Air gap in cavity wall

[B20P]

Hi, I have just taken down my conservtory which was built on top of a cavity wall which has an 80mm cavity. Currently the cavity has a 25mm PIR board, but due to the new building regs I was thinking of upgrading this to 50mm PIR to get closer to the U-Values.

Would a 30mm air gap be sufficient?

I will also install insulated plasteroard intenally to reach the U-values but thought that upgrading the insulation in the cavity, I'd lose less of the room.

Thanks in advance

 

[noseall]

Hi, I have just taken down my conservtory which was built on top of a cavity wall which has an 80mm cavity

What are you building back, in place of the connie?

 

[B20P]

What are you building back, in place of the connie?

Just a regular room so it'll be warmer in winter and cooler in summer. Block walls and then a slate roof.

 

[noseall]

Just a regular room so it'll be warmer in winter and cooler in summer. Block walls and then a slate roof.

Habitable? I.e. no separation from the main dwelling and no independent heating?

 

[B20P]

Habitable? I.e. no separation from the main dwelling and no independent heating?

Yes habitable, at the back of our house. We had a heater in there so will use that. Why do you ask?

 

[Alastairreid]

Yes habitable, at the back of our house. We had a heater in there so will use that. Why do you ask?

Because you will need to comply with current building regs.

 

[B20P]

Yes I know, that is why I'm asking with the air space in the cavity. Will try to get hold of the council's building regs department today. Just a thought I got last night, thought someone on the forum would have an idea from past experience.

Thanks

 

[dormermike]

Are you considering the roof replacement system and the Hup! blocks for the walls out of interest? Sounds like not if you are putting a cavity in.

I'm in a similar quandry, conservatory, 30 years old, need something doing with it. The roof replacement guys all say their systems meet current BC reqs, but i'm not convinced.

 

[martygturner]

I would check the foundation depth first, foundations for conservatories were basically unregulated as its a light weight structure. As for gap, the standard is, 100mm gap with say 90mm of insulation leaving a 10mm air gap. Remember that the u valve can be achieved by various methods, you could leave the gap free use internal and external insulation to get to the average U vale for your structure. or combine with say 50mm in the cavity and 25mm on the inner wall, ie insulated plasterboard.

If your foundations are not tradtional and reach building regs there are alternatives, Structural insulated panels for starters

 

[B20P]

Are you considering the roof replacement system and the Hup! blocks for the walls out of interest? Sounds like not if you are putting a cavity in.

I'm in a similar quandry, conservatory, 30 years old, need something doing with it. The roof replacement guys all say their systems meet current BC reqs, but i'm not convinced.

Did consider roof replacement but got a mate that's a builder who is gonna help and he estimated a similar cost.

Yes blocks for the wall, have a inner and outer leaf of hollow blocks with 25mm PIR up to about 3ft atm. So hoping to continue the cavity up. Hoping to upgrade the insulation in the 80mm cavity from 25mm to 50mm.

 

[B20P]

I would check the foundation depth first, foundations for conservatories were basically unregulated as its a light weight structure. As for gap, the standard is, 100mm gap with say 90mm of insulation leaving a 10mm air gap. Remember that the u valve can be achieved by various methods, you could leave the gap free use internal and external insulation to get to the average U vale for your structure. or combine with say 50mm in the cavity and 25mm on the inner wall, ie insulated plasterboard.

If your foundations are not tradtional and reach building regs there are alternatives, Structural insulated panels for starters

Thanks for getting back to me. Foundations are fine as the builder made them to accommodate two floors (ground and first floor) just in case, this has been confirmed by building regs.

Okay, I have an 80mm cavity, would I be able to put 50mm PIR in there and leavr 30mm air gap. Would building regs be happy. I'd then make up the rest to comply with the U values via either PIR internally or insulated plasterboard.

Thanks in advance

 

[martygturner]

I would have a look at building regs for isulation required....it can be found here : https://www.cncbuildingcontrol.gov.uk/faqs/much-insulation-need-extension/

You may need to up that as your floor may not have the required insulation in the slab as it was non habitable... again its best to ask your building regs office, they have faced this question many times and are not some sort of stais secret police, they will help you get something that does the job.

 

[dormermike]

B20P how deep are your foundations out of interest? Mine are 800mm, and the floor slab is insulated, and cavity dwarf walls (breeze internal, brick outer tied into main building), seems to have been built with a possible extension in mind, rather than just a conservatory.

 

[noseall]

Thanks for getting back to me. Foundations are fine as the builder made them to accommodate two floors (ground and first floor) just in case, this has been confirmed by building regs.

Okay, I have an 80mm cavity, would I be able to put 50mm PIR in there and leavr 30mm air gap. Would building regs be happy. I'd then make up the rest to comply with the U values via either PIR internally or insulated plasterboard.

Thanks in advance

You need to satisfy latest (June 2022) Part L reg's.
You may be ok with 50mm partial fill, but this would mean a similar (50mm) PIR thickness fitted to the face of the internal wall.
Check either your designer or BCO, whether the air gap is sufficient.
With partial fill you have to build the blockwork first.

 

[^woody^]

The amount of air cavity required depends on the type of insulation board and the manufacturer's guidance