Re-building ceiling to solve condensation problem, advice needed

[mashley]

Hi all,

We posted last year because we were having damp problems in the ceiling of our self-built extension. We hired some builders to insulate, construct and plaster the ceiling as we were unable to do this ourselves. We subsequently found that the ceiling was under-insulated, had no vapour control layer, and had air gaps between the edges of the celotex with no filling. After a frost we would get patches of damp on the ceiling which with the help of the members of this forum we found were due to condensation. Unfortunately our architect was also a cowboy and did not provide adequate drawings with specifics of how the ceiling should be constructed.

We are about to rip the ceiling down and re-build it ourselves with the advice we have received from this forum, and we just want to check and clarify a few things before we start.

I have attached a diagram of the ceiling in its current construction, and also of what we intend to do when we rebuild it. We would really appreciate it if someone could have a look at these diagrams and answer the following questions for us.

1. Is each layer in the intended layout in the correct order?
2. Is the second layer of batten (between the VCL and the 40mm celotex) necessary?
3. Will 40mm celotex be sufficient or should we use thicker?
4. With regard to the VCL:
   1. Is any particular type recommended (e.g. PVC, thin foil, foil bubble sheets etc)
   2. Is foil-backed plasterboard/celotex necessary if we are using a VCL?
   3. Is the VCL necessary if we use foil-backed celotex and use foil tape along the joins?

Here are the diagrams of the current and intended structure (click thumbnail for full size version):



Thank you all very much in advance for any advice you can offer.

 

[freddiemercurystwin]

So all you really need to do is remove the existing plasterboard, fill any gaps between the existing Celotex and rafters with expanding foam, fit 40mm Celotex underneath, tape over the joints with foil tape, fit plasterboard, foil backed if you really want to but not really necessary as the foil on the insulation will act as the VB.

The only void should be between the breathable membrane and the insulation.

 

[Mikefromlondon]

Not sure what you wrote last year unless you can place a link here to that thread. What guarantees that the extra expense of taking down your ceiling and and adding filling EP in the gaps between rafters and then adding another layer of 40mm celotex will resolve the damp issues. Are you trying to block a roof water leak by adding more layers? The cause may be a roof leak not condensation.

I am going by the fact I have a similar construction to your original method, and I have never had any damp issues.

Only exception for condensation is if the rest of your house is warm and humid, and this extension roof and ceiling space is unheated and cold, then obviously any moist laden air from other parts of your house entering this room could condense and form damp like conditions, if the room is kept warm at the same temperature as the rest of your house, then your ceiling is already adequately insulated and should not form any condensation, so my feelings is that you may have a leak in your roof and water is seeping through causing damp.

Is the damp patch everywhere or in a confined area?

 

[mashley]

Thank you both very much for taking the time to respond.

So all you really need to do is remove the existing plasterboard, fill any gaps between the existing Celotex and rafters with expanding foam, fit 40mm Celotex underneath, tape over the joints with foil tape, fit plasterboard, foil backed if you really want to but not really necessary as the foil on the insulation will act as the VB.

Is there any benefit to adding a separate vapour barrier? Or is the foil backing of the celotex and the foil tape adequate for this? For example, would adding the foil bubble sheeting in addition to the other changes you described insulate us better or provide better vapour control?

The only void should be between the breathable membrane and the insulation.

Does this include between the 100mm and 40mm celotex layers? Our understanding was that they should not be in contact with each other.

Are you trying to block a roof water leak by adding more layers? The cause may be a roof leak not condensation.

The roof is definitely not leaking. This only ever happens in freezing or frosty conditions. There are never damp patches when it rains, even in the extremely heavy rain and strong winds we have had recently.

I am going by the fact I have a similar construction to your original method, and I have never had any damp issues.

I should have added that it is a vaulted ceiling in a large kitchen with underfloor heating. I understand that vaulted ceilings require different construction, particularly in kitchens and bathrooms.

Only exception for condensation is if the rest of your house is warm and humid, and this extension roof and ceiling space is unheated and cold, then obviously any moist laden air from other parts of your house entering this room could condense and form damp like conditions, if the room is kept warm at the same temperature as the rest of your house, then your ceiling is already adequately insulated and should not form any condensation, so my feelings is that you may have a leak in your roof and water is seeping through causing damp.

