How much insulation is enough?

[ey143]

For cavity wall insulation on a new extension, what is usually the thickness of insulation required as a minimum and what type is best used v commonly used?

 

[theprinceofdarkness]

As part of the building regs, you will have to bring the whole building up to some overall figure of insulation (including windows etc.). So you will have a choice of where you put your insulation or double glazed windows. In general , to stop having to re-engineer the whole house, the insulation in a new extension is made thicker then it is really justified. Or you could have the cavities insulated in the main house and use less insulation in the new bit.
Or if you are an Eco Warrior, put in 200mm in the new cavities and roof!
Frank

 

[^woody^]

You tell us what you are building the wall out of, and the construction and then we can tell you what insulation you could use.

 

[xr4x4]

100mm cavity slab is normal.

 

[ey143]

LBC bricks on the outer skin and grey blocks on the inner skin with cavity in between. Would 50mm on the side cavity walls be sufficient and 120mm warm deck roof be ok?

All insulation will be Celotex type insulation boards including the 50mm insulation in the side walls.

Builder said there needs to be a 50mm air gap.

Thx.

 

[tony1851]

Full-fill cavity batts are easier and less prone to trouble than rigid board insulation.

 

[ey143]

Full-fill cavity batts are easier and less prone to trouble than rigid board insulation.

Pls expand on what you mean by "trouble".

Thx.

 

[tony1851]

They have to be carefully cut to fit, and if they are not properly fixed, the board can fall slightly out of position. They have been known to fall
against the outer skin, compromising thermal performance, and risking water penetration.
Corners and reveals can be a problem with careless bricklayers.

 

[ey143]

Noted, thanks. But when they go through the brick ties and have those round retainers on, how do they fall off?

The 50mm Celotex is sufficient and will probably outperform the 100mm Rock wool stuff right?

 

[tony1851]

Done properly, rigid board is OK - just that it takes a bit more care to do right, that's all.

 

[noseall]

Rigid board cavity is a pants idea and even more pants to get Bob on. Who in their right mind would build the block-work fist.