Breather membrane below dpc - big no no?

[CB76]

Hi,

Planning on building a 4m x 7m timber summerhouse with shiplap or similar cladding and would like to know if its a big no no to take the the breather membrane down to the ground? The reason I would like to do this is to hide the 150mm exposed external foamboard I am fitting below dpc. I am looking for a quick and cheap way to cover the exposed foam. Have looked at covering with marine ply painted black but even that comes out quite expensive just to hide 150mm exposed foam.

The foam board is fitted to a single skin 3 course concrete block wall which is going below ground by 500mm and this to allow extra height inside building so I can keep within the maximum 2.5m height outside. On top of the concrete wall will be 100mm timber frames. I am thinking running dpm up wall and under timberframe.

First time on a forum and hope I've explained issue clearly.

 

[John D v2.0]

Diagram would help but the first 150mm above ground is a critical position as it's in the fitting line for splashing rain and UV from the sun.
As such you would need something suitable there, a breather membrane isn't. Not much in the way of timber is suitable for direct ground contact either.
Normally you'd either prop it up on bricks if you want it to last, or if it's a more proper building build a couple of courses of brickwork and dpc on top.

 

[Notch7]

Hi,

Planning on building a 4m x 7m timber summerhouse with shiplap or similar cladding and would like to know if its a big no no to take the the breather membrane down to the ground? The reason I would like to do this is to hide the 150mm exposed external foamboard I am fitting below dpc. I am looking for a quick and cheap way to cover the exposed foam. Have looked at covering with marine ply painted black but even that comes out quite expensive just to hide 150mm exposed foam.

The foam board is fitted to a single skin 3 course concrete block wall which is going below ground by 500mm and this to allow extra height inside building so I can keep within the maximum 2.5m height outside. On top of the concrete wall will be 100mm timber frames. I am thinking running dpm up wall and under timberframe.

First time on a forum and hope I've explained issue clearly.

I used wide dpc to staple onto the outside of my shed to hide the bottom timbers, then covered the outside with breathable membrane overlapping the dpc. When I clad the outsdie, the cladding will come down just below the breathable membrane, so you cant see that, just a bit of the black dpc. I used 300mm width.

Breathable membrane isnt usually uv stable so it will degrade if exposed to light long term.

 

[^woody^]

I dont think you understand the concept of a breather membrane, but if you just want to cover the insulation then lay polythene on it.

 

[CB76]

Hi,

Thanks for the replies. I have uploaded a drawing to show my thinking. Although more expensive I was thinking about using a uv breather membrane at the bottom as I understand normal breather membrane not good with the sun. I may use polythene suggested by woody.

As the inside floor level will be below ground level would it be okay to run DPM from under the concrete slab up the outside of wall and folded over wall to form DPC so under timber frame ?. I don't think it would be possible in one piece so I would have to do in two pieces of dpm. One for slab and one for wall with overlap being taped.

Cheers

 

[Notch7]

Im not sure if the detailing will work.

It seems to me you are 100% reliant on the dpm wrapped up the outside to keep out damp penetration. Any cut or tear of the dpm where it wraps around the concrete slab will create a leak path.

Since you dont have a cavity wall or the cavity extending below FFL, it seems risky.

You have a thermal bridge where the blocks sit on the slab, so your vapour barrier might cause condensation as its not on the warm side.

Im sure somebody will be along to give professionsl advice shortly......

 

[^woody^]

Here is the principle.

BTW, building off hardcore will cause the building to settle and move. More so of that hardcore is soaked by a drain.

You don't need sand under the concrete slab.

 

[CB76]

Hi,

Thanks Notch7 for quick reply.

Its early days with working out the details and you have pointed out an issue which I have been thinking about. Would it be worth taking the 25mm external foam so it wraps around the side of the concrete slab to reduce damp penetration and thermal bridging? I was hoping the 25mm external foam insulation next to the dpm and block wall would suffice with stopping damp penetration.

Sorry to sound stupid but don't fully understand what you mean by the vapour barrier isn't on the warm side as I had put it on the inside. Not clear in the drawing but was planning on starting vapour barrier above laminate flooring or which ever flooring I decide on. Is this correct?

