Airtightness tips for dormer loft conversion

[ectoplasmosis]

We're having a dormer loft conversion built right now, and are almost at the point of fitting PIR between joists/studs.

 

I'd like to wrap the lot in airtightness membrane, but not sure which type to get, or best-practices for applying it. Are there guides for doing this, especially for around window/rooflight reveals? 

 

Which membrane performs well without breaking the bank?

[Redbeard]

Are you looking at A/T membrane inboard of the PIR? Do you have a wind-tightness membrane externally? 

 

For internal A/T membrane I would (and did) use Intello. Not cheap but seems to do the business.

[LiamJones]

If your doing PIR between and under joist, you can tape the joints in PIR under the joist for air tightness, assuming foil coated.

[Iceverge]

1 hour ago, ectoplasmosis said:

fitting PIR between joists/studs

 

Groan......

 

Just use some mineral wool and a layer of PIR under the rafters. 

[ectoplasmosis]

29 minutes ago, Iceverge said:

 

Groan......

 

Just use some mineral wool and a layer of PIR under the rafters. 

 

The rafters are ~100mm deep, with a wind-tightness membrane above, then slates. Would it be possible to full-fill between rafters with Dritherm 32, or would an airgap be necessary? 

 

Edited December 18, 2023 by ectoplasmosis

[Mr Punter]

You would need long screws for the plasterboard.  Not sure why @Iceverge is against PIR between rafters and OK under.  I would just PIR as the rafters are quite skinny.

[Iceverge]

1 hour ago, Mr Punter said:

You would need long screws for the plasterboard.  Not sure why @Iceverge is against PIR between rafters and OK under.  I would just PIR as the rafters are quite skinny.

 

Too slow to get a good fit. Lots of cutting and waste. Differential shrinkage of timbers and PIR. Poor for noise. 

[Iceverge]

2 hours ago, ectoplasmosis said:

 

The rafters are ~100mm deep, with a wind-tightness membrane above, then slates. Would it be possible to full-fill between rafters with Dritherm 32, or would an airgap be necessary? 

 

 

Depends on the roofing felt/membrane re full fill. 

 

How tight are you on space? 100mm PIR is probably as cheap as 80mm. 

[Gus Potter]

2 hours ago, ectoplasmosis said:

The rafters are ~100mm deep, with a wind-tightness membrane above, then slates. Would it be possible to full-fill between rafters with Dritherm 32, or would an airgap be necessary? 

Examine the detail of this closely.  It does work and often design this way myself in principle but use PIR between the rafters and an insulated plasterboard under with a vapour control layer.

 

 

Under the tiles is usually a breathable (wind tightness) membrane, say Tyvek, part of it's function is also to resist wind driven snow. You seem to have that.

 

But note.. the detail shows timber on top of the rafters.. NOT OSB3 for example as it is not vapour permeable enough. Usually I do something like this with a 100mm timber sarking board with a 5.0mm gap between each board. The timber sarking provides stiffness to many of the roofs I do so as it has a structural application anyway.

 

Ideally we want to leave a 15 - 20mm gap between the top of the insulation and the underside of the sarking. This is an unvented space, not like the 50mm vented space you have traditionally.

 

You could partially fill with Dritherm 32 in principle so long as you leave the 15 - 20mm space above it so long as that space is breathable. Don't know how warm it will be though so best check.  I would also check the location of the air tightness membrane as you could get condensation on the underside of that... bad news. The bottom of the joists will be cold (they are a repeating bridge) so that is where you'll get bands of condensation.

 

I would put the vapour control layer just above the plasterboard.

 

 

 

Edited December 18, 2023 by Gus Potter