new large timber dormer

[dave1967]

Afternoon all

I will be finishing off the dormer, can any one give me an idea if my plan will pass bc. it needs to be done with sound a major problem as on a flight path at  certain times.

2 layers of 12mm soundboard staggered,  100mm rw5 in the 120mm frame, 20mm air gap then, decent vapour barrier outside, then celotex to meet u value and  breathable membrane , ecoscape composite cladding over the top. 

Thanks for any advice in advance.

 

[Redbeard]

Hello.

 

I take it rw5 is Rockwool.

 

What's the Lambda value? 

 

I*think* you are describing a hybrid warm dormer cheek, not the roof. Is that correct? I still question the position of the VCL, which is normally on the inside of (all) the insulation. I am also unsure of the need for an air gap between the 2 layers of insulation. Why do you suggest that? If  am right that it's the cheek, with

58 minutes ago, dave1967 said:

ecoscape composite cladding over the top. 

how is the roof being insulated and covered?

 

The rough rule of thumb is that two-thirds of the R value should be outside the structure and a third within it, in a 'hybrid warm roof (/or cheek) situation. I'd suggest you get a condensation risk analysis done, which will be more accurate than a rule of thumb.

[dave1967]

4 minutes ago, Redbeard said:

Hello.

 

I take it rw5 is Rockwool.

 

What's the Lambda value? 

 

I*think* you are describing a hybrid warm dormer cheek, not the roof. Is that correct? I still question the position of the VCL, which is normally on the inside of (all) the insulation. I am also unsure of the need for an air gap between the 2 layers of insulation. Why do you suggest that? If  am right that it's the cheek, with

how is the roof being insulated and covered?

 

The rough rule of thumb is that two-thirds of the R value should be outside the structure and a third within it, in a 'hybrid warm roof (/or cheek) situation. I'd suggest you get a condensation risk analysis done, which will be more accurate than a rule of thumb.

Hi

It a large timber dormer 4 m in length with a 1.9m set of doors. It will be tiled and was planning to just use rw slabs between joists and rolls of insulation in the loft area up to required amount.

[Redbeard]

But you say: 

''It will be tiled and was planning to just use rw slabs between joists'''

 

Joists? This is a floor? Or ceiling joists?

 

You also say:

 

2 hours ago, dave1967 said:

celotex to meet u value and  breathable membrane , ecoscape composite cladding over the top. 

This implies that to meet thew target U value (0.3 if it's a wall; 0.16 - or maybe 0.15... if it's the roof we're discussing) you need to add Celotex *outside* the timberwork. Which bits are tiled? Is it a pitched roof dormer? Or is the roof flat and the cheeks (sides) tiled?

 

Could you do a sketch, photograph it and post here?

[dave1967]

Heres a picture. Its timber construction

[Redbeard]

Thanks.

 

''ecoscape composite cladding '' being for walls, AFAICS, I assume your lay-up in post 1 refers to the cheeks (walls) and the front walls beside and above the double doors.

 

I still do not understand your insulation lay-up.

 

In the absence of sufficient detail to make a judgement, 

5 hours ago, dave1967 said:

can any one give me an idea if my plan will pass bc.

 

Yes, if the walls have a U value of between 0.28W/m2K (if it counts as an extension) and 0.3 (if it counts as 'renovation of a thermal element') and the roof U value is 0.15 - 0.16W/m2K.

 

Windows/doors will need to be 1.4W/m2K.

 

I still think you need to be aware of the risks involved if you have a non-standard lay-up.

 

[dave1967]

Ok thanks so vapour inside after pboard.

[Redbeard]

On 21/09/2024 at 20:26, Redbeard said:

In the absence of sufficient detail to make a judgement, 

On 21/09/2024 at 14:36, dave1967 said:

can any one give me an idea if my plan will pass bc.

 

Yes, if the walls have a U value of between 0.28W/m2K (if it counts as an extension) and 0.3 (if it counts as 'renovation of a thermal element') and the roof U value is 0.15 - 0.16W/m2K.

 

Windows/doors will need to be 1.4W/m2K.

 

Just spotted my error (above). It should have said "Yes, if the walls have a U value of between *0.18W/m2K* (if it counts as an extension)... etc.

 

Extension walls were 0.28 till 2022 but tightened up considerably to 0.18.

 

And re VCL, yes, Plasterboard, VCL, frame and insulation in whatever lay-up you choose, but remember if you do a 'hybrid' (some insulation within the frame and some outside (or inside)) it would be wise to have an interstitial condensation risk assessment done.

 

*Can anyone else comment from recent experience as to whether their new dormer (on an old building) was judged to be an extension (hence 0.18 for walls) or renovation (in which case 0.3).*

 

[dave1967]

Thank

I will be aiming for .18.