Is the damp patch everywhere or in a confined area?

There is no significant difference in the temperature and humidity of the main part of the house and the extension. However, the damp patches do seem to appear soon after we put the heating on on a frosty morning. It's also notable that if we look at the roof on a frosty morning, the frost on our roof is melting while the main roof of the house and those of our neighbours' houses are still covered in frost.

The damp patches are in a few very specific spots and never appear anywhere else on the ceiling. The locations of the patches correspond to the locations of the noggings in the roof which lead us to believe that heat is escaping through the ceiling, meeting the cold air above the insulation causing condensation which then runs down the celotex until it hits a nogging, runs down the nogging and through the gaps in the 100mm celotex before finally dripping onto the plasterboard.

Our hope is that sealing the gaps around the 100mm celotex, adding sufficient additional insulation and controlling the escape of warm moist air using a vapour barrier will remedy the problem, but we want to be sure that we have the method correct before we begin.

Thanks again for the help and support!

 

[tony1851]

Our understanding was that they should not be in contact with each other.

Where did you find this? A gap between the two insulation layers is not necessary, and you will reduce the free air space below the breather membrane.

As advised, just remove plasterboard, fit 40 PIR directly under joists; tape all joints, and fit foil-backed plasterboard.

 

[Mikefromlondon]

Vaulted ceilings can trap warm moist air, depending on the slop and size of the roof, and prevent normal air circulation, so perhaps air vents might help to circulate air, though you will lose some heat energy, might be worth experimenting with installing an air brick higher up near the apex, rather than going the extra length of double insulating .

If you don't want to install an air brick or two, as an experiment, I would use a small table fan running on slowest speed pointed towards the apex to force air circulation and then see if you are still getting damp patches, not only that circulating air disperses heat as well rather it accumulating and getting trapped in the vaulted area. A table fan running at slower speed would only draw less than 20watts, and run it in winter times only. If this stops damp patches, also circulating air will help drying any damp patches that you already have.

Insulation effectiveness of your vaulted ceiling and comparing it with how fast or slow the snow melts against your other main tiled roof, or that of your neighbors, you cannot really compare like for like, the two comparisons are so different, since main roofs have a lot of empty space in the loft, and lofts are well insulated to like 150mm thick layer of mineral wool, and any small amount of heat which manages to enter this space is also driven away by air ventilating through eves, hence you won't expect a great deal of heat in those lofts to melt snow as fast as you would in your vaulted ceiling with 40mm celotex insulation, and constant heat trapped against the ceiling and so the snow would naturally melt far more quickly.

So think again, will doubling your insulation and the extra length of replastering new ceiling boards in solve your problem? it might reduce but might nevr solve totally as long as damp air remains trapped up there.

 

[mashley]

A gap between the two insulation layers is not necessary, and you will reduce the free air space below the breather membrane.

We were told by another contractor that the celotex should have been raised slightly so it was in the middle of the rafters and not in contact with either the breathable membrane or the insulation (or in our case plasterboard) below.

As advised, just remove plasterboard, fit 40 PIR directly under joists; tape all joints, and fit foil-backed plasterboard.

Thank you. Is there any advantage to using an additional vapour barrier such as the sheets of bubble foil? We don't mind over-insulating at this point after suffering a very cold first winter!

Vaulted ceilings can trap warm moist air, depending on the slop and size of the roof, and prevent normal air circulation, so perhaps air vents might help to circulate air, though you will lose some heat energy, might be worth experimenting with installing an air brick higher up near the apex, rather than going the extra length of double insulating .

...

So think again, will doubling your insulation and the extra length of replastering new ceiling boards in solve your problem? it might reduce but might nevr solve totally as long as damp air remains trapped up there.

Thank you for all the information you posted Mike. We know that it is vastly under-insulated as we had the U value calculated and it does not meet building regs. We will add the additional insulation as this is required anyway, and if we still experience problems will look into adding vent tiles.

 

[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.

 

[Mikefromlondon]

Amazing what Insulin can do to a diabetic ceiling!

 

[sh4d0w]

How is this now! very interested to know if the problem is still a bay.