Question on attic insulation

893bet

My attics is fully vented. Airtight layer on first floor ceiling. I am planning on 300mm knauf on most of the attic floor however along the centre part I am doing 150mm between joists and putting loft board down( these boards are 70mm PIR type insulation with timber board attached).

Some one planted the seed with me today that they might trap moisture beneath them?

Is this a concern?

Calahonda52

Raise the rafters to take flooring and the run the knauf through and then put plain boards down with a 5mm air gap between then as opposed to T&G

893bet

Calahonda52 wrote: »

Raise the rafters to take flooring and the run the knauf through and then put plain boards down with a 5mm air gap between then as opposed to T&G

Twice the work for roughly the same cost plus I save on height also!

Calahonda52

893bet wrote: »

Twice the work for roughly the same cost plus I save on height also!

Not the case re cost but I will let it pass.

describe what u mean by fully vented
where will insulation stop?
how will 300 be laid?
what depth are roof rafters, roof, not ceiling?
Whats plan for dealing with cables, pipes, cold water, tank, MHVR ducts, attic hatch?

893bet

Calahonda52 wrote: »

Not the case re cost but I will let it pass.

1)describe what u mean by fully vented
2)where will insulation stop?
3)how will 300 be laid?
4)what depth are roof rafters, roof, not ceiling?
5Whats plan for dealing with cables, pipes, cold water, tank, MHVR ducts, attic hatch?

1) Simple A roof. Vent trip along soffit.

2) I don't understand the question sorry?

3) 300 will be laid in two layers perpendicular. Along the centre run then where I want to put a floor there will be one row of 150mm between joists, then 80mm PIR laid flat on joists and then osb on top. I just want to ensure moisture won't get trapped by this PIR.
4) 6 x 2s
5) cables all ran away from where I plan to deck out, same with MVHR pipes. Will be building a stand for cold water tank.

Thanks

Calahonda52

if the insulation runs too far out into the soffit space then it will block the air vent
If it is not out to outer edge of wall plate then you run the risk of mould at the ceiling wall interface.

second pic is from a CCTV camera screen, no insulation so mould as photo one in bedroom
if the end of the insulation is not airtight then the wind from the soffit vents will pass through it, reducing its U value
if the wind can get in under the layers then the top layer is useless.

Re the PIR etc, leave a 5mm gap every 400 or so longways across the ceiling joists

893bet

Thanks. What would you recommend doing to ensure both layers of insulation are utilised fully and their u value not reduced?

Calahonda52

893bet wrote: »

Thanks. What would you recommend doing to ensure both layers of insulation are utilised fully and their u value not reduced?

re the ends near soffit
google Breathable vent card, look at images
bring them well up over insulation so as wind is well up from insulation.

Re the rest, that is why I suggested building up the rafters and sheet with 300/400mm wide strips, with 5mm vents
Building up along ex=isting ceiling joists is eager but has thermal bridging
cross building is better but harder