Insulation board for sloped ceiling (skeiling)

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Hi all

My house has sloped ceilings (skeilings) toward the edges of the rooms with exposed rafter feet on the outside. The ceiling meets the wall at roughly 45 degrees for about the first 75 cm, before levelling off.

Having taken the plasterboard off I see that there is no insulation whatsoever on the loft side of these sloped ceilings, so I am going to install insulation boards between the rafters (100mm rafters so I’ve bought 50mm insulation board - Kingspan), and then I will be installing insulated plasterboard afterward.

My question is -
how do I install the Kingspan; do I cut an edge of the insulation board at 45degrees so it fits flush with the top of the inside side of the wall, or do I just put it in straight as it’s cut?
Thanks
 

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Hi,

Cut at an angle, so it sits over the edge of the wall.
If cut straight, there is a risk of a cold bridge and condensation forming at the wall/ceiling boundary.

Is there adequate ventilation for the new void/s?
 
Hi,

Cut at an angle, so it sits over the edge of the wall.
If cut straight, there is a risk of a cold bridge and condensation forming at the wall/ceiling boundary.
Thanks for your reply. Would I stick anything down to the top of the wall to fill the tiny gap or is pushing it down as flat as possible sufficient?
Is there adequate ventilation for the new void/s?
It’s a “cold loft,” so air can still get around from the loft, as I’m only using 50mm insulation board (rafters are 100mm).
 
op of the wall to fill the tiny gap or is pushing it down as flat as possible sufficient?
Fill any gaps as best you can with expanding foam. But if you take care when cutting, the gaps should be minimal.
It’s a “cold loft,” so air can still get around from the loft, as I’m only using 50mm insulation board (rafters are 100mm).
Ideally there should be a 'cross flow' for the 50mm air gap, so that there are no areas with poor air flow. This is typically achieved with over fascia, or soffit vents.

A vapour barrier is also a good idea. Your insulated plasterboard could have a foil backing; but for belt and braces, you can also foil tape all the kingspan joints.
 
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Ideally there should be a 'cross flow' for the 50mm air gap, so that there are no areas with poor air flow. This is typically achieved with over fascia, or soffit vents.

A vapour barrier is also a good idea. Your insulated plasterboard could have a foil backing; but for belt and braces, you can also foil tape all the kingspan joints.
Thanks. My fascia / soffits are all wood and aren’t in the best condition (including a hole where the former overflow pipe for the water tank sat), but I plan to get someone in next spring / summer to have a look at tidying them up, so will get him to check then.

In terms of foil taping the Kingspan joints, would this just be taping two pieces together, or can I put foil tape to attach the Kingspan and the rafters,( and the wall?)

thanks.
 
Just foil tape it to the rafters.

I have a similar roof, and did what you are doing. All still seems well.
 
My fascia / soffits are all wood and aren’t in the best condition (including a hole where the former overflow pipe for the water tank sat), but I plan to get someone in next spring / summer to have a look at tidying them up, so will get him to check then.
Good plan, if you can get vented soffits that would be perfect.

And as @Mr Chibs says, if your whole ceiling ends up silver, you have a pretty good idea that moisture won't find its way through! :)
 

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