Insulating unvented roof void.

smartbodydublin

Hello, have been surfing for a few weeks and am still unsure as to how to proceed.

Would appreciate any solid advice.

Last two winters were a bit crazy in my crumlin house. I do most any DIY but am having trouble getting solid guide lines on insulating this 1940s built house.

It has bitumen sarking felt with slate tiles on top. No soffit venting, the rafters come down onto what appears to be a horizontal concrete top wall plate on the front and back walls. All these houses built by Dublin Corporation in this area from the late 30s to 40s appear to be like this. From the outside no vents on the underside of the horizonal plate can be seen. There is no ridge vent either. So...It appears to be an unvented roof with unbreathable felt.

I'm looking to do anything to (reduce, not prevent heat loss this winter), as I'm sure to do a really thourough job would be a serious untertaking. Any improvement would be be good..!!

My main concern with installing insulation is dampness to the roof timber with possible mold and dry rot in the future. So not how ideal the degree of final heat retention I get will be, but a compromise that allows a lower rate of heat loss and also doesn't encourage moisture build up.

I think there is not much vapour traveling up to attic from cooking as there is decent extractor. The shower is downstairs and light activated electric van vented with a wall heater used regulary to keep area dry.

I've sealed up perforations in the attic plasterboard from downstairs, removed and replaced downlighters and am trying to improve general airtightness both out of the house and up into the attic.

My current ideas on insulating the attic are-

A...I was thinking of a single layer of 6inch/150mm glass roll so the tops of joices are exposed to air to release moisture. I'm was thinking of pushing this right into the eves as there is no venting anyway??

B..Maybe use heavy double faced foil under the face of the rafters, having it well sealed to reduce vapour getting to rafters and directing a bit of heat back into house during winter and in summer hopfully this would reduce the sweat box conditions in the house by reflecting heat back out...(but maybe dry out/make brittle sarking felt much quicker as this would increase heat a bit in the above rafter area).

I'm hoping about doing various house jobs for winter so forgive me if I missed a good thread on this. I did look!!


Pat.

BryanF

smartbodydublin wrote: »

....No soffit venting, ...no vents on the underside of the horizonal plate can be seen. There is no ridge vent either. So...It appears to be an unvented roof with unbreathable felt......

I'm looking to do anything to (reduce, not prevent heat loss this winter),

A...I was thinking of a single layer of 6inch/150mm glass roll so the tops of joices are exposed to air to release moisture. I'm was thinking of pushing this right into the eves as there is no venting anyway??

B..Maybe use heavy double faced foil under the face of the rafters, having it well sealed to reduce vapour getting to rafters and directing a bit of heat back into house during winter and in summer hopfully this would reduce the sweat box conditions in the house by reflecting heat back out...(but maybe dry out/make brittle sarking felt much quicker as this would increase heat a bit in the above rafter area).

I'm hoping about doing various house jobs for winter so forgive me if I missed a good thread on this. I did look!!


Pat.

Pat,
can you just install some ventilated slates as low as possible in the roof and then a few ridge slates? that's the ideal solution and then you can insulate away!
I don't quiet understand the foil option, doesn't make any sense to me:)the only place you should place an effective vapour control layer is between the plaster board and below the joists. unfortunately a bit of grief if your living in the house. its maybe not your first priority if you can ventilate as above

smartbodydublin

Hi Bryan,

Thanks for reply. The foil was meant as a radiant barrier as the house is very warm in the summer. I had read that this 2 sided type foil would help cool a house in the summer by reflecting heat back out and help it retain heat in winter by reflecting it back in. I also read that as an unvented void the less vapour the timbers got the better, so that was the idea of making the foil as airtight as possible to have it also reduce vapour to the rafters with my upper floor ceiling sealing being my attempt at vapour control to the joices and purloins. I was going to silicone seal it as best I could at joins and onto the side walls. I was considering this as opposed putting solid foam in the rafters.

The vent slates sound very good. I had not considered them as having the non-breathable felt under the slate roof I thought anything less then eves vents, which would allow air to pass under the felt/inside the attic, was not going to help with water vapour removal.

