Internally insulating solid walls for better U Values

[padstar]

Hi, I am refurbishing my 1900's detached house and am giving a lot of thought to insulating the external solid walls internally using an insulated plasterboard with built in vapor barrier.

The existing rooms are a decent size so losing a bit of the floor area is not a problem but i do want to keep this as small as possible whilst achieving the desired effect. The walls are cold to the touch at the moment and this cant help with the heating bills or the enjoyment of the house long term so a worthwhile upgrade in my opinion.

I have a few questions if i may for people that know more about this topic.

1) Building Regs suggest 0.30 as the desired U value to achieve. Is this a must or just to get as close to it as possible? I understand that a solid wall generally achieves around 2.70.

2) Do i need to have the Building Control Officer involved. The walls are sound internally so i do not intend to take the plaster back to block work. My understanding is that if i leave the external walls covered as existing i do not need BC approval to undertake the works.

3) I do not want to return down the internal walls 300-600mm as this will form a step or reduce the floor areas further. Will this cause me issues?

4) I plan to board down to joist level then install 100mm EPS within the floor void?

5) What is the best product to use. I am currently looking at Kingspan Kooltherm and Celotex PL4050 as options that can be bonded to the existing walls.

Any help and feedback on this would be much appreciated.

 

[wwwebber]

I used Gyproc Thermaline 50mm insulation backed plasterboard in our previous 1880s house on the inside of all internal walls. Worked very well, house heated up quickly and was also a good soundproofer.

 

[mikeey84]

In response to point 2,, whilst removing the plaster (a thermal element) above a certain % does invoke the supposed need to inform BCO I'm pretty sure the phrase is "altering", so adding insulation above a % of wall would also require getting them involved.

I'm sure many people don't though

 

[padstar]

Do you recomend universal bonding compound (traditional dot and dab) or an acrylic sealant adhesive. Are the gypsum adhesives prone to moisture resulting in mould?

 

[mikeey84]

It's key to ensure that no warm wet air gets through to the brickwork, thereby causing mould.

I would use separate insulation and plasterboard. You can use anything suitable to bond the insulation to the wall, then tape and foam the joints to ensure they are airtight. Plasterboard can then be screwed through insulation into the wall over the top, or battons can be used to allow a service void.

I would put a squirt of silicone into the holes before the Rawl plugs to help with airtightness, but I'm not sure how effective this is

 

[SFK]

Padstar,
With regards to insulating 1900's house, this might be useful background reading (I have done similar):
SFK

https://jack-kelly.com/insulating_our_victorian_living_room

 

[JP_]

I used 50mm kingspan, fixed with Everbuild Pink Dryfix foam, with plasterboard fixed to that with same foam. All boards taped with aluminium insulation tape.
You get a double vapour barrier (hard to properly seal insulation backed plasterboard). And when I looked at prices a year ago, it was cheaper to get separates.

Didn't inform building control. I didn't remove old plaster unless it fell off - I what it with a utility bar a few times, if it comes off, it goes, if it doesn't it stays!

I don't return down internal walls. Here's the bedroom I did last year. Floor done too.

 

[padstar]

Jonbey - I have been looking at the Everbuild Pink Dryfix foam. Is it easy to use and does it stick well? How many boards do you get from a tin. I will investigate the insulation and board seperately option. Hadnt thought about doing it that way to be honest

 

[JP_]

It sticks really well. I stuck a spare bit of plasterboard to a piece of wood as a test and after it dried I could not pull them apart no matter how much I tried.
You can use with a gun or just use the nozzle provided (I use the nozzle, forgot to clean my gun!).
Follow the instructions, i.e. apply, wait 4 minutes, then hold in place for a while. If done too soon the foam will continue to expand and push the boards away from the wall.
I used more than it suggests, I probably do 2 boards per can. So not cheap, but for a DIYer it's a lot easier than dot and dab.
To be extra secure you can some anchors direct into the wall - some people recommend this, although it breaks the vapour barriers. But it is not really needed, it's no more likely to fall apart than dot and dab. I think the reasoning for it is in case of fire, to stop the boards falling off if a fire melts the adhesive, but with wooden floors and ceiling and roof, and the boards sandwiched between brick and skimmed plasterboard, I think if a fire has got hot enough to melt the adhesive, your house is probably gone already!

Some pics of sticking plasterboard to insulation. Because of the 4 minute wait it is probably a bit slow going for pros, but as a DIYer I am happy to take my time! It also helps seal air gaps etc.

 

[padstar]

Jonbey - Sounds like you have done exactly what i am after. Do you mind a few more questions?

How long ago did you do it and have you seen any de-bonding from the block work since?

Have you noticed the difference since insulating? If you were to do it again would you bother?

 

[SFK]

"Have you noticed the difference since insulating? If you were to do it again would you bother?"
Yes and yes.
It is expensive but made room much more pleasant to be in.
SFK

 

[padstar]

Jonbey how did you fix your back boxes? Did you mechanically fix back to the blockwork as usual?

 

[John D v2.0]

Jonbey how did you fix your back boxes? Did you mechanically fix back to the blockwork as usual?

Fixing to the blockwork would cause a thermal bridge, the back box would only go 15mm into the insulation layer so blowing it all away for a socket would be excessive. In my case i decided the best compromise was to dig out the celotex a bit and use a dry lining back box. Does make the socket poke out a bit and is less solid feeling but it's better in general in the situation.

 

[padstar]

could you stick metal back boxes onto a recessed insulation board with a bit of the pink foam?

 

[padstar]

So i have been looking at pricing the job for materials and it appears to be far cheaper to get the insulation and the plasterboard separately.

I can get the 50mm celetex boards for just under £20 each. The same in kingspan is approximately £8 a board more. As i will be requiring approximately 30 boards is there any benefit in spending extra money on the kingspan? They appear to do the same thing