Change pitched ceiling to flat and improve insulation

[Poppletool]

We have a cold north facing office room (2.2m wide by 2.2m long) which has a pitched ceiling thats 3.6m at it's peak.

The ceiling has pine cladding and under that is some pastic sheeting then insulation roll.

Looking at the heat calcs if I made the ceiling flat (say 2.4m) I would need a lot less heating.

If I used an insulated plasterboard with vapour layer would I need ventilation in the void that would created between the old pine ceiling and and the flat roof???

Thanks!

 

[motorbiking]

Rather than reduce the volume to be heated you could simply remove the pine roof and replace with PIR and then plasterboard or the insulated plasterboard. It's a standard cold roof, the additional insulation of say 50mm could be added to the current pitched ceiling. I suspect however, the reason this room is cold is probably due to walls/windows/floor and lack of heating. What is the rest of the room like?

 

[Poppletool]

Thanks for your reply!

The room has got two new windows so that should be ok. There are two external walls. One wall seeems to have two small cold spots so not sure if the wall insulation has moved about over the years. The floor is concrete so like the rest of the downstairs probably lacks insulation (it's a 70s house).

As it's just used as an office, Im not too bothered about the ceiling height.

 

[robinbanks]

Looking at the heat calcs if I made the ceiling flat (say 2.4m) I would need a lot less heating.

Don't really know what this means; if you insulated the vaulting it it would need a lot less heating. Installing a flat uninsulated ceiling will do little

Second what motorbiking said, but I'd use more insulation. Even standard building regs is 150mm+ of PIR these days. Line the walls internally while you're at it

 

[Poppletool]

Thanks for your reply. The intention is to insulate the flat ceiling.

The initial query was about ventilating the void and if it was necessary.

 

[robinbanks]

Ventilate it to where?

 

[Poppletool]

In effect the void would be similar to a loft space so could ventilate either through the roof or via the wall if required.

 

[robinbanks]

It essentially throws away any use the existing insulation (you said "pine cladding and under that is some pastic sheeting then insulation roll." which was kinda confusing with the word "below" but from that I interpreted that your ceiling buildup from inside the room to outside in the world is: pine cladding, plastic sheet, wool insulation between rafters,felt, battens, roof covering) offers; insulating the vault would keep it..

but yes, you could certainly ventilate the space, strip the cladding, sheet and insulation off, install a flat ceiling that you insulate with 150mm PIR or 300mm wool aand put the cladding/sheet back (or your chocie of finish)...

..or you could leave the existing buildup alone, ventilate, and put another ceiling in e.g. plasterboard, joists with wool between, PIR or wool over and essentially turn the roof into a cold construction

Ventilation would ideally have a through-draft aspect

 

[Poppletool]

Thanks and apologies for the confusion.

So if I leave the existing buildup alone and put another ceiling in with plasterboard, joists with PIR it would need ventilation with a through draft?

I had thought ventilation might not be needed as there shouldnt be any moisture getting into the void.

 

[robinbanks]

It depends on whether you're effectively barriering it from getting there or not. Was there a plan to install a vapour barrier in the new ceiling? A sensible construction detail for any application is not to have two vapour barriers with something in between that can be damaged by moisture. From your description you already have one VB; I personally would keep to having only none with other membranes being breathable. I this case that would involve pulling the existing ceiling cover down; it would take minimal time in such a small room but it would create mess. Insulating the vault leaves little to no air space capable of acquiring moist air for condensation

 

[Poppletool]

thanks robin

 

[motorbiking]

You really are better off adding insulation to the existing design rather than building a new void.

Insulation works best when it is together without a gap.