Insulation for sloped ceiling

lukegriffen

Hi, I have a 1950s house, and all the ceilings upstairs are sloped where it meets the wall.
The slope is about 20 inches, so overall a sizeable enough area & it can be difficult to reach in places. Any recommendations, do I just try and shove earthwool down into the slope?

Thanks in advance for any replies

_Brian

It’s a balance between getting it in and packing it in, as you compress it it looses its insulation properties.

Lumen

Would 50mm PIR be easier to achieve air tightness with?

_Brian

Insulation and air tightness are completely different.

Surely if op was potting a membrane underneath the timbers the type of insulation would be inconsequential

muffler

Ventilation will have to be maintained so you won't be able to just "shove" the insulation in.

Lumen

_Brian wrote: »

Insulation and air tightness are completely different.

Surely if op was potting a membrane underneath the timbers the type of insulation would be inconsequential

Sure, I'm just bringing it up as it wasn't mentioned. Any air pressure differences and the heat's going to go straight through that insulation without air tightness.

I have read that air tightness membranes work best in practice when properly supported, so I was just wondering whether having rigid insulation makes for a better job (plus the better u-value).

antiskeptic

You could put warmboard on the internal sloped sufaces. The 8ft board means few (one?) joint between boards. The boards have no tape chamfer at the ends but by placing the joint on a ceiling joist, leaving a 3mm gap, adhesive foaming to bond the insulation end-to-end and filling that gap with polyfilla it's possible to do a tidy job that will be both complete and not affect air ciculation above the ceiling.

Wartburg

Lumen wrote: »

Sure, I'm just bringing it up as it wasn't mentioned. Any air pressure differences and the heat's going to go straight through that insulation without air tightness.

I have read that air tightness membranes work best in practice when properly supported, so I was just wondering whether having rigid insulation makes for a better job (plus the better u-value).

Attention is needed regarding PIR insulation and additional air tight membranes. The aluminium foil is completely vapour resistant, whilst a standard air tight membrane (which acts as a vapour control layer too) is permeable. Depending on the so called sd-value, they allow a certain amount of vapour to pass the membrane. The lower the sd value, the more vapour can pass. As far as I remember, we´re talking about 5-6 gramm vapour per square meter per 24 hours in case the sd value is 10m. I know some companies, selling air tight membranes with an sd value of 2.3m. That vapour can´t pass the aluminium foil of your PIR and is trapped in between your membrane and the PIR insulation with easy access to the rafters. I think I do not have to explain any further where it could end in the worst case....