Rebuilding bay window and building regs

secla

Looking at the quote from builder it says to rebuild 300mm wall so I’m assuming he was planning to do a cavity anyway.

Doozergirl

secla said:

Looking at the quote from builder it says to rebuild 300mm wall so I’m assuming he was planning to do a cavity anyway.

You'd need a bigger cavity wall construction to meet the new building regulations (375mm?) unless he was also planning to put additional insulation on the inside. Either way, the wall is thicker unless you go for a modern method like Structural Insulated Panels, which would be a lot thinner.  

secla

Ah ok, I think the woman at building regs was saying the cavity would need to be 150mm not sure what that would make the wall thickness. Il have to get him back round to work out the best route forward 

stuart45

secla said:

Ah ok, I think the woman at building regs was saying the cavity would need to be 150mm not sure what that would make the wall thickness. Il have to get him back round to work out the best route forward 

New U values are 0.18, but there are different methods of achieving this.

ComicGeek

secla said:

Ah ok, I think the woman at building regs was saying the cavity would need to be 150mm not sure what that would make the wall thickness. Il have to get him back round to work out the best route forward 

150mm of Knauf Dritherm 32 full fill cavity insulation is one of the most popular methods to achieve the new standard. There are a couple of 90mm high performance insulation boards from Kingspan/Celotex that work in a 100mm cavity, but are a lot more expensive than Dritherm and more difficult to install. Depends on how important that circa 50mm saving is.

Or install timber studs with an external brick skin (even brick slips if you want to make it as thin as possible!) with mineral wool batts between, and insulated plasterboard internally sized to achieve that 0.18 U-value. That would be about 300mm overall. That would be my preference, as you could then look to continue the insulated plasterboard internally within the room, either now or at a later date - the junction between a cavity bay window and an internally insulated solid wall is always going to be a problem.

secla

Yup il have to speak to the builder as you say a 370mm cavity joining a 200mm solid wall could end up looking a bit strange 

FreeBear

stuart45 said:

secla said:

Ah ok, I think the woman at building regs was saying the cavity would need to be 150mm not sure what that would make the wall thickness. Il have to get him back round to work out the best route forward 

New U values are 0.18, but there are different methods of achieving this.

If the remaining solid brick wall were to be insulated at the same time, it may be possible to get away with a lower u-vale for the new wall. The approved document (often referred to as "Building Regs"), has a number of get-out clauses and is often open to interpretation.