Weatherboard rather than brick, effect on U values

[Post and beam]

My proposed house design is planned to have Cedral( £££ permitting) weathrboard on the gable ends and around the Dormers plus the down stairs dining room that sticks out the back.

My TF supplier targeted a U value of .15 by using their Kingspan Ultima 15 but this was based on an all brick facade.  I asked what was required to maintain this value with a lot more weatherboard than they anticipated. The answer is that the U value will just go out to .16

Not a lot of change you might say but going in the wrong direction.

So....

For those that have employed weatherboard what would/did you do to maintain the U value you wanted?

Perhaps extra thickness of external battens and 50mm of insulation

[JohnMo]

You need a ventilation space behind, which is mostly there to allow water to escape. If you fill the gap with insulation you will be asking for trouble.

 

Adding extra battens internal and insulation between?

[ETC]

Put the Cedral or other on top of the brick or block outer leaf. Get the same effect. No need for ventilation and U-value stays the same.

[JohnMo]

20 minutes ago, ETC said:

Put the Cedral or other on top of the brick or block outer leaf. Get the same effect. No need for ventilation and U-value stays the same.

That sounds expensive to gain 0.01 u value, additional foundation to carry the blockwork, the blocks and labour to build.

[ETC]

Why would you need another foundation? The outer leaf is already there just fix the Cedral or other to it.

[Post and beam]

11 hours ago, ETC said:

Put the Cedral or other on top of the brick or block outer leaf.

There will not be brickwork at these points it would be way too expensive, and in the case of the large box dormer on the front possibly an issue of support. This is not the answer.

[SteamyTea]

Can you grab the losses back elsewhere i.e. floor, ceiling/roof or the best one, lower ACH?

[ETC]

1 hour ago, Post and beam said:

There will not be brickwork at these points it would be way too expensive, and in the case of the large box dormer on the front possibly an issue of support. This is not the answer.

Irrespective of costs my solution is feasible (apart from the dormers which obviously don’t have foundations). The  gables will have an outer masonry leaf at ground floor level to build off and attaching cladding to an outer block leaf is achievable.

 

In any case have you looked at the impact of the reduced impact on your SAP. I doubt it’s significant.

[Iceverge]

Kingspan are playing tricks here. 

 

You have a ventilated cavity in both cases so bricks or weatherboard will make no difference to the U value in reality. 

 

They are simply there to keep the rain off the timber frame and maybe in the case of brick to add a little to the structural stability. 

 

Looking at their product which i assume is 140mm kooltherm between the studs and 40mm over the top just maybe scrapes a U value of 0.155 ish. Are you committed to their system? 

[Mr Punter]

Another vote for a thin layer of external insulation.