External wall vapour barier

22 Sep 2009
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United Kingdom
Any advise on the type of brush on vapour membrane/seal product that would be the best to apply on the internal face of a single brick wall and a compatable dap adhesive for bonding foil faced insulation board to the membrane, along with a bonding agent to bond plaster board the same foil faced insulation
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You are best buying insulation+foil backed plasterboards, then either fix them to timber battens or use a foam adhesive to fix them directly to the wall.

I know very little about brush applied membranes although synthaprufe springs to mind.
OP seems to be confusing vapour barriers and damp membranes

Better to explain what you are trying to achieve - are you just dry lining a wall? Is there a damp problem?
Thanks for your replies. I’m currently converting a attached garage in to a habitable space, half the current garage (external half) has a single brick course, to meet the building regs a single brick wall is sufficient providing the internal face has a suitable vapour barrier and is well insulated, as this is what I was told by the local building control who suggested a paint on type of membrane.

For me it will be easer/cheaper to line the walls with a hanging polyphen sheet/membrane, batten, and insulate between the battens and plaster board, would this be acceptable to BC

I already have a large stack of 80mm foil faced insulating board acquired from a nabour at no charge (retail £40 a sheet) that I intent to use and save some money
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If I have read your post correctly, you are proposing to put the polythene sheet again the inside of the brick wall. This is WRONG.

The purpose of the barrier is to prevent warm, moisture-laden air from the inside meeting cold air/surfaces and condensing and this is done by installing it on the warm-side of the insulation i.e. towards the habitable area.

Put your sheet immediately behind the pb.
I was led to believe the membrane against the brick was to prevent the moisture absorbed through the single brick from passing beyond the membrane

Should I consider installing 2 layers of membrane, 1 against the brick face and 1 behind the PB or would this cours the cavity between the 2 membranes to sweat creating a further problem
I don't know why moisture should pass through the bricks -are they very porous ? In any case , even if you were to put the poly against the wall, this would not stop moisture coming through, just prevent it from going further, then condensing and running down the sheet.

As far as I am aware ( not builder) the only conditions where water is conceived of penetrating walls - except when they are in poor condition/saturated by broken down-pipe etc - is when you have lashing rain and high winds which is more typical of coastal areas and it is for these conditions that cavity- walls were initially introduced.

As far as the second membrane is concerned, I am uncertain. Some people talk about the need to allow the wall to breathe, but if -as I have written above - moisture won't permeate from out to in, I'm not sure why it would then move from in to out
The principle of dry lining is to line the internal face of an external wall to stop moisture getting in - hence polythene.

When insulation is then put up the wall, and with no air voids, then this removes cold surfaces for air to condense on and so a vapour check membrane is not required.

If the wall was timber frame, then a vapour check would be required on the inside face to prevent moisture from entering the frame to rot the timber, and a moisture resistant breathable membrane would be required to the outside face to stop water getting in from the outside and to allow the timber to breathe.

If the OP does not place a physical membrane up against the wall, then water will potentially enter and drip down the wall or allow mould to grow behind the plaster lining.
Logically what you have stated would seem correct, however as the building control advised, I can either double up the existing wall by building another course of bricks, alternatively I can install a membrane and insulation to coincide with the B Regs.
They were not specific to the type or position of the membrane

The existing brick wall is of a solid none pourers type brick

As suggested in you replies, i would propose to install treated battens, Foil faced hard insulation 80mm, Plastic membrane sheet, Plaster Board, in that order

Please tell me if it is normal or common for moisture ( does this mean water ? ) to penetrate walls and if so, why ?

This is a genuine question - i'm not trying to be a smart-arse - because I have difficulty understanding how moisture will penetrate a normal single-skin brick- wall which is in good condition.

The why part of it relates to my long-ago physics lessons, from which I remember that natural reactions are governed by elements (whatever) taking the path of least resistance.

This for me would me that instead of trying to penetrate a brick-wall (difficult), moisture would simply float around outside (easy, less "effort" ).Grateful for any light you might be able to shed.
Moisture can permeate a masonry wall via several methods - capillary action, diffusion, sorption, wind and gravity.

Generally, the transfer of moisture depends on the spacing and size of pores within the material, and the action of external forces - heat and wind.

The denser the brick, or the fewer the voids or pores in the mortar then the less chance of moisture moving far. But unless the material is truly waterproof, then there will always be some movement of water.

Effectively the wall acts like a sponge, so moisture/water does not hang around on the external surface
Very good post that Woods. Brought back memories of days spent in the labs at Uni with trays filled with water and different types of bricks partially submerged. A set of scales, a stopwatch and a calculator and we were away! Good times! :LOL:

Then days in the library spent slavishly trawling for obscure BRE and British Standards docs to support your conclusions, which back then were all stored on fudging microfiche!!! Bad times! :cry:
Is your idea of a single brick wall a 9" thick brick wall, or do you have some idea that a 4" - 1/2brick wall is a single brick wall? Point is, if it is the latter 1/2 brick wall you will have some way to go to make it a habitable room!

As noted on a previas post,. I was advised by Building Control that providing a vapour barrier is created and the single 4" brick wall is well insulated, this be compliant with the current B Regs, the insulation values was stated but cannot remember the figures

Any advise on other works required to the single wall not mentioned by BC

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