Warm roof with overhang/canopy

[Sunil237]

My project is in planning stages so hoping to get some advice to change my plans as I think there are big flaws. Hoping for a simple solution that I have overlooked!!

The project is a rear extension width of house and out by 5.5m, plus an additional 1m overhang/canopy. I plan to use a warm roof and my builder has suggested to go with a posijoist system partly due to the span.

The detail image below is from my architects plans. The way I see it, there is no "tucking in" and a huge cold bridge as the soffit of the overhang is pretty much directly connected to the "warm roof" space of the room. Ontop of that, the metal of the Posijoist will also be a thermal bridge even if this were insulated.

 

I am struggling to picture where and how this should be insulated or if there is a better option.

 

Thanks in advance, happy to provide more info

[mjc55]

No insulation in posijoists where it over sails wall?  That's the minimum I would expect. also not quite sure why you would have insulation in exterior part of roof, what is it doing?

 

My plan is to finish posi joist on internal wall.  Outside overhang will be separated from inside.  On rear I have large overhang which will have separate support structure on exterior.  Need to work out on other the sides with 1 metre overhang.

Edited October 3 by mjc55

[Gus Potter]

On 11/09/2024 at 23:54, Sunil237 said:

My project is in planning stages so hoping to get some advice to change my plans as I think there are big flaws. Hoping for a simple solution that I have overlooked!!

The project is a rear extension width of house and out by 5.5m, plus an additional 1m overhang/canopy. I plan to use a warm roof and my builder has suggested to go with a posijoist system partly due to the span.

The detail image below is from my architects plans. The way I see it, there is no "tucking in" and a huge cold bridge as the soffit of the overhang is pretty much directly connected to the "warm roof" space of the room. Ontop of that, the metal of the Posijoist will also be a thermal bridge even if this were insulated.

 

I am struggling to picture where and how this should be insulated or if there is a better option.

 

Thanks in advance, happy to provide more info

Hiya.

 

I'll start with a general over view for folk on BH.

 

Simplistically the warm roof works by keeping the structural members on the warm side of the insulation thus they are protected from condensation. On top of the structural joists we put a layer of say OSB and on top a good vapour barrier (Alutrix 600 is one) and that stops moist air from getting into the insulation layer, condensing and causing problems. This has many benefits in that we can run services and put in down lighters etc all on the warm side without having to seal stuff up against moisture.

 

We can construct the walls by stopping moisisture from getting in and condensing in the wrong place.. we call this the dew point. Where the dew point is most likely to occur we can ventilate or similar. Say in a standard TF wall.

 

@Sunil237 too right there is a cold bridge but also we need to control the vapour transfer at the transition between the wall and the warm roof. While it is technically posssible to mitigate by theoretical detailing ( drawing stuff) it is not buildable in real life unless you want to pay thousands extra.. no builder I know is interested and you probably can't afford it. Your Architect will duck this big time unless they are able to take a pragmatic view, recognise that there is no ideal solution and design something that is a good compromise and buildable at reasonable cost.

 

I've shared the detail below for solid roof joists.. part of my IP property (yes it is mine as this is from basic maths design principles and also includes the rest of the structural behavoir) but what I can say is that every job is different. For pozi joists you could use a good air tight foam around the webs and flanges. Folks don't pinch it or you'll get into trouble but by all means use it to give you ideas. The detail below is based on pragmatic design, experience and knowledge of how each room in the house is going to be used and the structural design.

 

The below will sort out the worst of the cold bridges but not all, it is buildable at reasonable cost.

 

 

Hope this helps a bit and gives you ideas. The main thing is not to get too hung up on mitigating every cold bridge.. just do what you can and beef up the insulation levels where it is easy and cost effective to do so. The above should stop any really nasty cold bits which would result in say a line of mould on the wall or movement in the finishes due to significant moisture variations which can cause unsightly cracks in the internal painted walls and so on.

 

 

 

 

[Dave Jones]

good luck ever getting close to having a builder complete that. You need a rocket scientist ! You have 6 different types of membranes in that 'detail'

 

Have to keep it much simpler to stand a chance of getting it done correctly.