Pitched roof insulation & condensation

[polkablues]

I have recently had built a 3x8metre wraparound type extension to the rear of my property and have found this forum invaluable during the process. I had the builder build to a shell watertight finish. eg roof and glazing installed, screed floor and all steelwork and then it was over to me to insulate the roof and bring in electricians etc. All regs and planning are in place

I insulated the roof with 100mm kingspan between rafters and 50mm over rafters with taped joints, held up with roofing batten which I then fixed plasterboard to. All looked well until I noticed dripping from the top of each velux after ruling out a leaking roof I took down part of the ceiling and the underside of the breather membrane was soaking wet with I can only assume was condensation, it has now dried out with the insulation removed however all 4 velux over the roof are affected with the same drip some the whole roof must be full of condensation.

The roof has cromar vent3 membrane and Marley modern tile, a 50mm air gap between membrane and insulation which I though would avoid condensation.

I would be very grateful for some advice on trying to eliminate this issue, bar from taking the insulation down im at a loss I have posted some pictures of the ceiling [/b]

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[Blagard]

The roof has cromar vent3 membrane and Marley modern tile, a 50mm air gap between membrane and insulation which I though would avoid condensation.

Have you got eaves and top ventilation to the gap? If not that is likely the problem although I am surprised because you seem to have done the vapour barrier right on the kingspan by taping the joints

 

[polkablues]

Thanks for the reply, its a lean to roof with no ridge so there is definitely no ventilation there, the soffit and facia are of solid construction, the builder was confident that the breathable membrane did not require eaves ventilation. Now however im not so sure!

I also forgot to mention i am yet to insulate and board up to the velux windows but wanted to try and resolve this issue first. The builder suggested lining the whole ceiling in a vapour barrier and then refitting the plasterboard - i don't want to go to this effort if it wont cure the problem as i thought kingspan taped was an effective barrier alone!

thanks once again

 

[jeds]

Your builder is right - venting is not a requirement where a breathable membrane and vapour layer is properly used. It might just be excessive moisture trapped in the void that the membrane wasn't able to cope with. When stabilised it might never occur again. If it does the easiest way to cure would be to vent the void - top and bottom.

 

[polkablues]

Thanks for the reply Jed, I will leave the board and insulation down to allow it to dry out. Where the insulation has been removed there is now no condensation on the underside of the membrane?

I have also been running a dehumidifier over the past week or so to help dry out the new extension

Would adding a vapour barrier cure the problem of condensation without the need for vent tiles you think?

 

[jeds]

Sorry, I misunderstood. I thought you said there was a VCL in place. You do need a vapour layer in that construction.

 

[polkablues]

Thanks Jeds,

Hopefully that should cure it then. Is this the right product ? http://www.screwfix.com/p/vapour-barrier-2-5m-x-20m/12869

If not what type would you suggest

it must be the warm moist air getting through, although I thought 150mm of taped kingspan would have been quite an effective barrier - obviously not!

 

[limerockconstruction]

I am of the understanding that although lots of these membranes are sold as 'breather membranes' that there are only 2 fully breathable membranes on the market ie; those that do not require any ventilation whatsoever.

Roofshield is one of these, I believe that the other is one of the Tyvek products. (although this may not be the case anymore so please forgive if this is misinformation)

All of the others still require some sort of high and low level ventilation dependant on what level of BBA certification they have been given. A BBA certified breather membrane doesn't automatically give it full breathability.

Even if you don't have a ridge to ventilate from you could still install vent tiles high up.

I was also under the impression that the taping of the overboard of 50mm celotex DOES provide a VCL.

 

[jeds]

Just to backtrack again. Re-reading your original post you did say the under-boarding was fully taped which, providing it's done correctly with the right tape, does qualify as a VCL. My guess is as I said first off; it might be that this is a one off due to excessive moisture in the construction materials. Once stabilised it might be ok.

 

[polkablues]

Just to backtrack again. Re-reading your original post you did say the under-boarding was fully taped which, providing it's done correctly with the right tape, does qualify as a VCL. My guess is as I said first off; it might be that this is a one off due to excessive moisture in the construction materials. Once stabilised it might be ok.

Hi Jeds, I used aluminium tape to tape the 50mm underlay insulation over the rafters. I was wondering is the warm air was entering the roof space where the velux's are yet to be plasterboarded.

I have left a section of insulation and plasterboard down and that section is now bone dry. I will put it back up and see what happens next!

Thanks again for the advice

 

[polkablues]

I am of the understanding that although lots of these membranes are sold as 'breather membranes' that there are only 2 fully breathable membranes on the market ie; those that do not require any ventilation whatsoever.

Roofshield is one of these, I believe that the other is one of the Tyvek products. (although this may not be the case anymore so please forgive if this is misinformation)

All of the others still require some sort of high and low level ventilation dependant on what level of BBA certification they have been given. A BBA certified breather membrane doesn't automatically give it full breathability.

Even if you don't have a ridge to ventilate from you could still install vent tiles high up.

I was also under the impression that the taping of the overboard of 50mm celotex DOES provide a VCL.

Thanks for the reply, its really interesting about BBA, I found the certificate and 6.5 states no ventilation is required( if I'm reading it correctly) http://www.bbacerts.co.uk/CertificateFiles/47/4748PS1i1a29.04.2010.pdf

Im wondering if I finish the boarding up to the veluxs' this may stop it as if the moisture is getting in the gap between the velux and the rafter

ive tried to make the pictures visible in the post[/img]

 

[Architexeter]

Having read all the above.

I'd agree that taped boards form a VCL.

Use Tyvek in future

High level vents can be tile/slate vents as glidevale or similar.

You mention a lean to, please elaborate on this junction with the existing house, is it cavity, did you install a cavity tray ?

It may well be just that it all needs a bit of drying out time. Get some heat in the room as well.

 

[polkablues]

Having read all the above.

I'd agree that taped boards form a VCL.

Use Tyvek in future

High level vents can be tile/slate vents as glidevale or similar.

You mention a lean to, please elaborate on this junction with the existing house, is it cavity, did you install a cavity tray ?

It may well be just that it all needs a bit of drying out time. Get some heat in the room as well.

Unfortunately I let the builder collect the breathable membrane, naively assuming they did what they said on the tin, this now appears to be incorrect

The roof adjoins the rear of the property which is a 9 inch solid brickwork wall

I have had the under floor heating on prior to Christmas, the room temp is always now between 18-20 degrees dependant on time of day.

looks like some vents maybe in order then?

 

[polkablues]

Update,

Taken the plasterboard down and fitted a VCL taped and stapled to rafters, also ensuring all kingspan is foil taped etc, re installed plasterboard.

Pushed back some tiles today and the underside of the cromar vent 3 light was still soaking wet, I first suspected a leak however the top of the membrane was dry.

Im thinking venting is the answer, does this need to be to the top or underside of the felt?