So having the vents outside the felt would allow some vapour out even with the bitumen sarking?

It's a small 3 bedroom terraced house. How many low vent's and ridge vents would you suggest.

I can get to the top window to paint and fix but amn't a roofing type of DIYer, so would have to price this one out. I can imagine being told I need a full vent system now that you have me looking at this option, so would be good if you suggested the basic needs, so I've some idea what to price for..

Again, thanks for your time and help.

BryanF

forget the foil, I worry your just creating a water catchment (think of one of the survivor shows):)
if you remove your ceiling plaster board and put in a moisture barrier then you'll reduce the problem from below. then you put say 400mm of cellulose in the attic which is good at absorbing and delaying heat gains in such a space (*only install if there is definitely no leaks) then install the vents to allow adequate air movement (ventilation).

you are correct, the less vapour the timbers get the better, but i think what your suggesting still doesn't deal with non-ventilated space and so what have you solved?

I afraid you will need to talk to a vent supplier on this one, My understanding is, they are vents on the market that can introduce ventilation below the felt while still keeping out driving rain. as to the quantity of vents see http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,1647,en.pdfand ask the vent supplier what each vents allows in, it should be straight forward to work out your roof area and the percentage of vents that you can manage to install. and again you need to get an air cross flow which was why I suggested a low row of vents on each side and some in the ridge also.

smartbodydublin

Ok, the double sided foil is off the board, that alone is a good help. The sales blurb on it is very good and lot of posts around the net suggest it works well in vented roofs at least. But I get that it's metal based nature would be a fine condenser which I hadn't till now fully appreciated. With no mosture barrier I could end up with extra water supply up there at some point.

I looked at vents after you suggested it but managed to not consider that'd pierce and then be sealed around the felt which makes good sense! I'll price the slate job and will estimate based on 'the environ' document, how many vents are needed.

The moisture barrier as you explained would be a significant step in protecting the timbers.

There are 3 upstairs bedrooms with ceilings in good well finished condition. There is a drop ceiling over the single fligth of stairs and one landing coming to 2 more steps and final landing. I would not undertake at this point to remove them. I usually try to jobs properly as I hate repeating the same DIY work over. But in this case I'm not up the job in terms of time,mess and especially money as I'd need to pay someone to work with me. The roofing/slate job will be this years big out sourcing job... ha ha

An insulation option that will give me a few years of dry/safe timbers and reduce heat loss if even a little. A trade off in performance is fine as I lived in a freezer last two winters. It did improve as I stuck in ever more draught sealing foam and filled up holes last winter. Have done more sealing up of attic openings for this winter, so hopefully that will also make some differance.

I was suggesting 150mm glass fibre which didn't cover the joice tops completly as it would hold some heat leaving some drying surface open to the air. I repeat this only because I had thought I could then check it a few times a year for signs of mold and what ever it's condition I could pull back rolled insulation to allow timbers to dry better a few times a year when warm/hot weather. I'm limber still and use to moving about over the joices. Something along this low labour ongoing maintanence routine would suit as opposed any serious rebuilding at this point.

If nothing else is suggested I do appreciate your time and consideration on my home heating/freezer issue!

The vented slates should help greatly by themselfs now that I've a handle on them and how they'd apply to a bitumen felt underlay. Anything is a help when one is in a sea of confusion.

BryanF

step one sort out the ventilation, then install the 150mm mineral wool, as you said keep and eye on it, then maybe next year just add another 150mm, by keeping an eye you'll know youreslf. you could also purchase 2 or 3 RH sensors and leave them around the attic (they cost circa 50€ each). these can be put in an excell program and you can then assess how the humidty level are over a lengthy period of time.

smartbodydublin

Ok Bryan, will be looking into it. Will also look at the RH sensors, that's a very fine idea long term as well. All new to me.

Hope other folk with slate topped, bitumen underlayed, unvented roofs get to read this advice from Bryan, as I think a lot of attic timbers are going to have problems as likely 1000s of old houses have been insulated and sealed up without regard for moisture escaping except through attics like this one.