Insulating posi-joist ends

[jamieled]

I'm getting myself in a bit of a pickle trying to figure out what to do about the following, so I'm hoping the buildhub collective can help.

 

We have a 1 and 3/4 storey timber frame, built from JJI joists. The first floor joists are posi-joists, and sit directly on top of the ground floor panel wallplates (see pics below). The rim boards are single pieces of glulam, one for each side of the house, with a timber locator plate and bottom rail for the first floor above that.

 

0

Our plan for the rest of the house is to have blown cellulose installed through the OSB (not currently installed where the photos have been taken). However, the posi's are a problem, partly because of the fiddling required both between the joists, and also through them (as they are open). I'm not overly concerned about airtightness , partly because of the single piece rim boards, the weight above them and the fact that I can probably seal around the perimeter with airtight foam if it was looking like being a problem.

 

I realise now (a bit late on), that I should probably have installed some sort of airtight membrane prior to the posi's being installed, to lap onto the airtight layer on the inside of the house (in our case OSB).My current thinking is to forget blown cellulose around the first floor perimeter and just stuff some rolls of insulation in. I have a couple of queries:

 

1. There would be no vapour barrier in this location, and even using the traditional detail of a membrane lapped around the end of the joists and onto the walls above and below, there would be no way of preventing vapour entering the insulation. Can I take it this is not really a problem in this area? We are planning to use foil backed plasterboard on the walls and ceiling which would reduce vapour movement a bit anyway I suspect.
2. Has anyone successfully insulated around the first floor perimeter with cellulose, and if so, how was it done? My felling is that it can't be done without a lot of fiddling and that trying to tape round the joists to get an airtight seal is overly optimistic given the number of potential seals required. It might however, be possible to do some fiddling to create something to keep the cellulose in there.

 

 

 

 

[ProDave]

Too late for you I am afraid, but we lapped air tight membrane over the ends of the joists and round the outside of the outer ring beam, with all joints taped.  This then gets taped to the rest of the air tight membrane.

 

It has become known here as a Tony Tray.

 

Then the gaps between the joist ends, I was given bags full of rockwool offcuts by another local self builder and I stuffed all the voids with these rockwool offcuts.  A tedious job so one done gradually a bit at a time rather than all in one hit.

[jamieled]

@ProDave, if I understand the above correctly, your rockwool offcuts between the joist ends are open to the floor void (i.e. there is nothing between the rockwool and the first floor void).

[ProDave]

2 minutes ago, jamieled said:

@ProDave, if I understand the above correctly, your rockwool offcuts between the joist ends are open to the floor void (i.e. there is nothing between the rockwool and the first floor void).

Yes that's right.

 

This is the only photo I can find, not much help really.  This is one of the joists running parallel with the front wall of the house.  You can see the membrane wrapped around the joist that goes round the outer ring beam and back in one piece.

 

That gap was filled with rockwook offcuts by stuffing them through the gaps in the joists and into the void behind.

 

Warning: Posi joist webs have sharp edges.

 

Obviously this photo was before the insulation.

 

 

 

[oldkettle]

Here is a picture I took while watching American TV show - they installed a new support beam. Doesn't answer the insulation question, sorry.