metac and insulated board or sprayfoam?!

Idbatterim

Hi lads, need to insulate a roof, the ceiling height currently is 230cm, I dont really want to lose much / any height . So I can put in metac to the depth of the joists, i.e. 5 inches and insulated slab of 37.5mm or get it sprayfoamed for E500 and just slab it with 9.5mm plasterboard?

its E350 for first options or e530 for the second option. Would there be much of a difference in u values?

BryanF

Is this a new build? Where is the air-tightness layer?

Idbatterim

Hi Bryan,

no its not a new build. Its a cavity wall, block built garage with flat roof, being turned into a home office... its a flat roof of timber construction, with torch on felt, if that makes any difference...

I plan on just slabbing the walls with insulated board 50mm thick or thereabouts...

BryanF

As in block/cavity/block? If so just Pump the cavity with beads and plaster the internal wall. Plastering walls and sealing to floor will help avoid drafts.
Re roof what are joist depths and centers?, put breathable felt below the joists primed & taped to the plastered wall with foil backed insulated slab underneath. You need to cross flow ventilate the space between the joists from fascia/soffit of the flat roof.

Make sure you manage your heat and ventilation to avoid condensation leading to mould.

It’s essential you vent the roof. The above spec is mediocre at best, and isn’t suitable for a new build, but you’ve an existing bitumen roof to deal with.. if this was a bedroom, I’d be suggesting removal of existing roof covering to install a warm roof build up (insulation above the joists)

Idbatterim

Hi Bryan. Thanks for detailed reply. it’s cavity block built , not breeze block. Spray foam lad said they don’t spray foam cavity blocks as it’s not practical I suppose. If I’m not being clear enough , can take photo tomorrow ...

BryanF

So you meet a Spray foam installer. Ask for a manufacturer / installer joint warranty for what he’s proposing and he won’t answer the phone.

Metac in between the joists with plasterboard underneath will also rot the roof.

Can you answer my question above - roof: what are joist depths and centers? Maybe you could leave 50mm vent cavity by using vent cards and do as above with the vents in fascia / soffit. But not sure how much space you’ll left?

Calahonda52

As above plus:
there are two sorts of spray foam, open cell and closed cell.
Its very difficult to know which one they actually install, despite what they promise.
Open cell is cheaper and has lower insulation properties and is not suitable in this application.
In fact i would not used closed cell either as its too high risk here if not done properly.

Solar drive here is also an issue that needs to be taken into account.
Lastly, decrement delay as an insulation property need to be factored in, if you wish to avoid overheating in the summer

Idbatterim

There is a photo of joists bryan. Should I just use metac and foil back board ? Yeah the spray foam lad said he’d use open cell on roof...

Should there always be a gap and airflow between any insulation and the roof boards , regardless of insulation type ?

Calahonda52

That photo gives zero context

BryanF

After 50mm vent cards you ain’t going to get much metac in. Would you consider rafterloc insulation (PIR Branded product)