Attic Insulation Techniques - Partial Roof Replaced

[alic]

Hello,

I am in the process of renovating an 1890’s Yorkshire terrace house (originally a back-to-back terrace however now opened into a through terrace.

Currently, one roof pitch has been reroofed, with a breathable roofing membrane, new battens, and slate. To one side, the roof still has the original impervious roofing felt style construction. Typical of this period, roofing rafters are relatively small at 2.5”x3”. I am looking to convert the space to a habitable loft room.

Since I effectively have 2 roof constructions, I am unsure on the most appropriate insulation approach. At the moment, I am unsure if/when the second half of the reroof will be reroofed. Therefore, one side will retain the non-breathable roofing felt.

For the new roof pitch (with breathable membrane installed). I am considering a warm roof approach.

• 50mm airgap to membrane
• 50mm PIR between rafters
• 37.5mm insulated plasterboard (25mm PIR with 12.5mm plasterboard)
• Airtightness tape used to ensure airtight construction.

For this side, since the breathable membrane is installed, I consider it less critical to use breathable insultation techniques.

For the original construction, would anyone be able to advise is this would still be a suitable approach. I am concerned the lack of breathability of insulation materials if the existing roofing felt is impervious. Ideally I would avoid differing insulation techniques on either side. Or would it be better to go down a breathable hemp batts or similar route on this side?

Thanks in advance.

 

[alexander]

Have you taken into account that the floor joists will need to be upgraded to be able to support the weight for a usable room

 

[Lacan07]

Hello,

I am in the process of renovating an 1890’s Yorkshire terrace house (originally a back-to-back terrace however now opened into a through terrace.

Currently, one roof pitch has been reroofed, with a breathable roofing membrane, new battens, and slate. To one side, the roof still has the original impervious roofing felt style construction. Typical of this period, roofing rafters are relatively small at 2.5”x3”. I am looking to convert the space to a habitable loft room.

Since I effectively have 2 roof constructions, I am unsure on the most appropriate insulation approach. At the moment, I am unsure if/when the second half of the reroof will be reroofed. Therefore, one side will retain the non-breathable roofing felt.

For the new roof pitch (with breathable membrane installed). I am considering a warm roof approach.

• 50mm airgap to membrane
• 50mm PIR between rafters
• 37.5mm insulated plasterboard (25mm PIR with 12.5mm plasterboard)
• Airtightness tape used to ensure airtight construction.

For this side, since the breathable membrane is installed, I consider it less critical to use breathable insultation techniques.

For the original construction, would anyone be able to advise is this would still be a suitable approach. I am concerned the lack of breathability of insulation materials if the existing roofing felt is impervious. Ideally I would avoid differing insulation techniques on either side. Or would it be better to go down a breathable hemp batts or similar route on this side?

Thanks in advance.

What your suggesting is the standard approach for a warm roof insulation regardless of which particular felt is employed, so I would do exactly the same for both sides.

If memory serves I think the standard air gap is 50mm for bitumen and 20mm for breathable felts. (don't quote me).

Just remember with a warm roof insulation the aim of the game is to prevent the warm moist air from inside the room escaping through the insulation to the cold side where it could condense so make sure all gaps, bridges, fixings and blemishes are well taped for airtightness. In this regard I'm not so sure there's such a thing as a 'warm roof insulation' using breathable materials.

 

[alic]

Have you taken into account that the floor joists will need to be upgraded to be able to support the weight for a usable room

I have, the intention is to sister the existing ceiling joists with a deeper joist alongside suitable for the span. A new timber will be bolted to the existing with a new flooring to provide more of a floor diaphragm effect.

 

[DaveG]

Don’t mean to threadjack here but is there any reason you wouldn’t go for more than 50mm PIR?
I’m fairly sure that’s what my attic has and I’m not impressed with its thermal efficiency. Granted some of this could be down to how well it was done but I find in the depths of winter it can be as much as 7 degrees colder than the thermostat/other parts of the house. I could tackle this with a separate heating zone I guess but clearly there’s a high level of heat loss.
Id say summer is worse though. The heat can become unbearable on a summers day. My velux windows are around 20 years old and the blinds often won’t stay shut in the intense heat so that certainly doesn’t help but just wondered besides newer windows whether there’s anything to help with the temperature extremes like additional insulation? I’m looking into sarking boards as part of roof replacement this year which I’m hoping will have a noticeable impact too

 

[Lacan07]

Don’t mean to threadjack here but is there any reason you wouldn’t go for more than 50mm PIR?
I’m fairly sure that’s what my attic has and I’m not impressed with its thermal efficiency. Granted some of this could be down to how well it was done but I find in the depths of winter it can be as much as 7 degrees colder than the thermostat/other parts of the house. I could tackle this with a separate heating zone I guess but clearly there’s a high level of heat loss.
Id say summer is worse though. The heat can become unbearable on a summers day. My velux windows are around 20 years old and the blinds often won’t stay shut in the intense heat so that certainly doesn’t help but just wondered besides newer windows whether there’s anything to help with the temperature extremes like additional insulation? I’m looking into sarking boards as part of roof replacement this year which I’m hoping will have a noticeable impact too

Usually you'd insulate between the eaves to whatever depth you can manage taking into account the air gap required with at least 100mm on top. That would get you roughly into the U value required by BC.