Insulating gap under dormer knee wall

johnnyboy08 · 17-10-2019 12:06pm #1

Hi all,
I'm thinking about tackling the knee walls in one of the rooms of my dormer before committing doing them all and a big problem for me is air from the ventilated attic space being able to run between the joists.
There is rigid insulation board at the back of the knee wall between the studs that runs from floor level at top of joists to dormer roof.

My plan is to use pir (Kingspan Kooltherm K7) to block the gap under the existing insulation i.e. the height of the joist between the studs and seal around it but I have some questions.
1. what should I use to seal the bottom of the existing pir to the new? Siga tape?
2. what should I use to seal around the new pir and the joists?
3. do I need to worry about the joists breathing properly if I seal up that gap?

Thanks

MicktheMan · 17-10-2019 5:46pm

johnnyboy08 wrote: »

1. what should I use to seal the bottom of the existing pir to the new? Siga tape?
2. what should I use to seal around the new pir and the joists?
3. do I need to worry about the joists breathing properly if I seal up that gap?

Thanks

1. Siga tape
2. Siga tape - (btw, instead of PIR you can use ply (9mm) and pre-cut them if all/majority of the gaps are roughly the same size)
3. No issue

Calahonda52 · 17-10-2019 6:48pm

The jargon varies here but are the studs not supported on a bottom plate?
.
If so, the ply idea is better because if cut the right shape/size it can be screwed to the side of the bottom plate, using something like
https://www.ecologicalbuildingshop.ie/Ecological-Building-Product-Detail.asp?prodId=S196016&catId=255
to seal behind the ply and around the edges with the ceiling joists, assuming you cut them pretty neatly

.
.
If so again: your point 1 is not relevant as the upper PIR should be on top of the bottom plate and away from the ceiling insulation

johnnyboy08 · 17-10-2019 7:13pm

MicktheMan wrote: »

1. Siga tape
2. Siga tape - (btw, instead of PIR you can use ply (9mm) and pre-cut them if all/majority of the gaps are roughly the same size)
3. No issue

Thanks Mick, can you recommend which one please? They have a few.

johnnyboy08 · 17-10-2019 7:15pm

Calahonda52 wrote: »

The jargon varies here but are the studs not supported on a bottom plate?
.
If so, the ply idea is better because if cut the right shape/size it can be screwed to the side of the bottom plate, using something like
https://www.ecologicalbuildingshop.ie/Ecological-Building-Product-Detail.asp?prodId=S196016&catId=255
to seal behind the ply and around the edges with the ceiling joists, assuming you cut them pretty neatly .
.
If so again: your point 1 is not relevant as the upper PIR should be on top of the bottom plate and away from the ceiling insulation

Thanks for the information, I'll take a look tomorrow and put up a pic or 2 if I need more help. Cheers!

MicktheMan · 17-10-2019 7:56pm

johnnyboy08 wrote: »

Thanks Mick, can you recommend which one please? They have a few.

Depends very much on the surface but the rissan tape (green) is good. Their mastic is also excellent in combination with the tape especially if surfaces are rough.
They have a booklet which has good examples if memory serves.

listermint · 17-10-2019 11:49pm

I found contiga SL to be a more adhesive tape than sika. Sticks to anything.

MicktheMan · 17-10-2019 11:58pm

Siga

Have no idea of the comparison but I've found the siga range excellent

johnnyboy08 · 18-10-2019 11:12am

Right so, took a look in the crawl space earlier and the insulation in there was worse than I remembered.

I thought that I had pir on the knee wall but it's fibreglass. Worse still, it doesn't look very secure and has peeled away completely between some studs.

Looks like a bit more work to do then to replace that fibreglass on the knee wall with pir and seal around that and the studs.

I've attached some pics and measurements are:
Studs: 2" x 4" with 14" gap
Joists: 8.5" x 2" with 14" gap

IMG_20191018_100509.jpg shows the crawl space with fibreglass knee wall insulation and blown insulation
IMG_20191018_100644.jpg shows the stud base board and gap between the joists
IMG_20191018_101208.jpg are cross timbers between the joists. Bit confusing these, some have a gap of around 1 inch between timber and felt while others look completely sealed up. There is pir on the other side of that board running up into the attic and stopping at dormer ceiling where blown insulation takes over.

Calahonda52 wrote: »

The jargon varies here but are the studs not supported on a bottom plate?

You're right, the are.

Lumen · 18-10-2019 11:22am

Well, on the upside you can't make it any worse!

johnnyboy08 · 18-10-2019 11:35am

Lumen wrote: »

Well, on the upside you can't make it any worse!

LOL, that's true!

johnnyboy08 · 18-10-2019 11:37am

listermint wrote: »

I found contiga SL to be a more adhesive tape than sika. Sticks to anything.

How are these tapes with rough timber like joists and studs? Would sealant be a better option for those?

Lumen · 18-10-2019 11:40am

johnnyboy08 wrote: »

How are these tapes with rough timber like joists and studs? Would sealant be a better option for those?

I've found with the proclima tapes that the stretchier the better, their paper tape is a bit frustrating to use compared to the more expensive cloth one (and the glue is the same). The sealant (Orcon F in the case of proclima) tends to be used as a bead on a particularly difficult surface, and can be combined with the tape, but for joists you should be able to get away with stretchy tape.

It's bloody expensive though and you're going to need quite a lot of it!

edit: the other thing about the tapes is that they're not really designed to take a lot of air pressure. If possible you want to have something stopping physical movement of the tape when there is pressure on it, i.e. for it to be supported on both sides (front and back). I can't quite work out how this advice applies to what you're trying to do, but it should seem obvious when you're crawling around doing it. Basically, just imagine that there is a vacuum cleaner on one or either sides of the tape and think about how to reduce the tendency of that to bugger up the tape over time.

johnnyboy08 · 18-10-2019 12:17pm

Thanks Lumen, if anything I'd probably opt for sealant and tape between the joists and sealant only between the studs if the pir is butted tightly between them

Insulating gap under dormer knee wall

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