No dot and dab for retrofit insulated plasterboard?

[SJRSJR]

Hello

Was planning to install insulated plasterboard on a couple of external walls, and I had thought dot and dab and some mechanical fixings as back up was the way to go. However I've recently read that unless it's a new build (which it isn't) dot and dab should be avoided as it creates the potential for interstitial condensation in the airgaps.

Sure enough, this Retrofit Internal Wall Insulation Guide (p.42) says best practice is a continuous layer of adhesive. That's a bit of a problem because I was hoping dot and dab would help level up some of the wall's unevenness.

So is it just 'best practice' and the risk is minimal, or should I be avoiding dot and dab altogether? Thanks.

 

[Wayners]

Watch Charliediy on YouTube for options.
He covers this.

Last one I done I used insulated plasterboard stuck on with foam and hammer through mechanical fixings.

Depends on walls though. There are other options

 

[RandomGrinch]

Alternatively, you could keep the insulation and plasterboard separate?
The two bought separately are usually cheaper, the insulation joints can be foil taped for a continuous vapour barrier, and the plasterboard joints can be staggered away from the insulation joints.

 

[mrrusty]

creates the potential for interstitial condensation in the airgaps.

IMHO nowt to do with airgaps. Insulated PB has a VCL so most room air should be isolated from the much colder brick behind. However, my own theory, used in my own house with D&D insulated PB, is that you have to presume there will be some interstitial condensation in the now much colder brick behind the insulation.

The question is "what happens next". In my own case we (BCO - when I wanted another opinion- and me) decided that a ventilated cavity would be enough to mitigate the risk as any dampness in the inner skin brick would be evaporated away by the ventilation. However, if you have a solid wall, perhaps covered externally with impervious render, I would be much more cautious.

My theory and opinion. Others may well vary!! 5 years in with my D&D'd insulated PB external walls and so far so good.

 

[SJRSJR]

Thanks for the replies. I've watched Charlie's videos, he's obviously relaxed about dot and dab even though he's got a solid wall.
Mine is a cavity brick wall which is externally rendered. It has airbricks and judging by the draught coming through the exposed window reveals, it's well ventilated. So all in all that seems to be heading in the right direction.

Opinons are split on whether to buy the ready made stuff or do it separately but as I'll be paying someone to help, I figured it'll be swings and roundabouts.

Moving on, as I understand it, if 50% of the surface area of one elevation is being renovated, Building Regs will apply. I don't think it does here but I may as well aim for it if I can, particularly as it's an ongoing job and I may do other rooms along the way, which may eventually add up to 25% of the entire envelope where BR also apply.

So, I believe the target u value for a renovation is 0.3 or less. Despite Charlie's vid saying Kingspan has online tools to help, I couldn't find a u-value calculator for a simple unfilled brick cavity wall. Does anyone have a simple homeowner link, the ones I've seen go way over my head?

As I understand I need to add up the R values of the wall's components and divide the total into 1 to give you u-value. So to achieve a u-value of 0.3 or less the sum total of the R values should be 3.33 or more.

Anyone know the R value of two 1920s bricks, a cavity and a layer of render?! And do you have to factor in the windows? Thanks

 

[mrrusty]

Building Regs will apply

I had a BCO involved with the house for other reasons, but I discussed my insulation with him. I hadn't made a specific regs application for it, and he really wasn't interested in actual U values - his attitude "anything you can do will be better than it was". Personally, I would just get on with it, because if you get a bad BCO it could be a can of worms - depends on your attitude...

You can only do what's economically practical. We used 72.5mm insulated PB wherever we could, but on the stair wall we could only accommodate 25mm without moving the stair stringer.

Whatever you do, if you have a ventilated cavity don't be talked in to cavity wall insulation!! Do remember to take lots of photos as you'll need them to prove the insulation if you ever need an EPC.

 

[SJRSJR]

Thanks mrrusty, I'm definitely steering clear of cavity wall insulation!

Kingspan Technical Desk said 57.5mm would achieve the 0.3 target. Will double check it before I place any order.

Thinking ahead - when I come to do the bathroom does it make a sense to have standalone PIR board+12.5mm wedi board in the wet areas and PIR board+12.5mm plasterboard in the dry areas? It's got 2 external walls and one of them never sees the sun.

 

[mrrusty]

standalone PIR board+12.5mm wedi board in the wet areas

That's what I did. Battened with 2x3's on edge, filled with PIR and boarded with cement board tilebacker

 

[SJRSJR]

Thanks. Any reason why you battened and didn't dot and dab?

 

[mrrusty]

I D&D'd or foamed all the insulated PB, but in the bathroom I needed tile backer, and also to run pipes, leccy etc behind, so battening out, filling with PIR and overboarding was better on that wall.