condensation from flatroof extension

[nigs18]

We have built a flat roof extension to our bungalow and now have condensation dripping through the ceiling.

The construction is,
3 layers polyester based roofing felt on 19mm ply. 25mm s.w. cross firrings to give a 1:40 fall on 50 x 200mm sw joists@400mm cts.
80mm celotex insulation between the joists and a further 50mm under the joists. Batterned to take 12mm plaster board and ceiling dry lined.

The condensation first appeared through the ceiling after the ceiling was dry lined. The condensation was forming between the ply roof and the insulation. We placed a dehumidifier in the roof which the reduced the condensation but was obviously not a cure.
The soffit was not vented on the flat roof as we had used breathable felt. However, the condensation did disappear after we vented the soffit with a 70mm soffit vents every 1m. We thought we had found our cure.
After heating the rooms, the condensation has returned and in places increased.
Does anyone have any idea how we can solve this problem without major reconstruction of the roof?
The gap between the ply and the insulation is approx. 150mm.

 

[evansbuilders]

Sounds quite a tricky one but i would say the polyester roofing material may be sweating and dripping through onto gyproc.airflow is normaly key but each joist space should be vented.

I personaly dont do alot of flat roofs so dont take my advice as gospil maybe wait a little to see if roofers familiar with that construction reply on here.just my persnal opinion thats what it could potentialy be

 

[freddiemercurystwin]

There needs to be a vent positioned between each joist not just at 1m centres.

 

[tim00]

I follow freddie ref. the ventilation issue.
Assuming a three wall extn:
Do the joists run from the "front" of the extn to the main house wall?
Or, do they run from extn sidewall to extn sidewall?
On three sides does the fascia stand off the wall - ie. say, a gap of 25mm?

 

[masona]

As mentioned, cross ventilation for circulation to stop any air pockets due to lack of flows

 

[^woody^]

Batterned to take 12mm plaster board and ceiling dry lined.

The condensation first appeared through the ceiling after the ceiling was dry lined.

The condensation was forming between the ply roof and the insulation.

The soffit was not vented on the flat roof as we had used breathable felt.

How have you done the battening? Any air voids?

What do you mean by drylining the ceiling when its a new roof?

How do you know where the condensation is forming - have you been able to see into the roof void?

What do you mean by having used breathable felt on a flat roof?

 

[nigs18]

How have you done the battening? Any air voids?

What do you mean by drylining the ceiling when its a new roof?

How do you know where the condensation is forming - have you been able to see into the roof void?

What do you mean by having used breathable felt on a flat roof?

we placed 50mm insulation over the joists and and put batterns on the insulation so plaster boards could be attached. So there is small air void between insulation and boards. The big air void is between the 70mm insulation( placed in between the joist) and the ply (approx 150mm gap) On the top of the air void is the where the condensation forms

We dry lined the plasterboards to make a "ceiling" in the room
We can see through the joist gaps from the original loft. we can see light coming through the soffit vents.
We have a tile upstand around the flat roof (giving an impression of a roof) this is where the breathable felt is and why we didnt vent the soffit originally

Do the joists run from the "front" of the extn to the main house wall?
Or, do they run from extn sidewall to extn sidewall?
On three sides does the fascia stand off the wall - ie. say, a gap of 25mm?

The joists run from front of extension to main house wall. This is why we can see through the joist "gap" to the outside wall (and can see light coming through the vents).
Fascia stands off the wall

We do have firrings allowing cross flow although they are quite small on the lower side of the flat roof.

Thanks so far for all the help. We plan to add more vents tomorrow as I get the impression that this can only help!

 

[freddiemercurystwin]

You appear to have cold roof construction (as opposed to a warm roof), this has a void under the decking (plywood), this void needs to be ventilated. A breathable membrane is not applicable to flat roof construction and is doing nothing, your moisture is condensing on the underside of the plywood and there is not enough airflow to extract it.

 

[nigs18]

JUst one thought that came to me last night, no sleep worrying about damp ceilings!
I now understand that the warm damp air from the rooms is permeating through the plasterboard etc into the void.
Is it worth using any special sealant type paint on the ceiling to reduce this? They are bedrooms with simple white ceilings so it would be simple to do this. Is this worth it?
Thanks loads

 

[freddiemercurystwin]

No, otherwise you'll be back in here in a few months wondering why condensation is now appearing on the ceiling! Allow the ceiling to breathe a little and excess be extracted to the outside.

A house needs to be able to breathe, sealing up houses to contain all possible heat and eradicating moisture build up are not compatible.

You could stop breathing but that maybe going a bit far.

 

[nigs18]

38 vents now in a 9 x 3 m extension. Soffit looks like swiss cheese.
Havent had a look yet, fingers crossed

 

[tim00]

Perhaps i'm missing something, but how will your new vents achieve cross-ventilation when your joists are running against you ie. blocking cross ventilation? Bored holes in the joists and furrings might help a little. Perhaps you have some kind of roof construction that i dont understand.

Your answer to my question ref. the fascia standing off indicates that whatever ventilation is available is coming up in the gap behind the fascia. This is not providing cross-ventilation.

 

[nigs18]

I am also confused with the cross ventilation vents and how air can flow across the joists although there is a gap the the firrings has made. Boring holes would be a problem to do as everything is in place now.

The new vents have made a difference and it seems to be drying out slightly. However no one is living in the rooms yet and the heating is only low. Also one or two of the joist "gaps" have noggins half way through stopping airflow.

Vents in the tiling of fake roof perhaps?

My brother in law who deals with condensation problems in industry suggested extractor fans on/near soffits, running for a few hours a day! Is this recommended?

We have followed the architects plans and not going in completely blind!
Thanks for everyones help

 

[freddiemercurystwin]

It is too early to tell if your new vents have made a difference or not.

For the sake of mine (and I suspect anyone else trying to fathom out exactly how your roof is built), please can you post a photo of the roof and indicate which way the joists run and where you have added ventilation. You need to provide a vent at each end of the void between each joist if that has not yet been achieved it needs to be.

 

[^woody^]

My brother in law who deals with condensation problems in industry suggested extractor fans on/near soffits, running for a few hours a day!

We have followed the architects plans and not going in completely blind!

Sorry to say, but ignore your brother-in-law.

As for the architect, he's designed you a poor roof.

You are getting condensation because you have some moist air getting up into the void from the room below

Having a plasterboard ceiling, and then an air gap and then the insulation is a recipe for interstitial condensation - moisture is going to permeate the plasterboard and linger in the void, then it will ether condense or then travel through to the bigger void below the deck. That potential will not go away even if the main roof void is ventilated

You can get through-ventilation via the soffit vents and then forming a gap where the flat roof joists meet the main house roof and then ventilating the main roof void. But this wont deal with the gap between the ceiling and insulation