Housing - Renovation: Insulating where a wall meets the roof rafters

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- CommentAuthorkristeva
- CommentTimeMar 11th 2021
Hi all

I'm at the stage in my project where I've had to start renovating upstairs and just looking for some opinion on a few matters if I may. Two of my rooms (bedroom & bathroom) protrude into the lower rafters of the roof - see photo. The only barrier between the room and this section of the roof is lath & plaster, and access to the void behind from the loft seems to blocked by a purlin so I'm unable to insulate from above. These sections will obviously be a cold spot so I'm thinking about either over-boarding or just knocking it out and starting again.

I wonder if over-boarding will facilitate too much weight on the structure, and what material I should use - do I need to go with wood fibre here or would Cellotex backed plasterboard suffice for a bedroom & bathroom?

Cheers
- CommentAuthorkristeva
- CommentTimeMar 11th 2021
bathroom
- CommentAuthorphiledge
- CommentTimeMar 11th 2021
Bite the bullet and take out all the lath and plaster. You can then insulate between and under rafters.
- CommentAuthorbhommels
- CommentTimeMar 11th 2021
Posted By: philedgeBite the bullet and take out all the lath and plaster. You can then insulate between and under rafters.

+1 !!!
Insulation wise it is the weakest link (in my house). Very messy job so best to get it done now once and for all!
- CommentAuthortony
- CommentTimeMar 11th 2021
+one more
- CommentAuthorkristeva
- CommentTimeMar 12th 2021
Thanks everyone.

Is there such a thing as going too far with the insulation? I don't want the roof to 'sweat'.

Any recommendations for a cost effective 'breathable' insulation to put between the rafters?

Cheers
- CommentAuthorbhommels
- CommentTimeMar 12th 2021
Posted By: kristevaThanks everyone.
Is there such a thing as going too far with the insulation? I don't want the roof to 'sweat'.
Any recommendations for a cost effective 'breathable' insulation to put between the rafters?
Cheers

Instead of deciding beforehand on the insulation material, I would focus on getting a U value that is in line with the other insulation you are fitting. Then decide on how you are going to achieve said U value without sacrificing too much ceiling height. But I don't mind celotex, so perhaps I am biased.
- CommentAuthorNick Parsons
- CommentTimeMar 12th 2021 edited
''Any recommendations for a cost effective 'breathable' insulation to put between the rafters?''

Depends what you mean by 'cost effective'. If you mean cheap - no. If you mean ''it does the job I want, which cheaper things won't do, therefore the cost is an 'effective' cost'', yes. I like flexi wood-fibre. Pavaflex and Steico flex are 2 makes. Gutex probably make one too.

You will, unless your roof is unusual, need far more than will fit between your rafters. First you need a ventilation gap - 25mm if you have a breathable roofing membrane, 50mm if not. If you have 75mm rafters that has just left you with 25mm for insulation. So you need one of these. The top chord is your existing rafter, and the lower is positioned (via the plywood spacers) as far away as you need to get the right level of insulation/keep the BCO happy/not bump your head (delete where not applicable). A quick calc suggests 225mm of flexi W-F (with a lambda value of 0.039W/mK) would get you just under the 0.18W/m2K U value you need.
- CommentAuthorWillInAberdeen
- CommentTimeMar 12th 2021
What Nick said, but if the roof is original then it might not have a membrane.

Any reason particularly want it to be breathable? You'll get better insulation value and/or less thickness and less weight with PIR. The thing with breathable walls is to allow rain to dry both ways out of the brick, but AIUI that's not a thing in roofs.

The legal U value is tighter if you are in Scotland, and this is the green forum so everyone will encourage you to go better than the bare legal minimum insulation value...!
- CommentAuthorPeter_in_Hungary
- CommentTimeMar 12th 2021 edited
Posted By: kristevaAny recommendations for a cost effective 'breathable' insulation to put between the rafters?

Posted By: WillInAberdeenAny reason particularly want it to be breathable? You'll get better insulation value and/or less thickness and less weight with PIR. The thing with breathable walls is to allow rain to dry both ways out of the brick, but AIUI that's not a thing in roofs.

+1
Also I would support the notion of removing the lath and plaster. Messy (very) but probably worth the effort. Between the rafters it is generally easier to get a good fit with soft insulation (glass or mineral wool). If you use board insulation then usually to get a good fit you will need to foam it in. It would be possible to insulate with board type insulation under the rafters with plaster board over the insulation fixed through to the rafters.

