Loft insulation in old house with partially sloping/vaulted ceiling

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The original part of my house is around 170 years old. The rooms on the first floor have a flat ceiling which slopes down along with the pitch of the roof as it gets closer to the walls.

Above the flat part of the ceiling there is about 300mm of normal, glass wool, Knauf roll out insulation. But in the sloping part of the ceiling, the insulation is not so good. Squeezed between some of the rafters, pushed down into the small gap between ceiling and roof, there are rigid celotex boards (the kind with the foil backing). There is a small gap between the board and the breathable roofing felt.

But there is nothing at all between quite a lot of the other rafters in that space between ceiling and roof.

This diagram shows what it is like, where insulation is installed.
There is no eaves or ridge ventilation in the house (there never has been, I think, given the age of the house); the breathable felt seems to more or less do the job in terms of preventing condensation, though I am thinking of adding some vent tiles. For various reasons I haven’t been able to pull out the celotex boards to check if its nice and dry in those gaps.

My question is: what material should I use to insulate in the gaps between rafters above the sloping bit of ceiling?

I am concerned that I should use something breathable to prevent condensation in that small space, which is very hard if not impossible to ventilate; if I am right about that, should I replace the celotex with something breathable/permeable?

Thanks!
 
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Doggit

Your diagram tell you exactly how it should be done. Most rafters are 4" deep, and if you've got a breathable membrane, then you can get away with a 1" air gap between the cellotex and the membrane, so you could put in a 3" cellotex which would be equal to about 150mm of rockwool. But if only a few of the rafters have the cellotex, then that suggest it may have been added about the same time they did the loft insulation, but gave up the job part way through as being too hard.

Are you prepared to take the sloping part of the ceiling down, and do the job properly. Can you borrow an IR thermometer to test how effective the slope with insulation is, compared to the upper part, and then see how bad the uninsulated sections are.
 
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Thanks for the reply. I don’t know anyone with an IR thermometer, but I’ll look into it.

Taking the sloping ceiling down isn’t an option - the room’s been redecorated recently, and some of it is the original lathe and plaster which I’d prefer to keep. It is possible to shove the insulation down into the space from above in the loft, though as you suggest that would be awkward and less likely to get a perfect fit. But I can live with that unless I am going to create problems with condensation etc.

Based on what you say, you think that using non-breathable/non-permeable celotex board is fine? (The diagram I posted, which is by historic England, suggests that foil backed board may not be ideal.)

Part of me wonders whether to leave it all alone...
 
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Doggit

As, long as you can leave a gap between the cellotex and the tiles, then there won't be a problem, and it will improve the heat retention in the room. As there's rockwool in the upper part of the ceiling, then moisture can escape through there, so you don't really have an issue.

But it's down to you at the end of the day.
 
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The problem with earlier built 'raked' ceilings is little consideration was given to insulation and condensation related issues. And, because the timbers are often short in section (usually no more than 100mm deep), they will be vulnerable to cold bridging and 'shadowing' which will not be addressed even if your sliding the Celotex down the gap actually works.

It's a pity people didn't improve their homes more during cold Winters as this is when condensation related problems rear their heads. It's no surprise that loads of posts spring up when people start getting their Christmas deccies out the loft when its freezing outside.
 
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As, long as you can leave a gap between the cellotex and the tiles, then there won't be a problem, and it will improve the heat retention in the room. As there's rockwool in the upper part of the ceiling, then moisture can escape through there, so you don't really have an issue.

Thanks. Just to check - do you mean that leaving the gap you mention between top of celotex and breathable felt should do the job in terms of ventilation, even though there is no soffit/eaves ventilation?

And in case it makes a difference, the rafters are only 75mm, so 50mm of celotex is prob the limit.
 
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Doggit

even though there is no soffit/eaves ventilation?

Well you didn't mention that earlier did you; there's normally soffit or eaves ventilation to take any moisture out of the roof void, even with the 300m rockwool you've got there. 50mm of cellotex is better than nothing though.
 
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Sorry- I’m figuring this out as I go, and hadn’t initially realised that soffit vents were part of the reason why the gap between the celotex and roof is useful. The Structure of the roof suggests that there never has been soffits of any kind, really.

I am going to fit a couple of vent tiles, which will get the air moving, though I suspect it will do very little in terms of moving air in that gap above the celotex. Hopefully that, plus the breathable felt, will do the job in terms of minimising condensation in the roof generally and around the sloping ceiling, and the celotex on the sloping ceiling will help a bit in terms of insulating the room below.

Not ideal, and cold bridging seems almost inevitable, but based on this thread and noseall’s comment, there doesn’t seem to be much else I can do.

All that said, I am not aware of any actual problems other than a couple of coldish bedrooms; it’s about making sure I do what I can now to avoid any issues with condensation in future.
 
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Final question about the (lack of) eaves ventilation. Is it worth drilling a hole in the exterior wall between the rafters (so that there is a hole into the void where the celotex would go, making sure at least some of the hole would be above the top of the celotex)?

This would be like standard a soffit vent, to some extent, I think?

If this is worth doing, what size of hole or holes would I need to drill? (I would cover the holes with some kind of mesh or fly screen to keep out pests).

Thanks again.
 
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Doggit

I don't think drilling holes in the wall would work; the airflow needs to be able to flow up and over the cellotex, and it needs to be able to go in between each of the rafters.
 
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Thanks Dogget, I appreciate your time and advice. What I have in mind should allow at least the air to go up and over the celotex between the rafters, through the 25mm gap between the celotex and the roof (though the celotex would be tight against the side of the rafters). The sketch attempts to show what I am thinking.

Do you think this is worthwhile? If so, would one 25mm (ish) hole be enough?
 
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Doggit

Ahh, the wall should in fact be the facia,and you should be able to get vents to go in to the soffits on the underside. There is normally a wall plate on top of the wall that the rafters sit on, and then they attach the facia to the end of the rafters, and the soffit with vent holes underneath the rafters.

Go and have a look from outside,and then post a picture.
 
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I don’t think my old house is made that way, unfortunately.

From outside, there are rafters sticking about 30cm Out of the wall. On top of the rafters (so far as they stick out beyond the wall) is a wooden board, then felt, then the roof tiles. The rafters meet and go through the solid wall, and then into the loft (the wall is very thick). A wooden board is attached to the front of the rafter, which the guttering is attached too. That board is a bit like a fascia, and there is nothing I’d describe as a soffit.

Here’s a photo- it’s a bit dark, so I’ll post another clearer photo tomorrow.
 
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Fair enough! That’s the plan, two or three tile vents in the roof space.

It’s just those bits above the sloping ceiling (and the celotex) I’m worried about in terms of condensation, and getting air circulating down there between the rafters, above the celotex.

But if some sort of vent/hole put in the wall between the rafters isn’t going to make a difference, then as you suggest, tile vents is probably the only option.

Thanks again.
 
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