Kingspan type insulation on existing plastered wall...

17 Dec 2016
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United Kingdom
We moved into a bungalow last year.

One of the rooms is an extension, that has 3 exterior walls. It is consistently cooler than other rooms.

Upon inspection, we can see that the room has cavity wall insulation (it basically has a cavity between the bricks and a very small amount of yellow wool insulation), and rock wool above in the small loft space. There is no insulation in the floor, or anything else on the walls.

Because this room is so much colder, we would like to use Kingspan to fully insulate the walls and floor.

The existing walls are plasterboarded and skimmed. The question is whether we can simply attach some batons to the existing plastered plasterboard walls and place the Kingspan in, then another plasterboard on top and skim...or do we have to rip down all the plasterboard back to the bare brick? Would this cause any problems? Would it be better?

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Sorry for being a dunce, Woody...I'm not following with that post.
You'll obviously lose a bit less room size if you remove the existing plasterboard.
Details around sockets, windows etc. may prove the most difficult aspect.
As Woody indicates, questions about how best to install insulation are asked here quite frequently; other threads may not be describing exactly your scenario but they can still contain useful information.
How old is this extension? How do you know there is no insulation under the floor? The presence/absence of e.g. polystyrene under the slab is not obvious.
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Yeah, I had a search around but couldn't find a solid answer to it I'm afraid.

The extension is maybe 15 years old or so. It was built onto a garage before we moved in. So we don't feel like it was done to a top standard, it was considered a workshop rather than a room. There wasn't any insulation above for example, we had to do that. It was just plasterboard, then the loft space. From above, I can see the plasterboard onto the brick with nothing in between, and then the cavity wall with a small amount of yellow wool.

We drilled down into the floor, but yes, I guess not 100% conclusive.

But even so, we would like to insulate it to the levels of our new garage conversion which retains an incredible amount of heat.

It's just whether we need to strip back to brick first, or whether we can baton the existing walls, placing Kingspan in between and then more plasterboard on top.

Thank you!
Yes, you can do that if you want to.

Thanks. Is there any downside to doing that other than losing a bit of space? Is it likely to cause anything at all (don't really know what, just asking to make sure)? What size Kingspan would you suggest for this? 50mm, or would that be overkill considering?

The battens conduct more heat than the insulation. Fitting without them means less heat loss. It also means less work cutting the boards and foaming/taping.

See e.g. for what building regs currently require; maybe 0.30 W/m2K depending on where you live. (Regs may or may not apply to your project, but even so it's a good guide to best practice.) Guessing that your current wall might be 1 W/m2K, you'd need insulation providing 0.43 W/m2K. Using PIR boards (without battens) of 0.022 W/mK, you would need a thickness of 51mm to achieve that. So yes, your 50mm guess is about right. Of course using more only makes it better.
one way to do it, is to fit celetox type insulation continuously around the wall with no gaps or battens.

tape all the joints with foil tape

then screw on battens, 2 x1 laid flat at 400mm centres, or to suit plasterboard. (maybe fit 3x1 where you have radiator brackets)

Then fit plasterboard.

That way you have no thermal bridging at the studs and the foil face of the celetex acts as a vapour barrier, provided you tape all the joints properly.

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