Green Building Forum - Insulating Eaves Wed, 10 Jun 2026 10:07:58 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284685#Comment_284685 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284685#Comment_284685 Mon, 16 Nov 2020 16:49:00 +0000 ajdunlop I want to insulate the eaves for clean dry storage and am installing a central Mechanical Extract Ventilation unit and ducting that I want to keep within the building’s insulation envelope.
My house has a slate roof (original 1920s) with no felt. Rafters are 75mm.
There is some insulation between the joists so I was going to leave that in place as I don’t want all the heat from the 1st floor to end up in the eaves.

I have been reading up and have more or less decided to not bother insulating between the rafters as leaving an appropriate gap below the slates would only allow for 25mm between. Instead I was thinking of just attaching PIR under the rafters, probably 200mm to get good a u-value (~0.11 W/m2K I think) and to eliminate overheating in summer. I would tape the joins and foam where the PIR butts up to purlins and walls to get good airtightness.

I would prefer natural insulation but as far as I can see I would need a membrane above (to stop moisture getting into it and reducing performance) and below (to stop mosture from inside causing problems and for airtightness) and it would need to be very thick.

Does my reasoning sound ok so far?
Assuming it does I have a couple of questions.

Where the rafters meet the exterior wall’s inner leaf (external walls are unfilled cavity with sandstone outer and red brick inner) they rest on some wood on top of the brick. For most of the length of the wall there is some brick filling the gap between the rafters up to the slate. I worry that with my insulation under the rafters that any water ingress will run down the back of the insulation to these brick between the rafters and have nowhere to go. Therefore I was thinking of removing these to allow water to run into the cavity instead. Is this a good or silly idea?

I don’t mind having the silver surface of the PIR boards being the ceiling finish so I was thinking I should just screw the boards directly to the rafters. Although I have seen others describe using battens screwed through to the rafters being better. Is this assuming overbording and therefore doesn’t apply for me or are there other advantages?]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284687#Comment_284687 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284687#Comment_284687 Mon, 16 Nov 2020 17:03:12 +0000 ajdunlop You can see that one of the rafters has water damage. This was a slipped slate that has been repaired and the roof has generally been inspected.
You can see the wood on top of the the external wall's inner brick leaf that the rafters rest on and the bricks used to fill between this and the underside of the slate.
The insulation will be the length of what is shown in the photo and a bit to the left beyond the knee wall.]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284688#Comment_284688 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284688#Comment_284688 Mon, 16 Nov 2020 17:29:21 +0000 djh Posted By: ajdunlopinstalling a central Mechanical Extract Ventilation unit and ducting that I want to keep within the building’s insulation envelope
Why do you care about where the duct runs? As long as it doesn't go somewhere uninsulated and then back inside again.]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284689#Comment_284689 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284689#Comment_284689 Mon, 16 Nov 2020 17:42:17 +0000 ajdunlop Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284691#Comment_284691 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284691#Comment_284691 Mon, 16 Nov 2020 18:54:07 +0000 philedge
Id lay a sea of mineral wool over the existing ceiling joists. Not sure how thick the insulation is on the vertical wall but looks easy to add a layer of cellotex to that if its thin]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284692#Comment_284692 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284692#Comment_284692 Mon, 16 Nov 2020 19:09:41 +0000 ajdunlop Posted By: philedgewhere the insulation sits against the rafters leave a place for moisture to wick into and stay!
That’s one of the reasons for going for PIR boards with foil, will moisture not mostly run off it rather than wicking into it? Also with nothing between the rafters there should be good ventilation.]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284693#Comment_284693 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284693#Comment_284693 Mon, 16 Nov 2020 19:23:31 +0000 tony
Rafters look great and will stay that way, better without sarking.]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284702#Comment_284702 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284702#Comment_284702 Mon, 16 Nov 2020 19:55:52 +0000 djh Posted By: ajdunlopHumid air extracted from rooms will condense in the cold metal ducts.
The duct needs to run downhill towards the outside because of condensation in any case. Use plastic ducts instead of metal. Use insulated ducts for any bits that run uphill.]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284705#Comment_284705 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284705#Comment_284705 Mon, 16 Nov 2020 20:16:11 +0000 ajdunlop Posted By: djh
The duct needs to run downhill towards the outside because of condensation in any case. Use plastic ducts instead of metal. Use insulated ducts for any bits that run uphill.
The ducting is mostly internal ducting coming up from lower floor rooms to a central ventilation unit that will also be in this attic space and needs to be kept above 5oC operating temperature.
I’m sure I could insulate just the ducting and somewhere for the unit but we would also like the clean and dry storage space.]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284709#Comment_284709 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284709#Comment_284709 Mon, 16 Nov 2020 21:59:08 +0000 philedge Posted By: ajdunlop
That’s one of the reasons for going for PIR boards with foil, will moisture not mostly run off it rather than wicking into it?

