I have recently made an access to the eaves in our attic (rest of the attic has bedrooms and landing.
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?

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.

Personally, if its an old slate roof its likely to fail again and Id want to be able to see whats going on with any water ingress. Insulating the underside of the rafters will hide everthing from veiw and where the insulation sits against the rafters leave a place for moisture to wick into and stay!

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

Yes, go for it, might need penny washers to help squeeze pir to rafters, need to join new and existing insulation together, I would insulate the knee bricks with pir offcuts.

Rafters look great and will stay that way, better without sarking.

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.

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.

Then you get cold eaves cupboard

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.

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.

I was slightly surprised to find the brick at the ends of the rafters above the wood they rest on on top of the inner leaf of the external wall. I had thought with this sort of old roof that as much ventilation as possible was the mantra and so a gap would have been left there. There are a few rafters with nothing between but I don't know if that was intentional or because they have fallen out.

Dangerous and expensive unless you can source wool cheaply. Sheep’s wool need to be treated against insects and treatment need to last lots of years.

Ok, I have maybe gone off that idea again. It feels like as I get close to deciding what I am going to do I have a last minute throw everything back into the mix wobble.

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.

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?

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?

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.