Insulation in a gable end cavity?

[newbridgewolf]

We've just been calculating how much we need for the last batch of full-fill Rockwool insulation up to the roof - block and brick build (200mm cavity). The builder was suprised that the architech had not spec'd insulation for the gable ends. Its a cold roof (for the most part) and the cavity batts finish level with the top of the loft insulation (see below) encapsulated within - what I understand is called - the thermal envelop. Airtight stratergy achieved with medite boards on the first floor ceilings and wet plastering for the external walls.

 

The builder says he's never not run the cavity insulation up to the apex even for cold roof's.  I can't see anything in the regs, or why this method but it's what the builder says he has 'always done'.

 

I have a lot of respect for the builder who sometimes turns the architect's drawings or spec. on its head, usually for good reason. I'm not sure he's right on this one however.  

 

Have I missed something and if so, why would you insulate a gable end cavity for a cold roof? 

 

 

  

 

 

[ADLIan]

With insulation at horizontal ceiling level the gable cavity insulation should extend something like 300mm above the top level of the loft insulation. Should also use a cavity tray to protect the top edge of the cavity insulation and change of construction (in practice I’m sure this is rarely done!)

 

Alternatively insulate all of gable.

Edited May 20 by ADLIan
Added last line

[ETC]

6 hours ago, ADLIan said:

With insulation at horizontal ceiling level the gable cavity insulation should extend something like 300mm above the top level of the loft insulation. Should also use a cavity tray to protect the top edge of the cavity insulation and change of construction (in practice I’m sure this is rarely done!)

 

Alternatively insulate all of gable.

Huh? Wall insulation extending above top of ceiling insulation - where does that come from?

[ADLIan]

Here are 3 examples one from the now out of date Accredited Construction Details (so been good practice for some time) and one from URSA and Knauf current publication. There are many more.

[ETC]

Thanks for sharing. You learn something new every day.

[DannyT]

On 20/05/2025 at 15:54, ADLIan said:

Should also use a cavity tray to protect the top edge of the cavity insulation and change of construction (in practice I’m sure this is rarely done!)

This is how we are told by NHBC to do it at work now but I can tell you now, my gable ends won’t be sitting on any cavity tray. Why would you want a non bonded slip joint running the length of your gables in gale force Winds?