half story roof insulation

cravings

hello all

i have a 1 and a half story semi detatched cottage, probably built around 1907. have 2 upstairs rooms, one with the original lath and lime plaster ceiling, and the other, which the ceilings were stripped out of.

time has come to reslate the roof. i have looked this up and read a fair bit, but i must admit, the whole breathable / airtight thing is confusing.

current makeup is: slates, battens, rafters, ceiling. (in the room with a ceiling). the rafters are only 110mm deep. it's semi detatched so i need to knit the slates into the neighbour's roof, so counter-battening isn't an option. so i understand i need a 50mm air gap between insulation, and membrane.

so my current thinking is... slates, battens, membrane (have tyvek supro), air gap, 50mm isover metac.. then this is where i get confused. the rooms are small so i don't really want the ceilings (and when i say ceilings i pretty much mean the sloping sections) to come down lower than they are. i hope to be able to keep the lath and plaster in the room that has it, in the room that doesn't... do i need my internal celing layer to be breathable (calsitherm or similar?), or can i just put up gypsum warmboard? won't have a lot of room.. the existing lath / plaster ceiling is about 20-25mm thick.

there's no sign of any vents in the soffit boards, i just presume it's not airtight enough there to cause a problem.

here's some pics. the flat bit of ceiling is only about 1.6m long so it's quite a small room.

thanks if anyone has opinions and advice.

Paul Kiernan

You need to check where your ventilation is coming from.

If there's no eaves ventilation you should leave the space between the rafters open if you can so in the room with retained lath/plaster put bare minimum (20-30mm) of natural insulation e.g. woodfibre or sheepswool. In the other room you can use insulated gypsum but don't put anything (or very little) between the rafters.

If there is eaves ventilation then same applies except that you can get away with just leaving 50mm air gap.

The flat section you can put lots of insulation between and above the joists just making sure there's adequate air flow into the sloped sections.

cravings

Hi Paul. Thanks very much for replying. The eaves will almost certainly be replaced, so I could add vents there as part of the plan. I'd definitely replace them if it was what was holding back being able to insulate.

So having the roof space ventilate down there would allow me to leave the narrower air gap above?

Thanks again, really appreciate it.

BryanF

How big is the roof? Is budget a problem? What's stopping you putting in more insulation above the rafters? Then battening, assuming your not changing the rafters?

Paul Kiernan

It's all down to the air flow. Eaves ventilation will create a nice flow up along the top of the insulation. My house is much the same age as yours but rafters are bedded in wall so it's difficult to get any air flow. Your first photo appears to show a nice air gap down towards the eaves.

Are they the original slates? Do you really have the replace them? From the inside they look great, lovely lime parging and no signs of any damp. Having said that, to insulate the sloped sections you probably have to remove either the slates or the ceiling.

cravings

hi guys

BrianF: budget is of course an issue.. the years' work hasn't gone to plan. i do have the money to do this though, but i'd rather not spend more than i have to. the outside size of the house is about 5.5M front to back, and it's a bit over 8M wide. it's semi detatched, and it's one of a few pairs of houses the same in a row, so i don't want to change the outside size of the roof, so i have to keep the slates above the rafters in their original arrangement.

Paul.. as you've described with your own house, the rafters are pretty much buried in the wall. the front and back walls are about 60cm thick, old stone / rubble and brick around the windows.

to the left in those photos is the back side of the house. the back of the roof really isn't bad. not aware of any leaks and it looks good generally. and also the long term plan for the house would be to extend out the back in future. so for now i'm hoping to leave the back as is, because it will be coming off in future. but the front side of the house has 2 dormer windows, the visible one needs a full rebuild really, it's been leaking for years before i came along. and the front of the roof in general is very messy compared to the back. i did pick up about 1000 slates in pretty good condition which are a perfect match for the ones here, so that's good.

Paul: so if there's no ventilation at the eves, how would leaving a bigger air gap in the roof space help? if it's not ventilated other than through the slate structure itself, what difference will not insulating the gap make?

thanks so much for the knowledge guys. i'm just trying to understand the best approach to improving this house while maintaining it's character. so i can be better informed while talking to roofers etc.

photo of where the rafters meet the wall plate, and one of the worst dormer. my phone is the worst camera i've ever had..

Paul Kiernan

If the tight gap between slates and ceiling is open at both ends (eaves and attic) you'll obviously get a much better airflow that if it's closed at the eaves end. You then need to leave more space for sufficient air movement to disperse any moisture.

Is there no ventilation to attic at all? Vents in the ceiling? gables? ridge?

You need to look at how those upper rooms will be used and ventilated. A vapour control layer will prevent moisture entering the attic space but it'll then need to be extracted some other way. Were you planning on insulating walls as well?

Have you seen this "Room to Improve"? It's a good example of how timber which has lasted for centuries can be destroyed in a decade or two due to bad condensation control.

cravings

seems like there's no intentional ventilation no. i've taken some better photos with a camera (the camera on my phone is pretty poor).

the rafters are bedded into the wall / the wall is built up into the rafters so there's no space for ventilation there really. and there's no gap up at the ridge tiles or anywhere... and no, (well obviously the ceiling is missing in one room...but other than that..) the ceilings are solid by design with no vents. as i said, the house is about 110 years old now. these upper rooms will both just be bedrooms.

the long term plan for the house would hopefully be to extend out the back, on both floors. so for that reason i don't really want to spend on redoing the back of the roof just yet. plus the back doesn't really have any problems so i can live with it for now. (the hope was to fit some temporary insulation in just for the moment).

so as a first stage, i was hoping to get the slates on the front half (and obviously enough over the ridge for the membrane to fold over) lifted off and redone, and to slot some insulation in behind the existing ceiling while i'm there.. but it's now sounding like without ventilation that's not advisable. and to get the dormers redone, with new windows.

so ok.. i need to come up with a ventilation plan for the roof space. i like the old natural slates.. have never really liked the look of the vented modern slates when i've seen them, but i'll see if i can come up with somewhere inobtrusive to slot some maybe. am i on the right lines there?

thanks again for all your advice

link to better pics

cravings

and yes, i think i had seen that room to improve (i watch most of them alright), but it was good to see it again, thanks.

i'm hoping to do the right thing by this house alright..

Paul Kiernan

cravings wrote: »

so ok.. i need to come up with a ventilation plan for the roof space. i like the old natural slates.. have never really liked the look of the vented modern slates when i've seen them, but i'll see if i can come up with somewhere inobtrusive to slot some maybe. am i on the right lines there?
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I think so. I think ventilation is key if you want to insulate. You could put a vent in the gable wall maybe and get some vents in around the dormers. You'd be best off with at least two vents to get an airflow through the attic. Then even two inches of insulation would reduce your heat losses by 90% or more.