Bang for buck standard EPS is usually reckoned to be more cost effective than other board insulation because you get a cheaper given u value per pound sterling but at a slightly thicker insulation. Usually the only reason to use PIR or graphite EPS is if other reasons drive the choice e.g. constraints on thickness.

If you decide upon the u value you want,calculate what is needed in the various options for insulation then look around for sale items and offers is often the most cost effective method of insulating a small area.
- CommentAuthorkristeva
- CommentTimeMar 14th 2021
Posted By: Nick Parsons''Any recommendations for a cost effective 'breathable' insulation to put between the rafters?''

Depends what you mean by 'cost effective'. If you mean cheap - no. If you mean ''it does the job I want, which cheaper things won't do, therefore the cost is an 'effective' cost'', yes. I like flexi wood-fibre. Pavaflex and Steico flex are 2 makes. Gutex probably make one too.

You will, unless your roof is unusual, need far more than will fit between your rafters. First you need a ventilation gap - 25mm if you have a breathable roofing membrane, 50mm if not. If you have 75mm rafters that has just left you with 25mm for insulation. So you need one of these. The top chord is your existing rafter, and the lower is positioned (via the plywood spacers) as far away as you need to get the right level of insulation/keep the BCO happy/not bump your head (delete where not applicable). A quick calc suggests 225mm of flexi W-F (with a lambda value of 0.039W/mK) would get you just under the 0.18W/m2K U value you need.
Thanks for this Nick.

Fortunately I have breathable roofing felt. I'll remove the lath & plaster and see how much space I have to play with.
- CommentAuthorkristeva
- CommentTimeMar 14th 2021
Posted By: WillInAberdeenWhat Nick said, but if the roof is original then it might not have a membrane.

Any reason particularly want it to be breathable? You'll get better insulation value and/or less thickness and less weight with PIR. The thing with breathable walls is to allow rain to dry both ways out of the brick, but AIUI that's not a thing in roofs.

The legal U value is tighter if you are in Scotland, and this is the green forum so everyone will encourage you to go better than the bare legal minimum insulation value...!

Thanks Will, I guess I was worried about it sweating and getting damp, or leaks. I like the idea insulation has the ability to dry when temperature allows. Isn't rock wool type material a bit of nightmare if it gets wet?

on a similar note I'll be plastering the walls at some point. Is there much point in lime plastering non-external facing walls on the upper floors do you think?
- CommentAuthorkristeva
- CommentTimeMar 14th 2021
Posted By: Peter_in_Hungary
Posted By: kristevaAny recommendations for a cost effective 'breathable' insulation to put between the rafters?

Posted By: WillInAberdeenAny reason particularly want it to be breathable? You'll get better insulation value and/or less thickness and less weight with PIR. The thing with breathable walls is to allow rain to dry both ways out of the brick, but AIUI that's not a thing in roofs.

+1
Also I would support the notion of removing the lath and plaster. Messy (very) but probably worth the effort. Between the rafters it is generally easier to get a good fit with soft insulation (glass or mineral wool). If you use board insulation then usually to get a good fit you will need to foam it in. It would be possible to insulate with board type insulation under the rafters with plaster board over the insulation fixed through to the rafters.

Bang for buck standard EPS is usually reckoned to be more cost effective than other board insulation because you get a cheaper given u value per pound sterling but at a slightly thicker insulation. Usually the only reason to use PIR or graphite EPS is if other reasons drive the choice e.g. constraints on thickness.

If you decide upon the u value you want,calculate what is needed in the various options for insulation then look around for sale items and offers is often the most cost effective method of insulating a small area.

Thanks for run down Peter, I was thinking of a combination of something between the rafters and PIR plasterboard attached to the rafters for skimming.
- CommentAuthorkristeva
- CommentTimeMar 17th 2021
I've now removed the lath & plaster, the depth of the rafters are 90mm. There is a breathable membrane but its above the old bitumen felt. I guess I'm restricted to approx 40mm insulation between the rafters unless I employ a composite beam.
- CommentAuthorphiledge
- CommentTimeMar 17th 2021
If youre sure theres a breathable membrane above the felt you can carefully cut the bitumen felt away from underneath. Do you know why the breathable membrane was fitted over the felt?

Just double check the breathable membrane is specified as an under tile membrane and is self supporting.
- CommentAuthorkristeva
- CommentTimeMar 17th 2021
Posted By: philedgeIf youre sure theres a breathable membrane above the felt you can carefully cut the bitumen felt away from underneath. Do you know why the breathable membrane was fitted over the felt?

