Damp patches on ceiling of new extension

[mashley]

Hi all

We have had a 4m single story extension built on the back of our house. The roof has a 15 degree pitch and is tiled with Sandtoft plain interlocking tiles. The carcass is constructed from 2x7 timber with regular noggins. It is felted with breathable membrane, and insulated with 100mm Celotex cut to size and pressed in flush to the plasterboard. The ceiling is now plastered and painted.

Whenever there is a frost or very cold weather and the heating is on, we get damp patches in several specific locations on the ceiling. The patches appear to form in thin lines where the noggins are located.

Our builder has told us that he insulated the roof the same way as other roofs he has built and has not had this problem before, and our roofer has said that the construction and felting of the roof is the same as other roofs he has built. We are therefore at a loss as to why these damp patches are appearing.

We would appreciate any advice or guidance anyone can give to help us identify the cause.

 

[noseall]

Sounds like condensation. Just because the builder said "we've always done it this way" does not mean it is correct. Who designed the roof and is there a drawing? Is this a vaulted roof?


Things to consider....
Breathable membrane does not deal well with extremes of temperature and is not a magical panacea for ALL things condensation. Often vents are needed in conjunction with BM.

Has the celotex been cut snug and have all gaps been filled? Was foil tape used on joints? Was a sealant used to fill gaps? Was foam used to fill gaps?

Was a dedicated vapour barrier installed?

If this is a vaulted roof was there just one layer of celotex between the rafters and no underdraw? If there was an underdraw was this foil taped also?

Did he use foil sandwich celotex?

When did the extension roof go on?

Vaulted roofs need to be installed robustly and with care. This is especially relevant when fitting above bathrooms and kitchen areas.

 

[freddiemercurystwin]

It is felted with breathable membrane, and insulated with 100mm Celotex cut to size and pressed in flush to the plasterboard.

What is the exact roof make-up? If you don't know the thread probably won't be going anywhere.

 

[tony1851]

Whenever there is a frost or very cold weather and the heating is on, we get damp patches in several specific locations on the ceiling. The patches appear to form in thin lines where the noggins are located.

Textbook example of condensation.

There should have been a polythene vapour control layer immediately under the rafters, and above the plasterboard layer.

The breathable felt is irrelevant in this case. On cold, still nights, vapour going up above the insulation is not going to vent out, but will just condense on the cold side of the insulation and run down in rivulets.

 

[jeds]

Assuming you are talking about a sloping ceiling, that spec is not correct. You should have 100mm insulation between rafters and a 40 or 50mm continuous layer beneath as well. And even then I always specify vapour tape to the insulation joints prior to plasterboard.

 

[jeds]

Assuming you are talking about a sloping ceiling, that spec is not correct. You should have 100mm insulation between rafters and a 40 or 50mm continuous layer beneath as well. And even then I always specify vapour tape to the insulation joints prior to plasterboard.

PS. As others have said, the fact that your builder says that's how he always does it only means he always does it wrong.

 

[mashley]

@noseall
Many thanks for your response. I will do my best to answer your questions.

Who designed the roof and is there a drawing?

The roof was designed along with the rest of the extension by an architectural technologist, and drawings were produced.

Is this a vaulted roof?

Yes, it is a vaulted sloping roof.

Breathable membrane does not deal well with extremes of temperature and is not a magical panacea for ALL things condensation. Often vents are needed in conjunction with BM.

That's interesting. Our roofer suggested the possibility of adding vents to the roof. Do you think this could make a difference?

Has the celotex been cut snug

Most of the celotex fitted snugly and had to be tapped in with a rubber mallet. Some was looser so screws were used to prevent it from dropping out before the plasterboard was installed.

and have all gaps been filled? Was foil tape used on joints? Was a sealant used to fill gaps? Was foam used to fill gaps?

No, the gaps around the looser pieces of celotex were not filled. No foil tape, sealant or foam was used.

Was a dedicated vapour barrier installed?

No, this was neither specified on the drawing nor installed by the builder.

If this is a vaulted roof was there just one layer of celotex between the rafters and no underdraw? If there was an underdraw was this foil taped also?

I'm not sure... Could you explain what you mean by underdraw? I should then be able to answer your question.

Did he use foil sandwich celotex?

