Insulating internal solid brick walls of shed

[jimbob776]

Hello,
I'm converting some old stables/sheds into holiday cottages and have a wall insulation question..

The solid brick walls are very uneven being lime mortar and in quite poor condition being soft clay bricks. I considered fitting batterns to the walls and gain a plumb to work from but like I said the walls are quite poor in a number of places so opted instead to pour a new 4" thick concrete base (slightly thicker Upto 6" around the edge for the studwork to sit on) with DPM then build a 3"X2" stud wall onto the new concrete base using gypliner brackets just above the wall tanking membrane to provide greater support for the stud frame. In some places along the bottom the cavity is only an 2" but higher up where the wall meets the vaulted ceiling the gap is up to 4-6".. I now have a very solid studwork that I know will last well past my lifetime..

I planned to PIR 100mm rigid insulation between each 75mm stud thus 25mm would project out the rear but therefore out of sight. I would initially leave say 400mm PIR insulation from the top of each stud. Then I want to pour vermiculite insulation down the back letting gravity do the work filling all the remaining cavity void between the PIR and uneven brickwork. Finally, then putting the remaining insulation PIR into the top and plasterboard to finish.

Do you think this would be a good idea?? Would there be any risk of dew forming on the brickwork when I am completed. I guess not as I'm eliminating any void by using the vermiculite but just wanted to put it out there and gain some advice..


Thanks

 

[DIY_Doctor]

I'm no expert, but I would have thought you should use 75mm PIR within the studs, use the money saved to buy a further 25mm to place across the front of the studs to mitigate cold bridging and cover the whole thing with a vapour control layer to minimise the chance of moisture getting to the cold brick wall. If your brickwork is solid brick you would want ventilation of the cold void between it and the studwork, perhaps by leaving a gap at the top to aid airflow or if you have a ventilated space above your ceiling insulation, allow it to ventilate via that. Alternatively I cant see why fixing an external airbrick to essentially do the same thing wouldn't also work (think of the void as being like a cavity in a cavity wall).

If your brickwork was sound and your VCL airtight, you probably wouldn't need to worry about ventilating the void, but if your brickwork and pointing are in poor nick, you'll be getting water ingress or damp which would be impeded in drying out if the internal void was not ventilated.

 

[George_engineer]

Yeah I think that's a better idea. Essentially you are recreating how a masonry clad timber frame structure is.

Ideally you would have a breather membrane on the outside of the timber - if it is possible to get that in then do so.

Build up:
Outside - 238mm brick - min. 50mm ventilated cavity - breather membrane - 75mm timber + insulation - 25mm thermal bridging insulation - vapour control layer - 25mm service void formed with battens (for electrics and plumbing so the VCL is intact) - plasterboard

 

[jimbob776]

Sounds great. I'm going to start by fitting some air bricks around 1.5m-2m apart quite high up and once there in I'll set on with the recommendation.

I had alternatively thought about using Rockwool or hemp so as to try and eliminate altogether the cavity element but on reflection a vented cavity seems the best option.

Many thanks for your advice

 

[George_engineer]

Do check the planned u-values with Building Control - I was just use the thicknesses mentioned above. 100mm PIR should be enough to get down to 0.18 W/(m^2 K) but the presence of the timber can push it over the edge. Depends on the thickness and spacing. That said, all you could realistically do is increase the 25mm layer.