Materials for roof insulation

[Garald]

Background: doing a major renovation. The architect thought we'd just leave the roof insulation alone (the roof is pretty much the only part of the house that was already insulated...) but apparently the fact that we are poking eight new skylights into it forces the question upon us, in that the first step of the process was to remove the existing insulation (rock wool).

 

The foreman is thinking polyurethane (I don't know yet his opinion on PUR vs PIR). That's a possibility - we want as much insulation as possible in the existing space. (The ceiling is very low as it is; we can't really make it thicker without making our backs curve permanently.)

 

What are other possibilities? Pros and cons?

 

 

[joe90]

You will need to maintain a 50mm air gap behind those tiles so some pir between rafters then whole sheets over it then plasterboard (with long screws). 
 

Are you going to strip the tiles and felt the roof whilst your at it,? May give you the opportunity to raise the roof minimally to give you more insulation (and/or headroom) but so the planners don’t notice the difference.

[Garald]

14 minutes ago, joe90 said:

Are you going to strip the tiles and felt the roof whilst your at it,? May give you the opportunity to raise the roof minimally to give you more insulation (and/or headroom) but so the planners don’t notice the difference.

 

Not sure we'll go that far. (Why would we do that - in case it hasn't been felted already?) Raising the roof is a major, expensive operation - we may raise the roof by a lot in about 5-10 years, once I am again in possession of something known as "money". 

[Garald]

(I guess there isn't any felt because we don't see any?)

[joe90]

7 minutes ago, Garald said:

Why would we do that

I would not do all that internal work without felt.

8 minutes ago, Garald said:

Raising the roof is a major, expensive operation

Not if you just put some batten or insulation over the roof joists before renewing the battens over felt! Raises the roof a couple of inches and I doubt the planners would even notice, but as you say budget dependant eh 🤔

[joe90]

7 minutes ago, Garald said:

(I guess there isn't any felt because we don't see any?)

Yes.

[Radian]

How's it ventilated at the eaves? Judging by the grime on the rockwool there is some airflow but without a proper risk analysis for condensation, the choice of insulation can't just be left to chance.

[Garald]

2 minutes ago, Radian said:

How's it ventilated at the eaves? Judging by the grime on the rockwool there is some airflow but without a proper risk analysis for condensation, the choice of insulation can't just be left to chance.

 

Excellent question - will ask the architect (who is installing a proper positive-presdure system everywhere, thinks about condensation, etc.).

[Garald]

58 minutes ago, joe90 said:

 

Not if you just put some batten or insulation over the roof joists before renewing the battens over felt! Raises the roof a couple of inches and I doubt the planners would even notice, but as you say budget dependant eh 🤔

 

So it wouldn't raise the *ceiling* - it would just raise the roof slightly so as to fit more insulation between the tiles and the ceiling, right?

[Dave Jones]

rood insulation cant be bodged as you do not want the dreaded black mould. 

 

Tiles need to come off, counter battened to give you the correct air flow then breathable membrane and put tiles back.

 

Or just stuff the rockwool back in and plaster over .....

 

[ETC]

15 hours ago, Garald said:

 

Excellent question - will ask the architect (who is installing a proper positive-presdure system everywhere, thinks about condensation, etc.).

This won’t help with roof ventilation.

[ETC]

You should strip the tiles, fit a breathable membrane above the rafters, place PIR (100mm minimum) between the rafters leaving a 50mm ventilation space above the the top of the insulation and fit 63mm insulated plasterboard across the face of the rafters.

 

You will also need continuous eaves and ridge ventilation - 25mm at the eaves and 5mm at the ridge.

Edited September 24, 2022 by ETC