin, breaks down through UV and weather processes, if it were me, I'd be opting for B, but the choice is yours
Nick - the point of the dpm is to prevent soil moisture getting into the structure. I had a bit of difficulty seeing your point until your second post, and having used the format that you are using on 3 different sheds, I realise that you are potentially over-egging the cake.
Am I right in that you think that the concrete blocks will pass moisture into the structure? It doesn't actually seem to happen, but the way I got round that is to carry the cladding down over the blocks. I screwed the cladding on using wall plugs but doubleside taped 3 mm plastic shims from Screwfix on the inside to create an airgap.
You've had an answer about the vapour barrier (it's there to stop air borne moisture condensing in the insulation), but do remember that you should arrange an air gap between the insulation and the cladding to allow the cladding to breath. I've had it from a good source that OSB board is so loaded with adhesive that if you line the wall with that, then a membrane is not necessary.
As an added recommendation, look at Onduline for the roof cover. Costs no more than felt and is ten times easier to apply. Mine's been up for two years now and looks brand new still.
Oh - and by the way, the downside of the vapour barrier is that the moisture in the warm air in the workshop will have no where to go when the workshop cools off except on your expensive metalwork, so arrange some ventilation.
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