It was the gap between the foil and the wood where youd get any water wicking. How likely you are to get that depends on the roof condition and chance of further leaks.]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284710#Comment_284710 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284710#Comment_284710 Mon, 16 Nov 2020 22:32:09 +0000 ajdunlop Posted By: philedgeIt was the gap between the foil and the wood where youd get any water wicking.

Ah yes I see, sorry I misunderstood.]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284711#Comment_284711 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284711#Comment_284711 Mon, 16 Nov 2020 23:46:10 +0000 Jeff B
It looks like you have a decent thickness of insulation across the dwarf wall but again you could add PIR board on top. There was already PIR board between the studs in our place but I went for IWI in the bedrooms to increase the insulation and limit the cold bridge effect of the studs and the purlin.]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284712#Comment_284712 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284712#Comment_284712 Tue, 17 Nov 2020 07:24:20 +0000 tony Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284715#Comment_284715 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284715#Comment_284715 Tue, 17 Nov 2020 09:56:22 +0000 ajdunlop Posted By: philedgeIt was the gap between the foil and the wood where youd get any water wicking.
What about putting some kind of small spacer between the rafters and insulation so the bottom of the rafters could still breathe and any moisture on the back of the insulation would be kept away from them?


Thanks for all your replies but I would still like to insulate in there to make it a useful space. The rest of the roof has had insulation under the rafters (and a bit of 25mm ancient stuff between) where the rooms are without issue (was converted in the 1980s).

Does anyone have any ideas around what to do at the external walls as described in my original post?]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284716#Comment_284716 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284716#Comment_284716 Tue, 17 Nov 2020 10:04:59 +0000 ajdunlop Posted By: tonyThen you get cold eaves cupboard
Everything that has gone in there up till now has been covered in black mortar dust from the underside of the slates as well as general pollution from outside. We have also had wasps nests in there. We would like to keep things like clothes in there in future so the cleaner the better.]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284724#Comment_284724 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284724#Comment_284724 Tue, 17 Nov 2020 11:46:33 +0000 tony Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284726#Comment_284726 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284726#Comment_284726 Tue, 17 Nov 2020 12:13:32 +0000 Jeff B Posted By: ajdunlop
Posted By: tonyThen you get cold eaves cupboard

Everything that has gone in there up till now has been covered in black mortar dust from the underside of the slates as well as general pollution from outside. We have also had wasps nests in there. We would like to keep things like clothes in there in future so the cleaner the better.

We had a similar problem with dust in our previous place (also a dormer bungalow). What we did was to attach polythene sheeting (i.e. clear DPM) to the rafters with drawing pins (!). This caught the dust which was blown in from outside and would have also stopped any water dripping into the attic although that wasn't an issue in our case as there was felt under the tiles. In your case I would propose cutting small squares of uPVC board, panel pin them to the underside of the rafters at suitable intervals and then pin the DPM to those, so as to avoid physical contact between the underside of the rafters and the DPM.

There was still plenty of ventilation through the attic and no problem with condensation/damp anywhere. I don't think I would like to store clothes in there though - better to build in some insulated cupboards with access doors so that the contents are exposed to the same conditions as the bedrooms themselves.]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284729#Comment_284729 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284729#Comment_284729 Tue, 17 Nov 2020 12:48:01 +0000 ajdunlop Posted By: Jeff BI don't think I would like to store clothes in there though - better to build in some insulated cupboards with access doors so that the contents are exposed to the same conditions as the bedrooms themselves.
That is effectively what I am proposing. The stud wall of the room you can see with mineral wool to the left is a bedroom that currently has no access to the eaves but might be useful to do in the future. However there is access and an uninsulated wall to the left just past the knee wall. Heat and (if I remove the draught excluder) air coming through this wall would keep the eaves area conditions similar to the attic landing.

