Warm roof design using pozi joists mid build - to many holes in the conditioned space layer

[andeebee]

I am mid way through an extension and house refurb

The extension is a timber frame using a warm roof on pozi joists and I am increasingly worried about the separation of the roof insulation layer from the conditioned space below due to the difficulty in sealing up the gaps between the pj and the top layer of the walls with PIR

To further compound the problems, I wanted an overhang around the extension. Whilst the PJ's facilitate this structure naturally on the one face, the two sides are forming the overhang using outriggers

Combine the outriggers, packing and the roof fall angle and you can currently fly a drone through some of the gaps between the roof deck and the walls

Whilst air tightness could be achieved using a vcl, heat loss from the break in insulation continuation cannot and I don't understand how we have got here

Possible solutions I have considered are commercial spray foam sealing from the outside of the roof cassette and the inside or my latest idea to fill the area below the roof deck with blown in insulation. 

Please share your thoughts and possible solutions

[andeebee]

More images...

[andeebee]

The gaps around the outriggers are due to added packing required to bring the roof deck level due to the discrepancy from the interface with existing building 

[Russell griffiths]

What’s holding up the top plates over that large opening, there’s not much of a lintel on show unless they have something on the outside. 

[JohnMo]

So basically all the gaps you see need to be filled. I would look to use a good expanding foam.

 

Some of the vertical filler pieces of PIR, don't look to butt up against the OSB roof cover. All PIR needs to be hard up against that.

[ProDave]

15 minutes ago, JohnMo said:

So basically all the gaps you see need to be filled. I would look to use a good expanding foam.

 

Some of the vertical filler pieces of PIR, don't look to butt up against the OSB roof cover. All PIR needs to be hard up against that.

And to add to that.  Once ALL the bits of PIR closing the gap are the right size (they reach the OSB layer) AND are properly sealed so there are NO gaps for air to pass through, that will have got you reasonably air tight so no fear of cold draughts coming in from outside, then you need to go outside and add as much more insulation as you can behind those relatively thin bits of PIR.

 

It is this sort of detail that needs to be got right NOW while you can get at everything.  There have been at least 2 threads on here recently where the builders have been left to it and done a shoddy job and cold air is getting in and it can be a VERY invasive job to fix it later if not done properly now.

[Big Jimbo]

Bad eaves detail, with a warm roof. How many times do we see that.

[ProDave]

10 minutes ago, Big Jimbo said:

Bad eaves detail, with a warm roof. How many times do we see that.

Yes it's shocking.  Put a well insulated roof on top of a well insulated wall, but detail it so badly that cold air just gets in and negates it.  Shocking that so many builders just do not understand that.

[Nickfromwales]

It’ll also be an acoustically poor solution, so with moderate to hard rain the sound will be notable to the room interior.

 

A blown-in insulation would be the very best solution for weaving around the metal webs and those associated impossible to get to gaps, and give a huge boost to sound deadening.

 

As @ProDave says, this is something you need to stop all wires with and get a solution to, before moving any further forwards.

 

Just had Gordon Lewis on site pumping my current clients roof with Warmcell (blown-in cellulose) and he’s done a fantastic job getting every nook and cranny packed full of the good stuff. 👌

 

Abandon the rigid insulation in the roof, and preserve that for the vertical walls only.
 

Arcitect will need to rerun the intersitial condensation analysis before pulling any trigger.
 

Airtightness is moot, the original house will underperform most likely, just all about draft proofing at this point; plus obvs managing repeat cold bridging, and maxing out on the (properly installed) insulation.