Warm Flat Roof ?

[RobFurn]

Garage conversion underway.
The roof is flat, Joists support the roof on the inside and above the joists is chipboard and poking out of the gaps in the chipboard is fibre insulation.
Does this mean I have a warm flat roof?
I need to insulate between the joists using celotex. As I understand it if I have a warm flat roof I dont need to have a 50mm vapour cavity right? I can just have he celotex as close to the chipboard as I like

 

[noseall]

To build a 'warm' flat roof you will need to fit the insulation wholly above the roof joists thus rendering the space below and the joists themselves warm.

So, you need to remove the chipboard and any insulation, then fix ply bonded 97mm insulated deck to the top of the joists. You will also be required to fix a further 25mm layer tight to the underside of the deck insulation.

Any other method, i.e. fitting insulation between the joists will mean you will need to vent the space between joists.

 

[RobFurn]

Oh bugger I thought (hoped) with the insualtion above the chipboard the It would be classed as Warm

Oh well think I will do as the Celotex data sheet says on garage conversions for cold roof upgrades and:
place 75mm between the joists leaving 50mm gap then overlay with 30mm below the joists. That should do it eh?

 

[noseall]

Lose the chipboard it is rubbish!!!

Any cold roof arrangement will require through ventillation for any air gaps.

Why your reluctance to re deck? You will end up with a superior simpler design.

 

[RobFurn]

Wont removing the chipboard and insulaton behind knacker the roof covering above. Or are you suggesting I completely recover the roof. That would be my concern. If I went down the rout of insulting between the josts would I have to install eaves vents and Mushtoom vents for air flow though the gap?

 

[Deluks]

Chipboard? Do you mean OSB board?

Sounds like you do have a warm roof, but fibreglass is not the norm. What's on top of the roof? (On top of the insulation?)

 

[RobFurn]

OBS board yes sorry that's it not chipboard just googled it, I'm not sure what's behind it but will investigate later and write back cheers

 

[RobFurn]

After removing a small section of the OSB board theres nothing behind it just felt, Who knows what the bits of insulation are sticking through the gaps in the OSB are. Now I have a sodding tiny hole in the felt and its dripping (brillient) oh well at least I know.

So whats the verdict, insulate between and over the joists with Celotex and leave a 50mm gap, if so whats the best way to achieve airflow through the gap?

 

[Deluks]

Who knows what the bits of insulation are sticking through the gaps in the OSB are.

Was the roofspace previously insulated? Could be just leftover tufts that got caught between the boards. Could it be mould?

To create adequate ventilation you'll need to vent between each of the joists. By removing bricks and replacing with vents, or venting into a soffit, which is also vented.

As Nose states, it would be better and easier to insulate above the deck, but you'll need to factor a new roof covering into the cost. The OSB can stay put.

The insulation he refers to sound like kingspan with a 6mm layer of WBP ply stuck to it (topside) which is to take the felt.

 

[noseall]

If what you have is a basic flattie then there is no reason why you can't still have an unvented warm deck.

You can remove the current felt covering then fix ply backed (97mm) insulation sheets over the existing. You can then fit the 25mm sheets to the underside of the osb.

The only downside to a warm deck is a rather deep fascia at the roof edge detail.

 

[RobFurn]

All excellent advice gents, thanks.
A warm deck may be the answer, my reluctance is cost and the fact that just above the garage attached to the house wall I have an AC condensing unit and pipework. Raising the roof may interfere with that, also when fitted will the plyback be capable of being stood on so I can maintain my AC?

If I choose the vented option I realise I would need to vent between each joist, so can this be done just at the soffet edge. Or is ventilation also required at the other side attached to my house as well?

 

[noseall]

If I choose the vented option I realise I would need to vent between each joist, so can this be done just at the soffet edge. Or is ventilation also required at the other side attached to my house as well?

I have a feeling they (building control) insist on through ventilation.

 

[RobFurn]

I they probably will.
Im getting some drawings knocked up by and an architectural technologist (friend of a friend) It will be interesting to see what he says on the subject. Those drawings wont be available untill next week so I will post back then.

 

[Deluks]

If I choose the vented option I realise I would need to vent between each joist, so can this be done just at the soffet edge. Or is ventilation also required at the other side attached to my house as well?

I have a feeling they (building control) insist on through ventilation.

Which would mean a funky vented upstand at the abutment, which would still get in the way of your externals.

Might get away with more in between and less on top, but you should check what the thinnest inso thickness that is acceptable above the ply.

 

[RobFurn]

Have checked other posts from this site and "Woody" suggests that in theroy that a gap is not necessary so long as all the space between joists is completely covered, a combination of rockwool hard up against the OSB covering all space then solid celotex over that. This way no condensaiton can form as your in effect creating a warm deck. If makes sense to me but I bet the BC wont like it.

On the subject of venting the abutment can I use mushroom vents like these http://www.ubbink.co.uk/roof_vent/flat/felt.htm
[/url] in every gap between joists or is there some regulaton stopping me. They wont get in the way of my AC as the AC unit is at the other end of the garage which Im not converting (part of the garage is going to be a small utility room)