PIR Question please

[Rusty Peg]

Hi all,
Looking for advice, garden room build using 18mm OSB and timber joists for walls, would I be better dropping from 125mm pir between the joists
to 100mm between the joists with 25mm across the front to help with thermal bridging, and changing from
6x2 to 5x2 timber to keep wall thickness the same.
Second silly question, I can buy 2 x 50mm pir cheaper than 100mm, will doubling up the 50's to get to 100 be better, worse or no difference in insulation values.
with thanks.
Rusty

 

[freddiemercurystwin]

100 between + 25 under will be better, using 2 x 50's is identical to 1 x 100, more cutting/labour though. Don't understand the wall question.

 

[stevie888]

Theoretically 2 50s should be a fraction better as part of the thermal resistance comes from going through a surface and you now have more,

In practice I'd say easier cutting, 2nd chance at closing any gaps and more flexibility in order. As an example of the latter I did my daughter's flat roof several years ago and 2 50s saved me hacing a half sheet of 100 at the end.

 

[Rusty Peg]

Thanks for the response in line with my line of thinking, problem with google is you get half internet saying never do this then the other half say always do this, end up second guessing yourself.

 

[^woody^]

Drop the frame timbers to 100mm to avoid any void in the frame.

Make sure that any slightly covex board is flattened so that there are no voids between the sheets.

Expanding foam or flexible mastic for any gaps to the frame, as that will be the biggest route for heat loss.

Buy a proper insulation saw (no saw teeth) for a tenner or so or blade for a jigsaw, and avoid all the bits and fine fibres. You don't regret it.