Cold roof insulation

[demogorgan]

Hi,

Trying to convert my garage into an office and am struggling to work out insulation, the joists run side to side - one side is flush with the house and the other is into the garden so can't have side to side ventilation.
The joists are directly into the brickwork with no vents and soffits are screwed directly to the joists.
Ideally I would switch to a warm roof but it's not feasible due to the position of the garage and how it ties into the house.

Would it work if I was to cross batten and attach PIR or insulated plasterboard to that and ventilate either end the opposite way as that could be vented front and back and the air could flow albeit over the entire roof rather than a channel?

Thanks in advance
Demo

 

[Wayners]

Have you seen.

 

[demogorgan]

Hi,

I hadn't seen this no, but I think this is saying I need to put a mushroom at ever joist run ?

Surely this would be bad on a felt roof ?

Thanks again

Demo

 

[noseall]

Hi,

Trying to convert my garage into an office and am struggling to work out insulation, the joists run side to side - one side is flush with the house and the other is into the garden so can't have side to side ventilation.
The joists are directly into the brickwork with no vents and soffits are screwed directly to the joists.
Ideally I would switch to a warm roof but it's not feasible due to the position of the garage and how it ties into the house.

Would it work if I was to cross batten and attach PIR or insulated plasterboard to that and ventilate either end the opposite way as that could be vented front and back and the air could flow albeit over the entire roof rather than a channel?

Thanks in advance
Demo

Every time I have attempted to construct a cold roof or a hybrid, I wish I hadn't. I fit warm deck now whenever possible.

 

[robinbanks]

Ideally I would switch to a warm roof but it's not feasible due to the position of the garage and how it ties into the house.

Show us some pictures of the outside. I can't understand this assertion

 

[demogorgan]

Hi,

See attached - the roofer that came out to quote for the new flat roof said there is not enough space due to this corner and how it sits.

Thanks in advance
Demo

 

[noseall]

Hi,

See attached - the roofer that came out to quote for the new flat roof said there is not enough space due to this corner and how it sits.

Nonsense. Here is one we did last week...

warm3.jpg

• noseall
• 17 Mar 2025

warm2.jpg

• noseall
• 17 Mar 2025

warm1.jpg

• noseall
• 17 Mar 2025

 

[robinbanks]

not enough space

To start with you'd probably get 3 inches just lifting the bottom row of tiles up to the same amount of kick as the others to the left of the pic are (though a kick as savage as that isn't something I'd strive for)

Good oppo to sort that gutter too; that's nasty dumping all the water from the main roof onto the flat

Anyhow; if you don't want all your insulation on the outside you can put some there and some inside.

Perhaps call on some other trades, find a competent builder rather than a roofer; most the roofers I've met don't really understand concepts like insulation, or anything that can't be installed with a hammer, for that matter

 

[noseall]

that's nasty dumping all the water from the main roof onto the flat

Nonsense.

 

[demogorgan]

To start with you'd probably get 3 inches just lifting the bottom row of tiles up to the same amount of kick as the others to the left of the pic are (though a kick as savage as that isn't something I'd strive for)

Good oppo to sort that gutter too; that's nasty dumping all the water from the main roof onto the flat

Anyhow; if you don't want all your insulation on the outside you can put some there and some inside.

Perhaps call on some other trades, find a competent builder rather than a roofer; most the roofers I've met don't really understand concepts like insulation, or anything that can't be installed with a hammer, for that matter

Sorry technical issues with getting back on, that kick is caused by the fascia boards and is the same for next door, they have no interest in changing it so assume can't do mine to be a different level?

How would you suggest I sort the gutter? I don't like it running onto the roof but I can't see how to sort It as it connects to the neighbours as well so can't change the direction of the flow I don't think and have no downpipe on my side of the fence.

I have 50mm PIR board that I was going to use, I would be stripping the whole roof off including old deck as shot in places, put 18mm OSB, the 50mm PIR and then 18mm OSB on top before getting someone to come and fit the felt roof - does that sound right or is the OSB to thin or thick etc ?

 

[demogorgan]

Nonsense. Here is one we did last week...

warm3.jpg

• noseall
• 17 Mar 2025

warm2.jpg

• noseall
• 17 Mar 2025

warm1.jpg

• noseall
• 17 Mar 2025

That looks good, what did you use to cover, felt or rubber ?

 

[robinbanks]

Personally I don't think you need to go so thick on the OSB; it's not a regularly trafficked surface and if space is at a premium then essentially making your own SIPS panels by bonding them all together would mean you could get away with eg 9mm. If the area underneath is going to be permanently heated /form part of the heated envelope of the house I'd try look to add more insulation by hybridising/putting between the joists

Ref what to do with the gutter, I don't have a clear idea on what happens across the entire run of it, because the pictures don't show much context but capping the end and looking to arrange a fall towards a new outlet that deals with the flow more towards the middle somewhere would be my approach

 

[noseall]

That looks good, what did you use to cover, felt or rubber ?

Felt always.

 

[noseall]

Personally I don't think you need to go so thick on the OSB;

We tend go with...

Joists

Furrings

11mm OSB

125mm+ celotex etc

11mm OSB (and some long screws!)

Felt.

 

[demogorgan]

Personally I don't think you need to go so thick on the OSB; it's not a regularly trafficked surface and if space is at a premium then essentially making your own SIPS panels by bonding them all together would mean you could get away with eg 9mm. If the area underneath is going to be permanently heated /form part of the heated envelope of the house I'd try look to add more insulation by hybridising/putting between the joists

Ref what to do with the gutter, I don't have a clear idea on what happens across the entire run of it, because the pictures don't show much context but capping the end and looking to arrange a fall towards a new outlet that deals with the flow more towards the middle somewhere would be my approach

Would you bond with an adhesive or just screw together?

It's a large garage currently and half of it will be turned into an office by building an insulated wall to separate the two, the office space would be heated when used - otherwise just the warmth from the servers that would always be on. I have plenty of of the PIR board so I could certainly add more into the ceiling same as I planned to do the walls - would I just push it tight against the bottom of the new deck better joists?

I will try and get more pictures of the gutter when I am next at home but basically my guttering is connected to next door and there is no down pipe for either side I believe and the fall from both sides goes to my roof. I have a surface drain roughly in the middle so I guess I could change the fall my side to go that way and put a downpipe there to deal with it.