Ventilated cold roof - can it ever be airtight?

[Stones]

Thinking back to some of our previous houses, they were built like thousands of others, with a ventilated cold roof:

Ceiling plasterboard (taped and filled), earthwool type insulation, ventilated loft void, sarking (OSB when roof tiles, 19mm softwood when slates), roofing breather membrane.  

No VCL or air tightness membrane whatsoever.  Clearly the idea is that any moisture will migrate through the permeable plasterboard and insulation into the roof space where it is ventilated away.  Provided there is a clear ventilation path, then this system works, and indeed I didn't experience any problems with this form of roof build up.

However, as we strive for super air tight builds, can this sort of construction still be used?  Clearly plasterboard ceilings can be taped / filled or skim plastered over, and care taken around electrical wire penetrations to minimise the escape of air from internal space to loft. 

The problems as I see them start when you introduce things like downlights.  I can well remember the standard procedure of clearing a large circle in the insulation around a halogen downlight, creating an uninsulated path for warm household air to migrate into the cold roof.  With the slim LEDs we have recently discussed, there may not be the same need to create an insulation free zone, but you would still be introducing an air leakage point as these slim lights are far from sealed units.

Surface mounted LED's would get you round this problem, but many of course prefer the look of recessed downlights.  Is there a solution for such cold roof spaces?  The only one I can think of would be creating some kind of airtight enclosure over the downlight fitting on the loft side of the plasterboard.

 

[ProDave]

It was a common problem in older houses that the roof space was not ventilated well enough, so as soon as you introduced more loft insulation, condensation became a problem in the roof space.

A lot of old cottages up here are built this way, then with walls lined with plasterboard, open to the roof space. If you remove a switch or a socket you are greeted with a howling gale of cold air coming through.

For a low energy house today I would not consider a cold roof. Warm roof all the way for me. There would be far too many details to try and make air tight, not least any light fittings and even the loft hatch.

[tonyshouse]

Cold roof me, insulation at ceiling level, airtight, less volume to heat and get heat losses from and it is a complete nightmare keeping wind out of a loft.

[Onoff]

If you wanted to get really complicated then you could go continuous VCL with plasterboard below that then create a false ceiling on battens with low profile, recessed down lighters in the void.

[Crofter]

Our house has the exact problem Dave describes- retrofit loft insulation and masses of condensation. I've managed to pry open the skylight for now, before I did that you would have water streaming down the chimney breast, collecting on the rafters, etc.

 

The medium term solution will be some air bricks in the gables, but in the long term we are converting the loft. The Rolls Royce option would be to strip the roof tiles, fit a breather membrane, and convert to warm roof. But a new roof covering would probably blow the budget completely.

[Stones]

11 hours ago, ProDave said:

 

For a low energy house today I would not consider a cold roof. Warm roof all the way for me. There would be far too many details to try and make air tight, not least any light fittings and even the loft hatch.

Appreciate the reasoning but a warm roof (for me anyway) only tends to make sense when you are using part of the roof as accommodation - room in roof, 1.5 storey etc. I would suggest that a cold roof still makes sense in certain applications as per Tony's comments.

7 hours ago, tonyshouse said:

Cold roof me, insulation at ceiling level, airtight, less volume to heat and get heat losses from and it is a complete nightmare keeping wind out of a loft.

Your comments seem generally positive towards a cold roof except for difficulty in keeping wind out of loft.  What method do you employ to mitigate this?

In our last house, we had a cold roof, timber sarking, breather membrane and slate. What we didn't have was soffit or ridge ventilation. 450mm of insulation and it worked perfectly, no condensation at all.

 

[mafaldina]

1 hour ago, Onoff said:

If you wanted to get really complicated then you could go continuous VCL with plasterboard below that then create a false ceiling on battens with low profile, recessed down lighters in the void.

That's what I was thinking of doing, mainly because I have 'inherited' the roof construction – shallow W.

[Crofter]

18 minutes ago, Stones said:

In our last house, we had a cold roof, timber sarking, breather membrane and slate. What we didn't have was soffit or ridge ventilation. 450mm of insulation and it worked perfectly, no condensation at all.

 

Breather membrane + no vents has got to be the way to go. It's what I would like to do on my house but I don't think the roof tiles would survive coming off and going back on again.

[tonyshouse]

My loft is cold and I don't care about draughts getting in there, I care a lot about them getting into the house and am keeping them all out.

[Stones]

13 hours ago, Onoff said:

If you wanted to get really complicated then you could go continuous VCL with plasterboard below that then create a false ceiling on battens with low profile, recessed down lighters in the void.

It's certainly an option worth considering, but the question for me is whether I actually need an air and vapour barrier for the cold roof.  I was having a look at air tight cable grommits online and they seem pretty basic, a piece of adhesive EPDM with a small hole in to push the cable through.  I have a roll of EPDM offcut, and with a roll of airtightness tape could fairly easily fashion what I need to.

 Around 1/3 of our roof is cold, the other 2/3 being warm (vaulted ceilings or room in roof)

13 hours ago, Crofter said:

Our house has the exact problem Dave describes- retrofit loft insulation and masses of condensation. I've managed to pry open the skylight for now, before I did that you would have water streaming down the chimney breast, collecting on the rafters, etc.

The medium term solution will be some air bricks in the gables, but in the long term we are converting the loft. The Rolls Royce option would be to strip the roof tiles, fit a breather membrane, and convert to warm roof. But a new roof covering would probably blow the budget completely.

I'm assuming it's a non breathable roofing felt beneath your tiles at the moment? When the retrofit insulation was installed, di the fitters block up any of the existing ventilation?

12 hours ago, Crofter said:

Breather membrane + no vents has got to be the way to go. It's what I would like to do on my house but I don't think the roof tiles would survive coming off and going back on again.

I suspect this is the main reason well insulated cold roofs work, as opposed to the issue you are currently experiencing.  We have Proctor Roofshield membrane specified for our roof.

11 hours ago, tonyshouse said:

My loft is cold and I don't care about draughts getting in there, I care a lot about them getting into the house and am keeping them all out.

So do you have a air and vapour barrier at behind the ceiling plasterboard?

[Crofter]

I assume the house has the original 1976 felt and tiles, and I assume that this is non breathing. No sign of any ventilation at all.

The retrofit insulation was added before we bought the house, but the previous owners only used it as their holiday house so it was unoccupied much of the time. Once we moved in, we noticed how bad the problem was. I'm sure that if left unattended for too long there would be loads of mould and eventually rot in the roof timbers.

[tonyshouse]

I do have a vapour barrier at ceiling level, condensation in lofts can happen even when moisture from the house does nor enter the loft.