Cavity wall thickness

[Matt60]

When I started reading the House Builders Bible, he mentions going for a 400mm cavity wall as a compromise between passive house standard and reducing room sizes. I thought that sounded pretty reasonable and I hope to build to a semi passive house standard too. However, both my architect and builder merchant have stated that if I go more than a 350mm thickness I will have to have lintels specially made which will add to the cost of the build considerably. 

 

I have been told that up to 350mm is fine and that off the peg lintels are available for that. All of this struck me as odd given that passive houses have even wider walls and I didn't think that 400mm was particularly excessive by modern standard.

 

Does anyone have any experience on this? 

 

Thanks,

 

Matt.

[Moonshine]

Also at 400mm you will have to go up to 600mm foundations, with 350mm wall you can just about get away with 450mn foundations.

 

I am personally looking to build 350mm with 150mm fully filled cavity.

 

[joe90]

What lintels?. I built 200mm cavity with 100mm block and brick inner and outer skins, had to go 500mm strip founds for the ground conditions (said building inspector) and had concrete internal lintels and brick arches in the outer, but these could have been. Concrete or something similar!!!

[Matt60]

Right, I see, everything seems to be pointing to a 350mm wall with 150mm cavity. Now you mention it I think something was said about needing larger foundations. I guess that 350/150 is still a larger cavity than normal, but didn't expect the 400mm wall with 200mm cavity to throw up so many issues. 

 

Thanks for the reply.

[PeterW]

600mm founds are pretty standard as they are a bucket width. 
 

Lintels at 200mm cavity are better done as separates, a lot of passive houses are 250mm timber frame with a brick skin if required. The standard lintels max out at 165mm so you can get a bit more width. 
 

What insulation are you planning ..?? 

[Matt60]

3 minutes ago, joe90 said:

What lintels?. I built 200mm cavity with 100mm block and brick inner and outer skins, had to go 500mm strip founds for the ground conditions (said building inspector) and had concrete internal lintels and brick arches in the outer, but these could have been. Concrete or something similar!!!

 

Right, I see what you mean. I think they were talking about Catnic type lintels that do the inner and outer. Obviously you're saying that you've done the inner and outer separately which makes sense. Thanks for this, I'll go back to them and pose this as a point. 

[Matt60]

11 minutes ago, PeterW said:

600mm founds are pretty standard as they are a bucket width. 
 

Lintels at 200mm cavity are better done as separates, a lot of passive houses are 250mm timber frame with a brick skin if required. The standard lintels max out at 165mm so you can get a bit more width. 
 

What insulation are you planning ..?? 

Hi Peter, thanks for your reply.

 

Yep, you're saying much the same as Joe90 has just pointed out. I don't know enough about this to have considered that they can be done separately but that does make sense. What didn't make sense was that my proposed 200m cavity being seen as some massive departure from the norm. 

 

Insulation? I don't know yet to be honest. It might sound like a bit of a simplistic approach but I'm just figuring on the wider the cavity, the more insulation and the better thermal efficiency. The following is what my architect has sent through with his note about the wall thickness at the bottom which I followed up with the builders merchant who also didn't spot the potential to use separate inner and outer lintels. The short answer appears to be Dritherm 32 which I think is the bendy stuff that comes in boards - to give it its technical description...

 

New Foundations:-

• 450mm and 600mm wide C25 mix concrete trench fill type foundations under the proposed 350mm wide cavity walls.
• Foundation depths to be checked and confirmed on site by Building Control Officer.

 

New Ground Floor Construction:-

• 75mm sand / cement screed.
• Selected underfloor heating to be installed within screed.
• 500 gauge separating layer.
• 150mm Celotex XR4000 insulation.
• 1200 gauge polythene DPM.
• Selected 150mm deep PCC beam and block floor.
• 225mm min. ventilated void below.

 

New External Brickwork Cavity Walls (350mm overall thickness):-

• Selected 103mm brickwork outer leaf.
• 150mm cavity to be fully filled with Dritherm 32 or similar approved insulation.
• 100mm Durox Supabloc (3.6mm2) or similar approved blockwork inner leaf.
• 12.5mm Gyproc Wallboard on dabs with skim coat finish.

 

New External Rendered Cavity Walls (350mm overall thickness):-

• K-Rend render or similar approved render.
• 100mm Hemelite Standard (3.6mm2) or similar approved blockwork outer leaf.
• 150mm cavity to be fully filled with Dritherm 32 or similar approved insulation.
• 100mm Durox Supabloc (3.6mm2) or similar approved blockwork inner leaf.
• 12.5mm Gyproc Wallboard on dabs with skim coat finish.