Just double check the breathable membrane is specified as an under tile membrane and is self supporting.

Thats a good point. Thanks.

I have no idea why they may have fitted a breathable membrane over the old, thinking about it now i'm not sure this arrangement occurs in other parts of the loft where I may have seen just the white membrane (the loft walls are lath & plaster and obscure most of it) I'll have to check tomorrow as I don't currently live at the property.
- CommentAuthorPeter_in_Hungary
- CommentTimeMar 17th 2021
Posted By: kristevaI have no idea why they may have fitted a breathable membrane over the old,

It could be that the breather membrane was fitted over the the old because the old had holes in it and it was easier to fit new on top of the old just on the damaged part(s) rather than change the lot.
- CommentAuthorkristeva
- CommentTimeMar 18th 2021
Posted By: Peter_in_Hungary
Posted By: kristevaI have no idea why they may have fitted a breathable membrane over the old,

It could be that the breather membrane was fitted over the the old because the old had holes in it and it was easier to fit new on top of the old just on the damaged part(s) rather than change the lot.

Yes I think you may be right. I have found a few holes in that section I've srevealed. In fact I wonder if the new felt / roof work was carried out in response to some major leaks that quite obviously occurred at the point where the old house meets the oast house. One of the rafters on the far left has had some water damage.
- CommentAuthorkristeva
- CommentTimeApr 27th 2021
Coming to the end of this small job now but just one more question if I may?

Looking at previous threads on this forum I came across Tony's idea to secure PIR insulation to the underside of rafters by fitting horizontal shallow batons over the top, which then enables you to secure plasterboard to the batons.

This obviously creates a bit of void between the PIR and the plasterboard. Would this be suitable for a bathroom do you think?
- CommentAuthorNick Parsons
- CommentTimeApr 27th 2021
A void is OK. Just make sure you have tight VCL, either by taping the foil-faced boards (I prefer air-tightness tape to foil tape) or by using a separate VCL. If you don't need the batten depth as a service void you can put an extra 25mm in the void.
- CommentAuthorkristeva
- CommentTimeApr 30th 2021
Posted By: Nick ParsonsA void is OK. Just make sure you have tight VCL, either by taping the foil-faced boards (I prefer air-tightness tape to foil tape) or by using a separate VCL. If you don't need the batten depth as a service void you can put an extra 25mm in the void.

Thanks Nick

Good idea regarding insulating the void, I'll probably do that.
- CommentAuthortony
- CommentTimeMay 1st 2021
I would insulate the ceiling 400mm quilt, the sloping soffit 25 ventilation over sheet insulation between rafters or 50mm over batts, 80mm sheet insulation under rafters. Then either 100mm Or more EWI or 80mm IWI all insulation joined up vapour/air barrier, no voids or service voids
- CommentAuthorNick Parsons
- CommentTimeMay 1st 2021 edited
''Thanks Nick

Good idea regarding insulating the void, I'll probably do that.''

Tightly fix a VCL across* the battens once you have insulated between them. Seal all joints and perimeters with specialist air-tightness tape and tape over all staples.

* I had written 'under', but it depends which way up your head is!! I mean between the battens and the plasterboard.
- CommentAuthorkristeva
- CommentTimeMay 3rd 2021
Posted By: Nick Parsons''Thanks Nick

Good idea regarding insulating the void, I'll probably do that.''

Tightly fix a VCL across* the battens once you have insulated between them. Seal all joints and perimeters with specialist air-tightness tape and tape over all staples.

* I had written 'under', but it depends which way up your head is!! I mean between the battens and the plasterboard.

Thanks again.

I've got some foil tape but i'll get some specialist air tight tape too.

If I choose to retain the service void is the VCL necessary, or would the extra security be welcome?

From what I've read about the subject it seems VCL are a vexatious issue. I've read a separate VCL should be used instead of a foil backed material like insulation and shouldn't be used with the foil material.

I'm guessing there shouldn't be any gaps between the plasterboard and the VCL either, so any insulation between the batons would have to be pretty flush?

Its all a minefield isn't it
- CommentAuthorkristeva
- CommentTimeMay 3rd 2021
Posted By: tonyI would insulate the ceiling 400mm quilt, the sloping soffit 25 ventilation over sheet insulation between rafters or 50mm over batts, 80mm sheet insulation under rafters. Then either 100mm Or more EWI or 80mm IWI all insulation joined up vapour/air barrier, no voids or service voids

Thanks Tony.
- CommentAuthordjh
- CommentTimeMay 3rd 2021
Posted By: kristevaI've got some foil tape but i'll get some specialist air tight tape too.