Cheers

 

[CB76]

Hi Woody,

Thank you for advice in the drawing. I thought the dpm would have to be under the slab and on the outside of the wall but happy if its better to fit inside, it would also make it easier to fit. Also happy to save money by not having to use sand.

In the drawing you have put a blue line representing a strip of dpc behind the cladding. Would that be okay to take to the ground to hide the exposed external foam?

In regards to the hardcore settling. Would it still be okay to go ahead with building on the slab? The total height of the outbuilding would be 3m.

Cheers

 

[Notch7]

Hi,

Thanks Notch7 for quick reply.

Its early days with working out the details and you have pointed out an issue which I have been thinking about. Would it be worth taking the 25mm external foam so it wraps around the side of the concrete slab to reduce damp penetration and thermal bridging? I was hoping the 25mm external foam insulation next to the dpm and block wall would suffice with stopping damp penetration.

Sorry to sound stupid but don't fully understand what you mean by the vapour barrier isn't on the warm side as I had put it on the inside. Not clear in the drawing but was planning on starting vapour barrier above laminate flooring or which ever flooring I decide on. Is this correct?

Cheers

The concrete blocks are sitting on the cold concrete slab, so will transfer cold where it can meet with warm air. Ideally you would insulation on the inside covering the blockwork.

 

[CB76]

Hi,

In regards to woody's advice in putting the dpm going up the inside of the block concrete wall. Would I be able to fit standard 12.5mm plasterboard next to the dpm or would the thermal bridging coming from the floor up the wall cause a problem? Also where I use vapour barrier over the timber frame on the inside can it be dropped down so it overlaps the dpm or would this cause a problem?
If possible I don't want to put insulation on the inside of the wall as that would have a knock on effect with the wall not being flush where it meets the timber frame whence why I put insulation on the outside. I was planning on having insulation inside the timber frame. Not sure what insulation at the moment, maybe rockwool for sound insulation.

Cheers

 

[^woody^]

An external DPC is at more risk of tears and punctures, so given the option internal would be preferable.

A slab would normally be thickened at the edge to take the loads from the walls. I would avoid any hardcore below the slab. If this is necessary to make up ground, then better to thicken the edges so the edge goes down to ground below the hardcore under the rest of the slab. As drawn, with continuous hardcore and a french drain, its a poor design.

25mm external insulation is realistically not going to do anything - the blocks will never be heated enough for the insulation to perform. If you think it is necessary then best have it internally and continue it over the frame.

Timber cladding is very susceptible to damp and rot from splashing. So a minimum of 150mm (better 225 mm) to ground level is necessary. The blue line in my sketch is lead or plastic to act as a drip because if it is exposed then you need something that is UV stable. The frame membrane would lap over this further up behind the cladding where it is shaded. But in design terms you should have a ventilated cavity behind the cladding (opn top and bottom) and this is covered with insect mesh.

 

[John D v2.0]

Excellent post

25mm external insulation is realistically not going to do anything - the blocks will never be heated enough for the insulation to perform. If you think it is necessary then best have it internally and continue it over the frame.

This is the whole thermal mass debate.
Off the top of my head 100mm concrete blocks will be r<0.1 and 25mm Polly would be r=0.5+ so
If you want a stable temperature and heated most of the time 25mm will be useful and well positioned.
However I see your point is that in an outbuilding it's going to be I used occasionally so quick warm up is more important, and it might not be heated long enough to make it useful.
So I agree in this case but not in general

 

[CB76]

Hi,

Thanks Woody for your advice. I have uploaded drawing with concrete slab thickened ends. As it is essentially a shed I was hoping I could get away with thickening the ends but best to do it right and shouldn't be much extra work.

I have moved the insulation next to the block wall to inside and I was thinking about using 30mm insulated plasterboard which runs all the way up to the ceiling. I'm not sure about the effectiveness of the 30mm insulated plasterboard compared to having a thicker insulation in the timber frame and having no insulation by the block wall. I have reduced the height of the wall inside so don't have to dig down as much and now only two courses of blocks.

Appreciate any advice to new drawing design.

Cheers

 

[^woody^]

That would get you an A+