 

New External Boarded Cavity Walls (350mm overall thickness):-

• Selected composite boarding to be fixed into 25mm or 38mm x 50mm tanalised timber vertical battens.
• 100mm Hemelite Standard (3.6mm2) or similar approved blockwork outer leaf.
• 150mm cavity to be fully filled with Dritherm 32 or similar approved insulation.
• 100mm Durox Supabloc (3.6mm2) or similar approved blockwork inner leaf.
• 12.5mm Gyproc Wallboard on dabs with skim coat finish.

 

New Pitched Roof (Vaulted Ceiling):-

• New zinc roof to be fixed in accordance with the manufacturer’s instructions on a selected underlay.
• 18mm WBP ply or OSB/3 to be fixed into timber rafters / raised tie roof trusses.
• 50mm min. ventilated air gap between the Celotex insulation and breather membrane.
• Insulate between rafters with 100mm Celotex XR4000 insulation or similar approved.
• Insulate under the rafters with 40mm Celotex TB4000 insulation or similar approved.
• 15mm Gyproc Duplex Wallboard (foil backed) with skim coat finish.

 

New flat roof (Warm Roof):-

• Selected single ply flat roof membrane.
• 18mm WBP ply or OSB/3.
• 120mm Celotex XR4000 insulation or similar approved.
• Vapour Control Layer (VCL).
• 18mm WBP ply or OSB/3.
• Flat roof joists (sizes to be determined by Structural Engineer)
• 15mm Gyproc Wallboard with skim coat finish.

 

New Pitched Roof (Flat Ceiling):-

• New zinc roof to be fixed in accordance with the manufacturer’s instructions on a selected underlay.
• 18mm WBP ply or OSB/3 to be fixed into timber rafters / raised tie roof trusses.
• Raised tie trusses.
• Insulate between the ceiling joists with 200mm fibreglass insulation and 200mm fibreglass insulation to be cross laid over.
• 15mm Gyproc Duplex Wallboard (foil backed) with skim coat finish.

 

New Pitched Roof (Above Garage / Vaulted Ceiling):-

• Selected slate tiles to be laid in accordance with the manufacturer’s instructions on 25 x 50mm tanalised timber battens.
• Selected Tyvek or similar approved breather membrane on attic roof trusses.
• 25mm min. ventilated air gap between the Celotex insulation and breather membrane.
• Insulate between rafters with 150mm Celotex XR4000 insulation or similar approved.
• 15mm Gyproc Duplex Wallboard (foil backed) with skim coat finish.

 

Please note that further to using 200mm wide cavities just double check with your builder or material supplier the availability of cavity walls lintels for this width of cavity as we don’t think there is many manufacturers that make a lintel that wide (400mm wide overall)?

Edited February 6, 2021 by Matt60

[tonyshouse]

My cavity is 305mm wide fully filled with R32 fibreglass batts, separate lintels to mitigate the usual thermal bridging issues  

[Matt60]

1 minute ago, tonyshouse said:

My cavity is 305mm wide fully filled with R32 fibreglass batts, separate lintels to mitigate the usual thermal bridging issues  

 

This is all making much more sense now, thanks. 

 

Is R32 the same as Dritherm 32 and I assume that "batts" is battons which is still but bendable rectangle pieces? 

[PeterW]

Consider blown beads at those thicknesses as they will be as efficient but may be easier to install. 

[Matt60]

21 minutes ago, PeterW said:

Consider blown beads at those thicknesses as they will be as efficient but may be easier to install. 

 

Thanks, do you know what the cost difference is?

[joe90]

I used dritherm 32, it is certified fir full fill and it can be installed DIY, I followed the bricklayers round installing it, just need to keep heavy rain out.

[Matt60]

4 minutes ago, joe90 said:

I used dritherm 32, it is certified fir full fill and it can be installed DIY, I followed the bricklayers round installing it, just need to keep heavy rain out.

 

Thanks, that's good to know about the full fill and its what my architect is recommended.

[wozza]

I also used Dritherm 32 in my extension - easy to use.

 

I would have also used thermally broken lintels but I only recently found out that they make them.

[joe90]

I did a very rough “thermal bridge” calculation on cavity ties and found it to be minimal.

[Matt60]

Thanks for all the replies to this thread, I really appreciate it, it's all much clearer now.

 

I will go back to the architect and builders merchant and suggest separate concrete lintels to deal with the 200mm cavity in Dritherm 32. 

 

Thanks again. ?