Alterative option to 150mm celotex + 50mm celotex?

[Iceverge]

Give me a cross section please of the house. 

 

I think I have an economical solution in mind. 

 

 

[Iceverge]

 

 

Raise the tie for the rafters, add 125mm PIR on the underside of the sloped sections, 

 

Full fill the rafters with mineral wool/cellulose and pur 400mm above the flat section. 

 

OSB above the rafters and batten and counter batten above for ventilation. 

 

Add a 100mm service cavity below the rafter tie for MVHR. 

 

Forget about any storage in the roof. It's simply much too small. 

 

Even with your original design it's only 1000mm high at the apex. 

 

 

 

[Mr Blobby]

On 02/09/2025 at 06:49, flanagaj said:

Point taken regarding the solar, but sadly, we have either a north facing roof or a south facing roof, so I think the decision is made on that one.  I am sure it will grow on us using the clamps.

 

Do you have a Garage?

[flanagaj]

50 minutes ago, Mr Blobby said:

 

Do you have a Garage?

Yes?

Edited September 3, 2025 by flanagaj

[flanagaj]

I was thinking about this again, and am simply going to ask the architect to design a cold roof.  I think I can fit the MVHR in the plant room and as long as the ducting is underneath the 400mm blanket of insulation I think it will be ok.  Unless I am mistaken, this approach will also mitigate the associated air tightness complexities that I would have had with PIR? 

Edited September 4, 2025 by flanagaj

[Iceverge]

Yes so long as you make sure to very diligently cover all the ducting with mineral wool and tape all penetrations of the airtightness layer