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.

[andeebee]

Thanks to all the above comments.

I have spent the last few weeks trying to fill the gaps left by the bad fitting warm roof, using PIR cut up into blocks and foaming them in

I used a airtight foam on the inside perimeter and a large area 'coverage' based foam on the outside.

Still not sure I have got all the gaps, due to the now inaccessible cavities between the outriggers and the roof edge

Had many sleepless nights adding up all the area of the blocks I have cut, to literally cover the daylight coming in. 

Had this not been addressed, at least partially by my efforts, it would have been like leaving a window open continuously.

I think the best way to have installed this would have been to fill the gaps between the outriggers, prior to the installation of the roof section, as you would have far more access.

The roof could have then been built onto a continuous layer

[Nickfromwales]

5 minutes ago, andeebee said:

Thanks to all the above comments.

I have spent the last few weeks trying to fill the gaps left by the bad fitting warm roof, using PIR cut up into blocks and foaming them in

I used a airtight foam on the inside perimeter and a large area 'coverage' based foam on the outside.

Still not sure I have got all the gaps, due to the now inaccessible cavities between the outriggers and the roof edge

Had many sleepless nights adding up all the area of the blocks I have cut, to literally cover the daylight coming in. 

Had this not been addressed, at least partially by my efforts, it would have been like leaving a window open continuously.

I think the best way to have installed this would have been to fill the gaps between the outriggers, prior to the installation of the roof section, as you would have far more access.

The roof could have then been built onto a continuous layer

Ouch.

 

The money shot for you will now be airtightness afaic, as with possibly compromised insulation volume you can still get a very well performing dwelling with AT and MVHR, as with a draughty house you can have 500mm of rock solid insulation and still be sat in the cold.

[andeebee]

I still have the nightmare of the extension to house interface to deal with, have been waiting for the existing house wall and roof to be made good before attempting.

This involves roof trusses and floor joists sitting on the wall plate which leads into the depths of a (well) ventilated loft, not helped by sub floor ventilation from open ended floor joists into loft space (Americans call 'Cape Cod' style house)

No consideration for bagging the pozijoists was made, until I suggested it (following installation, whilst staring up at the mess).

If I can isolate the new extension from the house, I hope to recoup some of my spend on insulation on the extension, otherwise it will be blowing out the other side of the house

The stress this is causing is huge, nobody seems to understand the problems I am seeing and how to deal with them. All design problems, that should have been considered up front.

How many architects wander round a site during a build thinking 'that doesnt look good, how could I make that better next time...?'

[Nickfromwales]

11 minutes ago, andeebee said:

How many architects wander round a site during a build thinking 'that doesnt look good, how could I make that better next time...?'

Not seen one yet, but I live in hope that they’re out there, somewhere, but seems like finding such a conscientious architect is like trying to find a Yeti in a snowstorm.

 

Seems you have to find the best possible one, and look further into what they’re doing for you to identify the gaps or omissions; not easy if you’re a novice, and something I am still amazed at when I’m introduced to a project after the architect has already ‘got the wheels in motion’.

 

Thats great, if the wheels aren’t fecking square ones. 

[Mr Punter]

I would be filling the awkward gaps with decent mineral wool.  It is far easier to use than cutting bits of PIR.  Fill the whole void where you have the outriggers and any gaps where new meets old.

[Marvin]

When I did our side extension flat roof connection to the eves side of the bungalow  I used OSB vertically all the way up, in between the roof joists, sealed the gaps with mastic and then put plastic over the OSB and sealed the edges where necessary before insulating, to protect the void. When PIR insulating I foamed every gap that was between any timber and the PIR.