I suppose I haven't really mentioned that currently these eaves are a source of draughts and cold to the attic landing as it hasn't been properly insulated and the attic door isn't very airtight. Pushing my insulation and airtightness to the rafters would solve this as well as giving us more good storage space. It feels easier to have nice continuous airtight insulation at rafter level than trying to get the access door to be airtight but still useable.]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284730#Comment_284730 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284730#Comment_284730 Tue, 17 Nov 2020 12:54:29 +0000 ajdunlop Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284840#Comment_284840 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284840#Comment_284840 Sun, 22 Nov 2020 18:18:22 +0000 ajdunlop How would anyone do this or would you steer clear?
Presumably I would need a VCL membrane under the insulation but what would you do above (on the underside of the rafters)? I would assume having something would be a good idea to stop wicking effect reducing the effectiveness of the insulation.
I am avoiding doing between the rafters to allow them to breathe and because space isn’t much of an issue.]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284843#Comment_284843 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284843#Comment_284843 Sun, 22 Nov 2020 19:47:55 +0000 tony Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284845#Comment_284845 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284845#Comment_284845 Sun, 22 Nov 2020 22:21:16 +0000 djh Posted By: ajdunlopOk so say I changed my mind and decided to use sheep’s wool insulation under the rafters...
How would anyone do this or would you steer clear?
Presumably I would need a VCL membrane under the insulation but what would you do above (on the underside of the rafters)? I would assume having something would be a good idea to stop wicking effect reducing the effectiveness of the insulation.
I am avoiding doing between the rafters to allow them to breathe and because space isn’t much of an issue.
I have trouble seeing the attraction of sheep's wool insulation versus recycled newspaper or even rockwool. But if you really want to use it the (a) make sure it is treated as Tony says (borax, I think) and (b) make sure it is sealed away from the world so creatures can't get in. Belt and braces. Airtight membranes made from something that insects can't attack seem like the best solution for this.]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284848#Comment_284848 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284848#Comment_284848 Mon, 23 Nov 2020 10:37:02 +0000 ajdunlop
Back to PIR...

The bit I am still unsure on is the filler bricks that have been added above the timber (resting on top of the inner leaf of the exterior wall) supporting the end of the rafters. These are filling the gap between this support timber the rafters and the bottom of the slate. I can't work out why these have been put there as I thought roofs like mine were built to have as much ventilation as possible.

Very bad diagram attached.

If I want to add PIR under the rafters I don't really want any moisture to end up behind this brick so was thinking of removing it to get a good airflow and so any moisture has somewhere to go.]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284849#Comment_284849 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284849#Comment_284849 Mon, 23 Nov 2020 10:42:48 +0000 tony
i would use fibreglass batts against the bricks and foil faced board under the rafters, no gap needed between wood and foil]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284852#Comment_284852 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284852#Comment_284852 Mon, 23 Nov 2020 11:17:55 +0000 ajdunlop Posted By: tonyi would use fibreglass batts against the bricks and foil faced board under the rafters, no gap needed between wood and foil

Thanks for the suggestion. Are you saying I shouldn't worry if my foil backed board butts up to the bricks filling between the rafters?]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284855#Comment_284855 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284855#Comment_284855 Mon, 23 Nov 2020 13:15:31 +0000 tony Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284859#Comment_284859 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284859#Comment_284859 Mon, 23 Nov 2020 14:13:27 +0000 ajdunlop Posted By: tonyYes but the insulation barrier should be connected and no gaps or cracks

Foam the cut (non foiled) edge of the board and then butt this to the bricks? I assume the hope is that any moisture that gathers there would ventilate off rather than causing any problems?]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284861#Comment_284861 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284861#Comment_284861 Mon, 23 Nov 2020 15:46:18 +0000 tony Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284862#Comment_284862 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284862#Comment_284862 Mon, 23 Nov 2020 16:33:14 +0000 ajdunlop Posted By: tonythere wont be any moisture
Assuming that there is no snow / rain blown under the slates? There is pack pointing under the slate joins but I worry that some will fall away over time. Although I suppose there is nothing stopping that happen already.]]> Insulating Eaves https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284863#Comment_284863 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16803&Focus=284863#Comment_284863 Mon, 23 Nov 2020 18:17:06 +0000 tony