Membrane to go behind cladding

[Pete Bannister]

I don’t think the maker's name was visible from within the roof Steve.

 

I’ve seen quite a few similar examples of 'failures' throughout the UK over the last 15 years or so. They would all probably have occurred with any vapour permeable membrane because there's always a construction or maintenance defect encouraging the condensation. The fact is at whilst these membrane offer great benefits, they also eliminate the ventilation which happens via the gaps in cladding (e.g. slate, tiles, stones or shingles etc.).

I’ve nothing against MVP membranes (having just spent over £100 per roll for top-end stuff on my own house) but folk often think that because they are called 'breathable' that they can forget about interstitial condensation risk in construction. They do so at their peril.

In a temperate climate, condensation will occur at some point in the envelope of the building for much of the annual cycle. It’s not practical to stop it happening but traditional building techniques have evolved to help materials survive this condensation when it happens. Ventilation of a traditional ‘cold’ roof construction won’t stop condensation occurring but it will greatly speed up its re-evaporation after the event.

If you are recovering an existing roof and need to keep water out of the building during the process, breather membranes are really helpful and a much cheaper option than a temporary roof

[jamesd]

Cheers for the help everyone, I'm also going to insulate the shed inbetween the stud work would this make a difference to the type of membrane i need? and what sort of insulation would you use? I was thinking of the kind you cut to size with a handsaw.

[Graham w]

kingspan sounds expensive i would use 100mm glass wool insulation between framework then on the outside of the frame have a sheet of ply to hold the insulation, on the ply i would mount a building paper or roofing paper usually rubberoid then at 600mm spacings vertically fix a 10mm x 40mm strap (i call them drip strips because they allow water to run off your paper) on the strap fix a 40mm x 50mm batton horizontally this gives your air gap, then mount your cladding

Edited November 5, 2014 by Graham w
la la la!

[mtt.tr]

tar paper?

[kevinjohnsonmbe]

Jamesd, previous comments are correct; you don’t need a waterproof barrier behind well-designed timber cladding (unless its a partial rain-screen type cladding). The product you identify is probably not a good idea for your shed.

 

A vapour permeable membrane ('breather membrane') will reduce air-filtration /draughts if heat gain or loss is your concern. As others indicate, that’s a better option. But even so called breather membranes are not always a good thing.

 

Photo show condensation-related mould growth in a traditional roof construction where a high-quality 'breather membrane' has been used. You can see the grey mould and if you look carefully you'll see the condensate droplets too.

 

Effective building is more about the way you use materials rather than the materials themselves.

 

Interesting pic Pete. Would the (or a) likely cause of this problem be insufficient venting above the vapour barrier to carry off the moisture? If what I've read is right, there is a need for airflow, via vented soffit and dry fix ridge tiles for example, to allow the air above the VB to carry the moisture away.

[codlasher]

Jamesd, previous comments are correct; you don’t need a waterproof barrier behind well-designed timber cladding (unless its a partial rain-screen type cladding). The product you identify is probably not a good idea for your shed.

 

A vapour permeable membrane ('breather membrane') will reduce air-filtration /draughts if heat gain or loss is your concern. As others indicate, that’s a better option. But even so called breather membranes are not always a good thing.

 

Photo show condensation-related mould growth in a traditional roof construction where a high-quality 'breather membrane' has been used. You can see the grey mould and if you look carefully you'll see the condensate droplets too.

 

Effective building is more about the way you use materials rather than the materials themselves.

 

If your roof finishes like this no wonder there's a condensation problem. This would happen with any membrane, be it traditional sacking felt or etc. Where you have a temperature difference you will have a dew point on the inner surface and presto condensation. Traditional roofs were ventilated for just this reason. This kept both the inner and outer surfaces at almost the same temperature so no problem. A warm roof such as in a modern house will need insulation to keep the dew point separate. The ventilation is elsewhere.

On a shed however, I would not be so worried unless as in the picture, there is no air flow between the wall plate and the rafters. Traditional 4'' x 2'' rafters 'birds-mouthed' over the wall plate will always have an air gap. It is then down to the constructor on how the ventilation is controlled on the external soffit.....which can be fixed if needed under the facia.:big grin:

Look at this series of pictures to get somme understanding of the warm roof concept https://www.google.co.uk/search?q=Warm+roof&client=safari&rls=en&tbm=isch&tbo=u&source=univ&sa=X&ei=AbJbVLvSNsTW7Ab-q4CAAg&ved=0CCIQsAQ&biw=1759&bih=848

codlasher

Edited November 6, 2014 by codlasher

[jamesd]

If your roof finishes like this no wonder there's a condensation problem. This would happen with any membrane, be it traditional sacking felt or etc. Where you have a temperature difference you will have a dew point on the inner surface and presto condensation. Traditional roofs were ventilated for just this reason. This kept both the inner and outer surfaces at almost the same temperature so no problem. A warm roof such as in a modern house will need insulation to keep the dew point separate. The ventilation is elsewhere.

