Cheapest most efficient way of insulating external walls

[Reetster]

Evening everyone.

I am building a substantial framed barn conversion. Walls, built on a reinforced slab,  are 150mm , insulated with a skin of 12mm ply, then felted.

There is then about a 150mm cavity before a further 4x2 frame which will support the timber cladding.

Im hoping to add another layer of insulation to this cavity, as it seems the right thing to do. What do we think will be the cheapest most efficient way of doing this? I was thinking of 100mm sheets screwed  into the back of the plywood, whilst maintaining a 50mm air gap.

Do I just plumb for a celotex type product, wood fibre or even a polystyrene board?

 

Many thanks!

[Hobbiniho]

im presuming that your 150mm frame is timber? is this going to be filled with insulation?, it seems quite unusual to have an air gap and then a completely seperate 4x2 frame just to hold the cladding up, you will have to be careful and detail this properly as you might run into interstetial condensation problems, there are quite a few people who have used woodfibre but this is usually fixed directly to the structural frame then either rendered or has battens and cladding creating a drained and ventilated cavity, do you have an closer up photos of the cross section of what you have?

Edited December 14, 2020 by Hobbiniho
ask for photos

[Mr Punter]

The reason for a ventilated cavity is to prevent water crossing into the structure.  The cavity ideally needs to be next to the rainscreen - in your case the cladding.

 

Given where you are with this, I would fill or part fill the 150mm cavity with Rockwool slab, fixed to the ply and cover with a fire resistant membrane.  You may need a starter profile at the base.

[Temp]

Efficiency has many meanings. 

 

100mm Rockwool Cavity Insulation Batt 

Thermal Conductivity: 0.037W/mK

R-Value: 2.7m2K/W

 

100mm Celotex GA4000 PIR Insulation Board

Thermal Conductivity: 0.022W/mK

R-Value: 4.5m2K/W

 

So I would expect..

 

100mm Celotex to be equivalent to about 168mm Rockwool.

or

60mm Celotex would do similar job to 100mm Rockwool.

 

Your mileage may vary.  Celotex is slightly harder to install without gaps as Rockwool is compliant but I think that's mainly an issue when fitted between studs rather than over ply.

 

 

 

 

[Reetster]

@Hobbiniho

yes, the 150mm timber frame will be insulated and will achieve building regs for insulation. Just seems a good opportunity to add another layer if  not too expensive etc

Plan view sketch attached

Thanks for your thoughts..

 

Wall Plan view.docx

[Reetster]

@Mr Punter

Thanks. Yes, Rockwool slab seems to make the most sense, probably best value too. How would you fasten it to the wall? Will it stay in place, total wall height will be approx 4.5m

Cheers

[Hobbiniho]

@Reetster that plan makes a lot more sense now, as has been suggested rockwool would work ok as would PIR probably the deciding factor would be what type of insulation you are using in your frame? im presuming PIR as you say it meets building regs, the different types of insulation have an effect on decrement delay but you also need to think about the positioning of them as to respect of the vapour permeability of them and also where your vapour control layer is going (usually as close to the inside as possible)