Yes, the celotex had foil on both sides.

When did the extension roof go on?

The roof was constructed, felted and tiled in the autumn, and the insulation, plasterboard and plastering was done a couple of months ago.

Vaulted roofs need to be installed robustly and with care. This is especially relevant when fitting above bathrooms and kitchen areas.

This roof is above our new kitchen :/

 

[mashley]

Textbook example of condensation.

There should have been a polythene vapour control layer immediately under the rafters, and above the plasterboard layer.

The breathable felt is irrelevant in this case. On cold, still nights, vapour going up above the insulation is not going to vent out, but will just condense on the cold side of the insulation and run down in rivulets.

So do you mean there should be two vapour control layers, one under the rafters and another above the plasterboard? Should this have been specified on the architect's drawings?

 

[mashley]

Assuming you are talking about a sloping ceiling, that spec is not correct. You should have 100mm insulation between rafters and a 40 or 50mm continuous layer beneath as well. And even then I always specify vapour tape to the insulation joints prior to plasterboard.

So do you mean that there should be insulation underneath the rafters themselves, actually below them?

 

[freddiemercurystwin]

Yep, was this signed off by BC?

 

[tony1851]

Textbook example of condensation.

There should have been a polythene vapour control layer immediately under the rafters, and above the plasterboard layer.

The breathable felt is irrelevant in this case. On cold, still nights, vapour going up above the insulation is not going to vent out, but will just condense on the cold side of the insulation and run down in rivulets.

So do you mean there should be two vapour control layers, one under the rafters and another above the plasterboard? Should this have been specified on the architect's drawings?

No - just one layer tacked to the underside of the rafters before the plasterboard is applied.
However, following on from a point made by jed; ideally there should be a second, thinner, layer of cellotex fixed to the underside of the rafters before the vapour barrier is applied. This avoids the 'pattern staining' you sometimes see.
As well as that, the 100 thickness your builder has installed might not be up to Building Regs, 125 - 130 is more usual.

PS Breather felt is OK up to a point, as long as there is some air movement and vents at top and bottom to clear the vapour away. But think of very cold winter nights with high pressure and no wind.

 

[noseall]

A VCL (vapour control layer) may or may not be necessary.

We fit foil sandwich between rafters, foam any gaps then apply foil tape to every join.
We then fit a second layer of foil sandwich across the rafters (underdraw) and foil tape these joints also including the one that abuts the wall. Again we fill any gaps.

I consider the way we fit the insulation robust enough to do away with a separate VCL.

 

[catlad]

Sounds like the roofer and your builder have a lot to learn you can't use ordinary plasterboard fitted directly to the rafters on a vaulted ceiling, it also sounds like a cold roof construction without ventilation.

 

[noseall]

When did the extension roof go on?

The roof was constructed, felted and tiled in the autumn, and the insulation, plasterboard and plastering was done a couple of months ago.

This may have some bearing although Autumn wasn't all that wet and is some months in the past. It may be that your building is still trying to sweat away some of its construction moisture and that it will settle down.

However, I fear that they may have not constructed the roof robustly enough.

 

[mashley]

Thank you once again everybody for your support with this issue. Over the last two days we ripped down the plasterboard to expose the 100mm celotex between the rafters. Some of the gaps were so big you could put your hand in, so it's really no surprise we had problems. Taking out some of these pieces of celotex revealed visible condensation and mould on the underside of the breathable membrane.

We have now used expanding foam to fill the gaps around the 100mm celotex, laid 50mm foil-backed celotex under the rafters, foil taped the joins, and fitted foil-backed plasterboard underneath. We have not plastered yet. However, this morning we had a heavy frost and were delighted to go outside and find that despite our heating being on, the roof was completely covered in frost and none had melted. There are so far no damp patches at all, whereas previously by this time in the day the frost had melted from the roof and dripped onto the plasterboard forming damp patches.

We are now ringing round plasterers to get the ceiling replastered as we are confident that this issue is now solved. Thank you so much again everybody for all of your responses and suggestions. We are so grateful that this site exists and that you have all given your time so generously to give us advice and to help solve this problem.

I am going to cross post this to our other thread on this forum about the same issue in case others with the same problem find the thread and are wondering what happened.