insane in the membrane

29 Mar 2009
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United Kingdom
sorry for the title, things are beginning to get to me...

So for my timber garden office I need the following vapor and breathable membranes but not sure what to use where:

floor joists (celotex in voids)>vapor barrier>chipboard floor>carpet

Do I just use a 1200 gauge polythene sheet here?

Insulated PB (contains own vapor barrier)>Celotex>WPB>Breathable membrane>baton>Cladding

Not sure what breathable membrane to use here and what spec it should be?

CEILING (warm deck)
Joists>WPB>Vapour barrier>Celotex>Rubber membrane

Again not sure what spec or product to use on the roof, is it simply a 1200 gauge polythene sheet or does it need to be breathable?

Thanks once again...
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A vapour barrier is fitted to prevent moist air from INSIDE the building migrating to the outside, thus eliminating condensation issues.

Membranes are there to stop water from the OUTSIDE coming in.

Breather membranes are also to stop water from the OUTSIDE from coming in but also allow water vapour from INSIDE to escape.
Thanks for the info but what should I use and where? Still a little confused by all the differences out there... :confused:
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OK, bitten the bullet and buying Tyvek housewrap but as 100m is the smallest roll I can get i'll have 60m left over therefore wondered if I could use this on both the floor and roof instead of 1200DPM or am I better sticking with plastic?


thanks for the clarification...


Seems like you're becoming my personal mentor :LOL: And also the person who brings me back down to earth (mostly with a bump)

So when you say 'any old' you're saying others are more than adequate and it doesn't need to be Tyvek? I don't want to get caught cutting corners to save a buck or two however a friend has a roll of Cromar Vent 3 left over from a job and said I could have it but thought I had to really use Tyvek Housewrap...

thanks for the link, i'll buy that if the Cromar Vent 3 isn't right for the job.
I only said Tyvek because thats what we use, there are always other products out there. Tyvek HomeWrap is particularly good, also worth noting that different 'Breather Membranes' have different gradings from BBA and are not all as brether as the next manufacturer.

I'm just not sure which ones for each application, I just know you can;t go that far wrong with Tyvek - and there's not that much money in it!

Good luck though.
Tyvek Housewrap is just another in a long list of suitable membrnaes, Tyvek have just been at the forefront of advertising campaigns and getting their names out there and with architects etc so this is what is commonly specified. I do it myself on my own drawings: Tyvek or other similar approved breathable membrane.
Appreciate all the advice guys – really do, think i'll go for item suggested off ebay as i'm about to shell out a whack on insulation (how Much????????)

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