Retrofitting cavity wall insulation

We’re having an extension done, one part of which is building a bedroom on top of the existing attached garage.

As part of this the whole roof is coming off the garage. They’ve also opened up the cavity of the “long wall” and you can see it’s completely empty. The rear wall is becoming internal so not a concern.

As our bedrooms going above, it would make sense to get the garage as warm as possible, and it seems like now would be as good a time as any to fill the cavity with balls or whatever as the roof is off it so access is super easy, I could just pour the stuff in the top rather than pay someone to inject it.

I’ve heard mixed things about retrofitting cavity wall due to breaching the cavity, so thought I’d ask for views. The wall itself will remain a garage wall, and it’s pretty sheltered (2m from a fence and then another 2m to next door), so I don’t think driving rain is a significant issue.

We can’t put slabs of insulation down the gap as there’s wall ties in the way

Had mine done with blown rockwool. Didnt make a massive difference to temps or damp but it did reduce the noise from the road a little.

Iirc glass fibre types like rockwool aren’t a great option. Think beads are less likely to cause issues.

Is the garage staying as garage or converted to habitable room? If it is staying a garage then insulating the wall to it is pointless. The extension bedroom floor should be treated as the thermal and airtightness envelope. Also make sure any door from garage to house is an external grade insulated door as this will be a major point of heat loss. Have a good think about where the extent of the thermal envelope should be. It is often not considered properly with integrated garages

the garage is staying as a garage (although getting a bit bigger). We’re putting a posh door on the front of it that’s insulated and sealed to make it a nice, warmer space, so whilst the walls between it and the rest of the house will be insulated (and the ceiling will be) it’ll be nice to get the garage warmer too

My first question would be, are the foundations of the garage suitable to take the weight of the new extension?

Be careful of trying to seal up a garage as it can lead to damp issues. You should make sure the wall from garage to house are all well insulated as they are often not and maybe add more insulation there. Also work on airtightness of garage ceiling to room above as it will likely leak a lot.

If it is staying a garage then insulating the wall to it is pointless.

If you’re keeping stuff in there that you want to stay nice, doing anything to lift the dew point is a good idea. Especially as if a fancy main door is being fitted the ventilation will be reduced.

We had the sticky polystyrene balls blown into our cavity on an 80’s build house a couple of years ago and it has made a big difference to warmth both in winter and staying cooler in the summer. We have just had one incredible wet winter / spring and no problems for us with water tracking across from the outer skin.

The only thing I would mention is that the balls only stick a little bit so if you were to try and put them in an open cavity from the top then any wind would have large amounts of little grey balls siling away into the garden. it really isn’t difficult to have them blown in after the work is done.

My first question would be, are the foundations of the garage suitable to take the weight of the new extension?

yep, they go down 1.5m, same as the rest of the house.

The only thing I would mention is that the balls only stick a little bit so if you were to try and put them in an open cavity from the top then any wind would have large amounts of little grey balls siling away into the garden. it really isn’t difficult to have them blown in after the work is done.

good tip, thanks!

Be careful of trying to seal up a garage as it can lead to damp issues

would it have any reason to be more damp than the rest of the house?

Yeah. No heating and reduced ventilation!

We had ours done with the blown wool stuff a few years ago, just before it kicked off in Ukraine and gas prices went crazy.

Made a big difference to how much heat the house loses overnight.  Before the boiler was running flat out from 5am until we left the house for work and still wasn’t even upto temp.  And when the heating went off at 8pm the house was chilly by bedtime.

Now unless it’s properly cold outside it barely drops overnight. And with WFH the heating is on all day but despite that and the price rises our bills didn’t go up anywhere near as much as the average.

No issues with damp related to it despite this winter.

The only thing I’d change if I did it again for another house, is I’d do it sooner!

I assume the house is reasonably modern if it has an attached garage. Check the outside of the garage to check for blown brick faces and damaged pointing. If that’s all good then I would add insulation.

We made a conscious decision not to do it in our previous Victorian property. It had a cavity but a lot of the external bricks were porus and pointing was crap. Over the years loads of crap had filled the cavities meaning we had damp/mould patches due to the walls being cold and water tracking through the bridged cavity.

Took buckets of crap out of the cavity where we found damp patches to fix the damp problem. Also fixed the pointing but never replaced any of the bricks who’s faces had blown.

Definitely didn’t want to add anything in there that would breach it again.

I assume the house is reasonably modern if it has an attached garage

yeah 8 years old – the brickwork is immaculate.

Yeah. No heating and reduced ventilation!

it doesn’t get mega-cold today – maybe down to 8-10C in the middle of winter. With a room above it and one behind, I figured it’d keep its heat even better as the only external wall is the one we’re thinking about adding insulation to.

The garage floor may well be uninsulated, keep an eye on that if the room is otherwise sealed.
How easy would it be to pop a small rad in there from the CH? Proper luxury man cave then, if the car will still be in there I’d run a dehumidifier too.

the garage today is effectively sealed – behind the front door there’s a foot of insulation and plasterboard with wood blocking up the bottom, then some racking – a small draft might occasionally get in I guess? I’ve got a heater in there, but we don’t use the central heating to warm it because its not used as often as the rest of the house.

No car in there at the moment, but we’re adding ~4m to the front of the garage (along with the fancy new door) to give me space to build a caterham with the kids once they’re old enough – so it’ll be around 10mx3m once we’re done

If your unheated garage is staying quite warm or at least not cold,  in winter it will be largely due to the heat loss from the house. Even more reason to insulate more between house and garage.