Converting a loft room.

[MICH43L]

Hi all,

I'm converting an attic room (roughly 4m square) which has only around 70mm depth joists, supported by large beams. Black bitumen type sarking felt. Two small velux windows currently fitted to work around.

I've fitted 7 round soffit vents at the front of the house, but there's no board at the back. There is also currently no ridge ventilation.

I've bought 4 plastic vented tiles (nearly £100!!) to get the extra ventilation.

So the idea is:

• Fit 2 tiles near the top of the roof and 2 near the bottom at the back of the house.
• Box off the ends and the apex of the attic to enable airflow.
• 80mm(?) Kingspan / Celotex below the joists.
• Vapour layer (what type? not sure if needed or just tape the PIR board?)
• Plasterboard (foil backed?) with LONG screws (assuming no issues with stability upon the PIR board)

There's just enough headroom, but I don't like the idea of wasting it with battens below the insulation (I'd rather put in thicker insulation).

Any thoughts on the whole thing?

thanks

 

[chappers]

without battens how are you going to create any cross ventilation. If you just stick two tile vents in between a couple of rafter spaces how is the air from the other spaces going to vent to those.
If you say you are going to box an area of the apex and not fit insulation between or under the rafters in this area then you are probably better off fitting soffit vents in between each rafter spacing both front and back and fitting your tile vents two at the front two at the back at high level in your boxed off space.

 

[MICH43L]

Battens were not really about cross-ventilation but fixing the Kingspan in place. I'm hoping that this can be held in place by plasterboard, long screws and butting up against the beams.

I think the voids should handle the cross ventilation, especially if the vent tiles are positioned above them:

Voids will be about:
18" high * 1m at the apex
1m high * 1m deep at the front
1m high * 2m deep at the back

 

[JohnD]

on a related Q, is there a non-dusty way to cut Kingspan cleanly?

 

[MICH43L]

Score it with a stanley knife and snap it.

 

[JohnD]

through the foil, you mean like plasterboard? And does that work even with a thick board like 100mm?

 

[MICH43L]

Through the foil, yes, just like plasterboard. The really thick stuff I'm not sure, you may get a kind of D shaped curve in your cross-section. Best thing is to try with an off-cut, maybe score both sides, but it does snap really well.

 

[chappers]

Battens were not really about cross-ventilation but fixing the Kingspan in place. I'm hoping that this can be held in place by plasterboard, long screws and butting up against the beams.

I think the voids should handle the cross ventilation, especially if the vent tiles are positioned above them:

Voids will be about:
18" high * 1m at the apex
1m high * 1m deep at the front
1m high * 2m deep at the back

Yep should be fine on both you will need 150mm drywalls and plenty of them, you may have to get them ordered or mail order, alternatively you could batten out your rafters by 25mm put 50mm between the rafters and 50mm over then you could use std 100mm drywalls, you would still have the same depth and just about the same u-values.
You won't snap thick board very neatly use a hand saw or a breadknife.

 

[MICH43L]

I like your thinking...

 

[John D v2.0]

on a related Q, is there a non-dusty way to cut Kingspan cleanly?

I found the best way was to get my wife to do it and leave the room!
I wondered if there was a way of using a hot cheese wire or something similar but in the end we just used a normal saw.
That was the 100mm and I did think 50mm x2 would have been easier.