On a shed however, I would not be so worried unless as in the picture, there is no air flow between the wall plate and the rafters. Traditional 4'' x 2'' rafters 'birds-mouthed' over the wall plate will always have an air gap. It is then down to the constructor on how the ventilation is controlled on the external soffit.....which can be fixed if needed under the facia.:big grin:

Look at this series of pictures to get somme understanding of the warm roof concept https://www.google.co.uk/search?q=Warm+roof&client=safari&rls=en&tbm=isch&tbo=u&source=univ&sa=X&ei=AbJbVLvSNsTW7Ab-q4CAAg&ved=0CCIQsAQ&biw=1759&bih=848

codlasher

 

Ok thanks i think i get the idea of how to insulate the roof but how about the walls?

I was thinking of just filling the spaces between the 4 x 2 studwork with insulation. So it would go feather board, membrane, insulation, thin ply to cover the insulation. would this be ok or would i get a lot of condensation. I was intending on not having an air gap just to make things less complicated.

[codlasher]

The very first shed that I helped build was in 1976, that hot, hot summer! Timber framed with one bay as a TV room for teenage children.

The whole building was felted using traditional tarred felt and then weatherboarded with 7" x 3/4'' larch from the sawmill. The bay that was to be the children's den was then infilled with fibreglass and plaster boarded. As the heating at that time was by an electric fire there was never going to be problems with heat differences. The shed still stands today and those children now have teenage offspring themselves!

So that is one way.

If you want to use this as a warm workshop and have a few more £'s and I can't remember if this is an alteration or new build. If it is an alteration I'd fix a 19mm (3/4") batten against the existing felt, on each upright, to create an air gap. Then I'd cut some Ecotherm/Kingspan type foil backed insulation to fit between the uprights. Bad cuts and joins can be foamy glued with Dow Insta-Stik. If you have more £'s you can then use insulated plasterboard to cover over this. Or up the game and add 50mm them either plasterboard or Sterling board.

A new build will work in a similar principal but you will be working this in reverse fitting the hard insulation between the uprights first then short bits of batten of foamy glue to hold this in place, then membrane, them weather board. Remembering to leave the bottom tilt fillet with an air gap between each upright. This theme continues right up into the rafters and roof. Then the breathable membrane will do its job.

Phew! Have fun....

codlasher.

[jamesd]

The very first shed that I helped build was in 1976, that hot, hot summer! Timber framed with one bay as a TV room for teenage children.

The whole building was felted using traditional tarred felt and then weatherboarded with 7" x 3/4'' larch from the sawmill. The bay that was to be the children's den was then infilled with fibreglass and plaster boarded. As the heating at that time was by an electric fire there was never going to be problems with heat differences. The shed still stands today and those children now have teenage offspring themselves!

So that is one way.

If you want to use this as a warm workshop and have a few more £'s and I can't remember if this is an alteration or new build. If it is an alteration I'd fix a 19mm (3/4") batten against the existing felt, on each upright, to create an air gap. Then I'd cut some Ecotherm/Kingspan type foil backed insulation to fit between the uprights. Bad cuts and joins can be foamy glued with Dow Insta-Stik. If you have more £'s you can then use insulated plasterboard to cover over this. Or up the game and add 50mm them either plasterboard or Sterling board.

A new build will work in a similar principal but you will be working this in reverse fitting the hard insulation between the uprights first then short bits of batten of foamy glue to hold this in place, then membrane, them weather board. Remembering to leave the bottom tilt fillet with an air gap between each upright. This theme continues right up into the rafters and roof. Then the breathable membrane will do its job.

Phew! Have fun....

codlasher.

 

Ok thanks that great, so by using some batten that will create a small air gap? its a new build and i'm using 3" x 2" for the stud work (its not a big shed say 12foot by 7foot) so what do you think to 2" of insulation and a 1" piece of batten to create the air gap between the membrane/featherboard and insulation.

 

Also not sure i understand this bit:

"Remembering to leave the bottom tilt fillet with an air gap between each upright."

 

I apologise for all the question, i don't want to have to rip the shed apart and do it all again!

[codlasher]

Ok thanks that great, so by using some batten that will create a small air gap? its a new build and i'm using 3" x 2" for the stud work (its not a big shed say 12foot by 7foot) so what do you think to 2" of insulation and a 1" piece of batten to create the air gap between the membrane/featherboard and insulation.

 

Also not sure i understand this bit:

"Remembering to leave the bottom tilt fillet with an air gap between each upright."

 

I apologise for all the question, i don't want to have to rip the shed apart and do it all again!

 

When you start your weatherboarding at the bottom by the wall plate you will have to 'tilt' the first board to create the correct shape to allow any driven rain to 'drip' off. This would normally be made by a piece of 3" x 2" cut diagonally and fixed on the wall plate, then the first board overlaps this by 1 1/4" to form the drip......If this is spaced out a little you will have formed a continuous air gap for the air to move in the gap you created. It doesn't need to be much as you don't want insects/mice to have access, just an air flow.

I am posting a picture of a barn roof showing 100mm insulation fitted between 150mm x 50mm rafters to give you an idea of how your shed should look. The foamy glue holds it in place.

There is a traditional set of trusses and double purlins underneath so the insulation is partly held in place by these timbers. The foamy glue just stops movement and any gaps. The Tyvek is laid on this followed by battens and then the roofing material itself. You can use temporary battens fixed to the inside 3" x 2" uprights to stop the insulation falling inwards. Then the foamy glue to permanently fix it in place.

codlasher