Sorting out a "Plasterboard tent"

[this is my username]

We have a single-storey extension with a pitched roof built around 2008. It has always been significantly colder than the rest of the house - even after you factor in that it is out there on its own with nothing above or below it. I suspect that the dot and dab walls aren't properly sealed at the top resulting in the dreaded "plasterboard tent".

We have lived in the house for 15 years and it has been on my "ignore" list for most of that time, but a wasps nest in the roof space and the subsequent recent arrival of a load of wasps crawling in through a gap below the skirting board has (a) confirmed that there is access from the roof space to behind the plasterboard and (b) forced me in to considering some action.

There is no access to the roof space, so my plan is to drill holes along the top of walls and around the patio door / window reveals and inject squirty foam. Before I start - are there any "unintended consequences" of injecting expanding foam that I need to consider?

I was thinking of using this stuff:

https://www.toolstation.com/soudal-gap-filling-exp...

and maybe putting a flexible tube on the gun to help get it behind the boards. If not, should I use the expanding foam plasterboard adhesive?

[OutInTheShed]

That's what I did, without the wasps.
I put a length of clear flexible tube on the gun, then a short piece of ali tube on the end of the flexi to use as a nozzle.
I am not sure I completely sealed the void behind the plasterboard from the loft, but it made an obvious difference to the room.

I drilled lots of holes, about one every 3 inches. about and inch below the coving. Filled with plaster and finished off with a bit of Easifil.

I was able to access the loft to know where the wiring was, be careful of that!

[this is my username]

OutInTheShed said:

I put a length of clear flexible tube on the gun, then a short piece of ali tube on the end of the flexi to use as a nozzle.

Aha - top tip! Thanks!

[RGG]

OutInTheShed said:

That's what I did, without the wasps.
I put a length of clear flexible tube on the gun, then a short piece of ali tube on the end of the flexi to use as a nozzle.
I am not sure I completely sealed the void behind the plasterboard from the loft, but it made an obvious difference to the room.

I drilled lots of holes, about one every 3 inches. about and inch below the coving. Filled with plaster and finished off with a bit of Easifil.

I was able to access the loft to know where the wiring was, be careful of that!

I've got a similar but not same project ready for the off.

The length of flexible tube.

What length flexible tube did you get to work just as well as the usual supplied piece of short tube?

[OutInTheShed]

RGG said:

I've got a similar but not same project ready for the off.

The length of flexible tube.

What length flexible tube did you get to work just as well as the usual supplied piece of short tube?

You have to hold the gun with the can on top, so I used about an 18" length of tube so I could get the end of it up to ceiling level.

A gun is easier to use one handed than the non-gun cans, leaving a hand free to poke the tube into holes.

It's not critical, a longer tube would just waste a bit more foam and cleaning fluid.

The ali tube nozzle was just random from my scrap box, I imagine plastic would do?
Just needs to go in the flexible tube.

I expect there is a 'proper thing' you can buy?

[No ideas for a name]

Be really careful with the amount the foam expands. If you go a bit mad with it, it will force the boards off the wall.
(I once injected a hollow spot and created a 'belly' in the whole wall.

I would use the low expansion 'adhesive' type foam.

[this is my username]

Job has been (sort of) done now.

Purchased a Soudal foam gun plus some Soudal fire-rated foam as this is a "low expansion" type.

Made up an extension using some 6mm pvc tube I had knocking about with some brake pipe on the end bent in to a "J" shape so that i could get it in to the hole in the plasterboard and squirt the foam sideways.

Drilled 10mm holes 100mm apart about 80mm down from the ceiling. About 30% of the holes were "blind" as I was drilling in to the plasterboard adhesive behind the plasterboard itself. I poked a long cable tie through a few of the holes to demonstrate that I did have a clear run from the hole up in to the roof space - so my "plasterboard tent" theory was correct.

Blew foam in to the holes, and I am now most of the way through making good with filler and paint.

I was a bit uncertain as to what to do around the window and patio door. I drilled a few holes around the window but they all came up "blind" so I gave up and just did the top of the walls. Will wait to see if there is any improvement.

Thanks for everyones help.

[biggiles]

Bravo, it's frustrating how houses (modern ones!) are built like this - presumably it's simple to do it properly at the building stage, and if done badly/wrongly, condemns the inhabitants to increased heating bills forever. (Let alone the environmental impact of the wastage).

[no eye deer]

One thing to be wary of is any cable drops. Any accessories will be a clue ( but don't assume anything) Also be wary of the safe zones at the top of the walls and in the corners etc if your planning on drilling in those areas.