Rafter level insulation query

Southern Comfort · 03-01-2017 12:56AM #1

I have 150 mm roof rafters in my attic. I'm hoping to put 100 mm of Metac between the rafters, allowing a 50 mm air flow gap over it. I'll then put insulated plasterboard on the rafters.

But I have a problem...... At the highest point where the rafters meet, there's a 150 mm board running the length of the attic. If I go ahead with my plan, this board will block and prevent the airflow over the insulation.

Photo attached.

How do I solve this problem? One solution would be to bore small holes in it to allow the air to flow through but this would probably weaken the roof (or would it?) I'd hate the roof to collapse in on top of us!!!!!

Any suggestions?

BryanF · 03-01-2017 04:24AM

Tile/slate vents

Southern Comfort · 03-01-2017 09:30AM

Thanks Bryan. Not sure how many I'd need. Probably about 20. Can they be installed from the inside like roof windows? Are they expensive?

BryanF · 03-01-2017 10:29AM

Is the felt breathable?
Is head height an issue?
Have you continuous eaves vents?

Belt and braces: vent card, wool between rafters, cross batten below rafter, then AT membrane, then board

Calahonda52 · 03-01-2017 11:08AM

OP, what are the mission objectives with this?
If you do as described, what's the plan for the "bird bath" in the pic?:D

Normal, good practice, would be to put a horizontal section up at the apex, using timbers cut to the right length and angle and screwed to the inside (attic side) face of the rafter, not the side.
By dropping it down enough, it makes finishing it off much easier and if done properly completely eliminates the cold bridging at the apex which is pretty inevitable when this sort of job is done wrong

https://www.nsai.ie/S-R-54-2014-Code-of-Practice.aspx

SC024 · 03-01-2017 09:36PM

Is there no collar ties below the picture? insulation wouldn't normally go upto the ridge board. It should turn across the collar ties & the back down the other side.

Rafter level insulation query

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