Cold Roof with Sloping Ceilings - Insulating

[thebadger7]

Hi folks

We're replacing our roof and insulating at rafter level at the same time. There are attic rooms with sloping ceilings and a void above the horizontal ceiling.

We're using woodfibre above rafters & sheeps wool between rafters (and above for the horizontal part).

Should the void space above the horizontal ceiling be ventlillated across from each gable end (detatched property)? It's on the cold side of the sheepswool, but warm side of the woodfibre sarking boards.

Any advice appreciated!

Thanks




QWe

 

[alexander]

you need to be venting between the membrane and the wood fibre board, you can achieve this by counter battening on top of the wood fibre board giving you an air gap do not vent between the sheep's wool and wood fibre board or you will create a condensation problem.
good luck

 

[thebadger7]

Thanks - yes there will be counter battens with ventillation above the wood fibre boards across the whole roof span

But the void is then the cold side of the ceiling-level sheepswool insulation, and I thought you needed to ventillate cold side of insulation

 

[stuart45]

Can't you run the sheepswool to the ridge instead of along the collars?

 

[Kearn]

It’s a tricky one - ideally you want to be solid filling the void above the collar to ridge to keep all insulation continuous. Otherwise the boards above collar are a bit pointless and worse still you’re creating condensation risks in the void below
However, it’s likely a decent volume to fill and may get you into the realms of too much insulation which brings other issues.

I assume boarding internally is a no go? I have a very similar roof which I am doing this way. Wood fibre batts between rafters, air gap above, wood fibre boards on the underside of rafters/collars. So effectively my red line from your diag goes under the green one and follows its profile.

 

[thebadger7]

Wouldn't gable end vents deal with the condensation risks in the voids below?
We're replacing the roof anyway and from what I've read boarding over rafters is the better performing option

We could use sheepswool under all the roof, so lines match and void is then on warm side

 

[Kearn]

Wouldn't gable end vents deal with the condensation risks in the voids below?
We're replacing the roof anyway and from what I've read boarding over rafters is the better performing option

We could use sheepswool under all the roof, so lines match and void is then on warm side

Yes, in theory, but then you’re reducing your insulation at the collar as it’s just the sheepswool, the boards are redundant as said.

If you ignore the collar and go to the ridge, there is no void? You’ve just got a vaulted room then.

 

[thebadger7]

Here's a pic of the room - the sloping ceilings are larger than the horizontal

 

[thebadger7]

We'd pack a decent depth insulation at the collar - much more than at rafters