Advice needed, insulating the soffit in a warm flat roof construction

happyhero

Hi I need some advice please on insulation. I am a retired electrician so I have some technical background when it comes to the makeup of house construction and I am quite hands on with doing stuff to the house.

I recently had a warm roof put on my leaking flat roof extension at the back of my house (6m x 4m approx.). I have a soffit at the back that overhangs about 300mm. This is insulated up to the facia in the top area part of the facia i.e. insulation level but not at the rafter level, basically the height of the rafters (7”) under this insulation/ply layer. I can easily see this from inside because I got the builder to leave the plasterboard off so I could do it myself after running cables etc.

Reading online I see this is a common weakness that lets the cold in and is often left unaddressed as they describe it so that it is insulated upwards but not behind the facia or on the soffit and now it’s getting colder you can really feel the cold coming in at those areas. I don’t want to put plasterboard up until I have addressed this cold problem, it seems to defeat the idea of having a warm roof.

The extension is a cavity wall with no insulation inside the cavity. The warm roof has been installed with no vents which I understand are not required on warm roofs. I understand that there can be problems where warm air meets a cold surface and condenses so I am considering this fact when I try to think how to insulate the soffit. I’m hoping to put recessed lights in the soffit so I don’t simply want to lay insulation on top of the soffit and at the back of the facia.

Also at the side of the roof both sides there is no overhang or about 1” inch if you count the facing PVC sheet. I have access to this area but the joists only allow me about 2” of access to squeeze anything in so I might be able to get some Celotex type insulation in there of about 2” thick with difficulty or alternatively fibreglass wool.

If I use fibreglass wool in the 2 sides would it be ok to completely fill the cavity over the wall up to the barge boards, I mean then warm air could not meet the back of the cold PVC, would that be acceptable, or terrible?

Where the 300mm soffit is could I put a sheet of 4” Celotex upright standing on the inner wall? Perhaps backing on to battens I could fix in place? Creating a cold box section but sealing the cold box section away from the internal joist section, would this be acceptable or terrible?

If I did the 2 insulation methods above I would have a complete insulated cap over the room, i.e. insulation from warm roof above and back and sides insulated with fibreglass packed in tight at the sides on top of the walls and Celotex closing the gaps into the soffit from inside the room.

It seems ok to me as there is nowhere for warm air to meet a cold surface, what do you think or what else should I consider?

Is it ok to totally fill a space with fibreglass, especially as there are no vents, do I need to leave any air space?

Could I use thinner Celotex and layer it i.e. create 4” of Celotex by layering 2 x 2” on top of each other or does that create new problems?

Any help or advice appreciated.

happyhero

Anybody, pretty please?

Newuser1987

You need to ensure that the wall insulation meets the roof insulation. You say your cavity wall doesn't have insulation so this is your weak point. Not really clear what insulating the soffit would achieve. Essentially you should have a junction like this.

happyhero

Newuser1987 wrote: »

You need to ensure that the wall insulation meets the roof insulation. You say your cavity wall doesn't have insulation so this is your weak point. Not really clear what insulating the soffit would achieve. Essentially you should have a junction like this.

Thanks for your input Newuser1987, your showing a picture of a cold roof, mine is a warm roof installation and the plasterboard is not in place yet.


To explain further, normally if you were in the room and looked up with no plasterboard in place you would see joists and the insulation inbetween but in a warm roof there is another whole layer. The top layer has the insulation so all you see is joists and the ply on top of them from the room. You dont insulate between the joists that you can see as the insulation is in the layer above the ply.


This means your roof is nearly twice as thick as a normal roof. Outside you will see a tall facia compared to a cold roof. Inside you will see the joists going over the cavity wall into the overhang and up to the back of the facia. Basically meaning you have a piece of plastic between you and the outside that you can see between the joists.



The warm roof does a great job above you but is let down by the cold coming in from the facia and to simply plasterboard the ceiling would be to leave a problem hidden and probably end up with a very cold ceiling.


I am amazed that they come up with something as good as a warm roof and leave such a weakness. There is loads on the net about how this problem is never addressed.



Also your picture is not really showing a soffit face with an overhang, but the side where the barge board is up close to the side of the outer wall.

Newuser1987

The image I put is a warm roof. You can see the joists with a ply deck and then the insulation above that. Your thermal line should be continuous from the wall to the roof as shown in the picture. The soffit/fascia is just cosmetic and has no bearing on the insulation of the space.

There isnt a weakness for a warm roof if done properly. You do not even need a fascia/soffit for a warm room. Fascia/soffits are traditionally used for cold roofs to allow air to circulate within the roof.

I would advise you study the detail I sent a bit more closely and you might understand more what is required to ensure your wall/roof junction is more robust.

Newuser1987

happyhero wrote: »

This means your roof is nearly twice as thick as a normal roof. Outside you will see a tall facia compared to a cold roof. Inside you will see the joists going over the cavity wall into the overhang and up to the back of the facia. Basically meaning you have a piece of plastic between you and the outside that you can see between the joists.

I just re-read your post and realised what you have done wrong. You should not be bringing your joists on to the outside wall/leaf. You have created a cold bridge which will open you up to condensation.

What you should have done is sit the joists onto the inner leaf only or hang the joist off the inner leaf. Then the outer leaf should have been built up to the warm roof deck as you will see in the detail I sent.

The reason why we use the terms cold/warm is to do with the keeping the structure either 'warm' or 'cold'. Currently you allowing your structure to reach the cold air.

happyhero

My God you are totally correct of course, please forgive me, I have totally misread your input.


I am really not very happy now as my roof has been incorrectly installed and I know it would be futile to try and get the builder back to make these massive costly changes. Is there a way I can make the best of a bad job?


The old roof was a cold roof and had joists up to the back of the facia with no overhang. I asked for a warm roof and to match the overhang of the floor above hence the 300mm overhang. But if the top layer is only insulation sheet and the joists are not supposed to go over the outer skin how do you create a strong overhang? I mean window cleaners etc. will walk on that roof.


I see now that the barge boards at the side should be cosmetic but they are not as the wall only goes just into the bottom of the fascia/soffit, i.e. the whole roof sits on top of the walls with the fascia/soffit plates just covering the brickwork a tiny bit.


Please Newuser1987 tell me some good news, what can I do to make the best of a bad job?


This is torture as it looks like such a neat job and it cost me enough.



Remembering I want to put soffit lights in can this all still work?


I’d be really grateful for any guidance here as to how to proceed to insulate and minimise future problems.


Is it not possible for the joists to touch the fascia as they did on the old roof without problems?


Help!!!!