Pitched Roof insulation, installation questions

spud2

I have a some questions/dilemmas with installing roof ( between rafters) insulation. I hope the photos will add clarity to the queries.

Installation:
I’m installing 50mm kingspan between roof rafters on the roof slope, and will be adding 50 mm backed plaster slabs to the roof rafters to complete the job, the second story of a dormer.

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Problem:
If I go below the wall plate with the insulation ,the angle this puts on the board will not allow the board ( just sitting on the wall plate) to be flush with the rafter edge. If left this way, there will be an air gap between the insulation between the rafters and insulation back plaster slab. This I believe is not desirable?. I could cut a notch out on the board but the 50 mm to help but it will but still would not be sufficient.

1 possible solution?
Is it a requirement to have the insulation to go beyond the wall plate? If not I would cut a notch on the edge of the insulation and fit into the front or inside edge of the wall plate and then it would be flush with the rafter. Additionally in my own opinion, this would be a better air tight solution ( not allow air to the wrong side of the insulation ) as opposed to fitting the board on top of the wall plate. Also I plan to have 50mm back slab boards for the vertical walls. So the roof insulation will be overlapping the wall insulation going just to the edge of the wall plate

Another Possible solution ?
Could I simply the fill the Gap that currently exists with rock wool ?

I know if I simply had thicker insulation I would not have a problem.

Also I believe Xtratherm have a new “quilt” type product for pitched roofs and because its not rigid it will fit snugly between the rafter even if they variations in width ( which I have). I need to look at this also. I could not find anything on their website.


Any suggestions/recommendations would be much appreciated.

Mike

ardara1

Fit all insulation as continuous as possible. The Xtratherm material is cut to size for rafter spacing - but insulate below the rafter also to meet the robust detail guidelines - (There is a specific figure to be used for this in tne new Energy Labelling method.)

typivally you need 100mm Xtratherm/Kingspan between and 40mm below to meet regulations 0.20 U-value

serotonin_sam

Have put in the insulation in a couple of dormers recently. At the end, cut a notch out so that it can then fit flush. Its time consuming but necessary.

Viking House

When you join two insulated plasterboards there is usually a rafter behind it, this is where the moisture gets in to the roof wood. Because the wood is the only hydroscopic/breathable member in your roof it tries to draw/vent this moisture to the outside. Because this happens slowly the moisture builds up in the wood and it starts to rot when the moisture levels rise above 18%.
I would never use Kingspan or Polysterene between the rafters because it is non breathable, traps moisture, its very difficult to cut accurately and because it is so light (30kg/m2) it looses heat very quickly making your attic very cold in the winter and very hot in the summer.
This link shows how heavier insulation materials hold onto heat for up to 10 times longer lowering your heating costs.
http://www.viking-house.net/decrement-delay