Good idea. There's more chance it will stay stuck for a long time.

If I choose to retain the service void is the VCL necessary, or would the extra security be welcome?

The service void doesn't make any difference. You're trying to stop valour getting behind the insulation and condensing. Either foil-covered insulation all taped together and around the edges or a spearate VCL will do the job. You don't need both.

From what I've read about the subject it seems VCL are a vexatious issue. I've read a separate VCL should be used instead of a foil backed material like insulation and shouldn't be used with the foil material.

VCL can be a bit tricky. Both to understand when they are or aren't necessary and to detail them right so they actually do the job they are supposed to. You can easily get confused by all the information out there, somne of which is badly written or just plain wrong.

I'm guessing there shouldn't be any gaps between the plasterboard and the VCL either, so any insulation between the batons would have to be pretty flush?

A gap between the plasterboard and the VCL is normal. It's often designed that way so that if anybody subsequently screws into the plasterboard there's less chance of making a hole in the VCL. I think when Nick talked about the VCL being 'tight' he was meaning it needs to be complete with no holes or cracks. There's no particular reason why a VCL needs to be tight as in taut. In fact it is normal to leave a bit of slack when a VCL goes around a corner in order that it doesn't tear or otherwise get damaged by any movement of the building.

So a VCL would go across the face of the insulation, then battens screwed through it, taking care that the screw holes are sealed somehow, leaving a service void in front of the VCL where wires or extra insulation can go, then the plasterboard.

Its all a minefield isn't it

Yup
- CommentAuthorNick Parsons
- CommentTimeMay 3rd 2021
''I think when Nick talked about the VCL being 'tight' he was meaning it needs to be complete with no holes or cracks.''

Yes! Thanks djh!
- CommentAuthorbhommels
- CommentTimeMay 3rd 2021
Posted By: djh
So a VCL would go across the face of the insulation, then battens screwed through it, taking care that the screw holes are sealed somehow

High quality double sided tape is your friend here (I used SIGA Twinet)
- CommentAuthorkristeva
- CommentTimeMay 4th 2021
Posted By: djh
Posted By: kristevaI've got some foil tape but i'll get some specialist air tight tape too.

Good idea. There's more chance it will stay stuck for a long time.

If I choose to retain the service void is the VCL necessary, or would the extra security be welcome?

The service void doesn't make any difference. You're trying to stop valour getting behind the insulation and condensing. Either foil-covered insulation all taped together and around the edges or a spearate VCL will do the job. You don't need both.

From what I've read about the subject it seems VCL are a vexatious issue. I've read a separate VCL should be used instead of a foil backed material like insulation and shouldn't be used with the foil material.

VCL can be a bit tricky. Both to understand when they are or aren't necessary and to detail them right so they actually do the job they are supposed to. You can easily get confused by all the information out there, somne of which is badly written or just plain wrong.

I'm guessing there shouldn't be any gaps between the plasterboard and the VCL either, so any insulation between the batons would have to be pretty flush?

A gap between the plasterboard and the VCL is normal. It's often designed that way so that if anybody subsequently screws into the plasterboard there's less chance of making a hole in the VCL. I think when Nick talked about the VCL being 'tight' he was meaning it needs to be complete with no holes or cracks. There's no particular reason why a VCL needs to be tight as in taut. In fact it is normal to leave a bit of slack when a VCL goes around a corner in order that it doesn't tear or otherwise get damaged by any movement of the building.

So a VCL would go across the face of the insulation, then battens screwed through it, taking care that the screw holes are sealed somehow, leaving a service void in front of the VCL where wires or extra insulation can go, then the plasterboard.

Its all a minefield isn't it

Yuphttp:///newforum/extensions/Vanillacons/smilies/standard/bigsmile.gif" alt="" title="" >

Thanks!

It was the 'Back to Earth' website that confused me, although I didn't take time to read it all. I'm happy to get a separate VCL if people think the foil backed insulation isn't good enough. But if not I'll just try and make sure its all well sealed with tape, etc.
- CommentAuthorkristeva
- CommentTimeMay 4th 2021
Posted By: bhommels
Posted By: djh
So a VCL would go across the face of the insulation, then battens screwed through it, taking care that the screw holes are sealed somehow

High quality double sided tape is your friend here (I used SIGA Twinet)

Cheers.

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