Insulating Small Pitched Roof From Below

[carson321]

Hi, 

 

I currently have a single storey wrap around section to my house containing utility and toilet that is part of the original early 80's construction, as shown. It has a double pitched roof over the utility, becoming mono pitched over the WC area.

 

Currently doing some renovation work and have all the ceilings down, this area has no insulation in the roof, which goes some way to explaining why it has always been freezing cold! 

What is the most practical way to insulate this roof from below, before I put new plasterboard up? I know I can just stuff some mineral wool up there, but how do I maintain ventilation? There were indicators of condensation (damp patches) on the back of the ceiling when I took it down, does that mean I already have poor ventilation and need to look at doing something to the roof construction? If I just stuff 300mm batts up there, I will have to avoid a large area around the edges to ensure I am not just contacting the felt under the tiles. 

 

Any advice or pointers greatly appreciated. 

[Russell griffiths]

Need some pictures and a diagram really. 
do you have a flat ceiling or is it all vaulted. 

[Redbeard]

Whether the roof has flat or vaulted ceilings I would suggest cut some rigid insulation at the eaves to form a 'ventilation profile', then (if it's a flat ceiling) 200-300mm of your choice of 'quilt' on top of the joists at right angles to them, then friction-fit semi-rigid mineral wool between the joists to finish. Then VCL, then plasterboard. If it's vaulted you'll need to 'build down' from a 50mm vent path to get a B Regs compliant level of insulation. (Crudely about 150 PIR or 270 'fluff')

 

BUT (re ventilation), The WC etc roof is a lean-to, isn't it, which presumably means there is no cross-ventilation from the eaves? So... Can you put vents in the end of the 'WC end' and the utility, to get air flowing that way. Keep the 'ventilation profiles' I refer to above as well, so you get a bit of extra supply ventilation to the void*. *Of course this only works if you have voids, and doesn't if you have vaulted ceilings. More info, please!

[Crofter]

Assuming it's a 'cold roof' construction, where there is a loft space left uninsulated, then you will need to add insulation above the ceiling and also add ventilation for the cold space- preferably at both ridge and eaves level. 

If I'm correct about it being a flat ceiling and cold roof, then there's not really much advantage to doing it at the same time as you board the ceiling- you'll have to board the ceiling first anyway. Although if your loft hatch is small, you could chuck the rolls off insulation up there first. 

 

If you can't (or don't want to) fit high and low level ventilation, you might be able to put vents in the gables instead. 

 

If you've actually got a vaulted ceiling, then not much of the above applies.

[carson321]

I shall take some pictures this evening and come back with some extra info as well. It will be a flat ceiling, not vaulted, and there will be no access to the roof space once the ceilings are boarded