Retro fit roof insulation - breathable vs non breathable

[ab12]

If you were doing a retro fit roof insulation to fit between rafters on a pitched roof and between joist below non usable  loft space which insulation type would you use

 

a) PIR  or 

b) Breathable 

 

It appears there is inherent risk of using PIR due to inevitable risk of interstitial condensation and future issues down the line as there it is probably impossible to fit PIR between the rafters so that not even the smallest gap is left between the insulation and rafters. Also its going to be tricky to get 100% proof the VCL above the insulation again leading to risk of vapour betting trapped between insulation and timbers.

 

With breathable insulation the moisture at least can be controlled in the sense that insulation will absorb the moisture and release it as conditions humidity conditions change so there is much less risk of causing future issues to timbers.  But the trade off is U-value similar to PIR are difficult to achieve.

 

By the way size of rafters tbc but may be 8 by 2 or 9 by 2.

 

Please share your experiences and thoughts on this topic.

 

[Roger440]

1 hour ago, ab12 said:

If you were doing a retro fit roof insulation to fit between rafters on a pitched roof and between joist below non usable  loft space which insulation type would you use

 

a) PIR  or 

b) Breathable 

 

It appears there is inherent risk of using PIR due to inevitable risk of interstitial condensation and future issues down the line as there it is probably impossible to fit PIR between the rafters so that not even the smallest gap is left between the insulation and rafters. Also its going to be tricky to get 100% proof the VCL above the insulation again leading to risk of vapour betting trapped between insulation and timbers.

 

With breathable insulation the moisture at least can be controlled in the sense that insulation will absorb the moisture and release it as conditions humidity conditions change so there is much less risk of causing future issues to timbers.  But the trade off is U-value similar to PIR are difficult to achieve.

 

By the way size of rafters tbc but may be 8 by 2 or 9 by 2.

 

Please share your experiences and thoughts on this topic.

 

 

Agree that some form of breathable insulation is better. Depends how much space you have.

 

I fitted 50mm PIR to the slopey bit, but i only have 3 inch rafters. If they were 6 inch i may have done different. I then installed rockwool in between/over the joists.

 

So only a small part is PIR. These were sealed up around the outside with the illbruck 330. I did quite a good job, but not as good as id have liked to do, but with the roof off, time is limited.

 

My last house had the same, but the PIR was "loose" in most cases. I lifted some up and it was completely dry underneath. There was just plasterboard underneath to the room. No VCL/membrane etc. So yes, its a "risk", but not sure its a big one.

 

Nail a VCL to the ceiling inside if you are worried.

 

 

 

[Redbeard]

I would argue the risks of condensation are low if the void above is properly cross-ventilated. Strictly in a roof it's not interstitial, as there are spaces, not interstices (IC would be an unwanted 'filling' in a 'sandwich' of, for example, solid wall and PIR). 50mm minimum vent path, and ensure that the eaves, soffit or fascia vents are of sufficient cross-sectional area (lots of guidance on the web).

 

How many old loft spaces with retrofit insulation and retained ventilation do you see with condensation issues? In my experience it tends to occur when cross-ventilation is compromised. 

 

Am I right in thinking that your insulation between rafters is above a habitable room? If not, why go to the significant extra cost compared with laying 300-500mm of cheap stuff on the ceilings of the rooms below?

 

Note that 225 (9") rafters, after the 50mm vent path, will only allow you 175 (7") insulation, which with a fully breathable product such as flexi wood fibre will not give you the B. Regs target value of 0.16W/m2K. I would consider DIY 'Larsen (roof) Trusses', with the rafter as one 'chord', spacers of OSB or ply, and a secondary (?50 x 50) lower chord. Make it as fat as you need, then.