Hi all,

We are currently submitting plans for an eco retrofit of our bungalow, which will involve a new habitable roof, EWI and MVHR. The work will be shell only to a high standard (> AECB) but we do not need certification. There is a single-storey 'annexe' (garage/bedroom) attached to the corner of the main property which would be very fiddly (expensive!) to include in the envelope. As budget is already an issue we would like to do the insulation work on this room ourselves.

The annexe bedroom has a cold roof with 120mm of PIR in between joists and I am planning 100mm of Kingspan under a floating wooden floor. A lot of heat loss is to the garage, with the connecting wall being just 70mmm in places. The other walls are 300mm cavity with the exterior finish a mixture of render (unsure whether breathable) and fake stone concrete fascia blocks.

Our plan is to fit IWI in the bedroom and en-suite, however I am nervous about how this will interact with the EWI on the main house. We will replace the three windows with triple-glazing (allowing installation of IWI on the reveals) and there will be a mechanical vent in the en-suite room. I don't see the obvious cold bridges with the main house as a huge problem (we are not after certification) but condensation problems would be a different matter. The interface between the cold roof and EWI clad wall above will hopefully be sorted during the main works.

1) Is it advisable to use moisture open IWI to mitigate against possible cold bridge/condensation problems? If so what product and thickness would you recommend?

2) Will we need an insulated airtight door between the annexe bedroom and main house? (we would like to avoid this if possible)

3) Could/should we incorporate this room into the MVHR system in the main house?

I have uploaded a rough image of the plans here: https://imgur.com/a/Tqr4eGe

thanks in advance for any help

Posted By: maxsmwe do not need certification

The value of certification to me was that it made a big difference to the tradesmens' approach - they were very careful. And of course the plans and as-implemented construction details had the benefit of an extra qualified pair of eyes.

I have uploaded a rough image of the plans here: https://imgur.com/a/Tqr4eGe

I can't open that because uBlock Origin lists pretty much all the many domains as something it won't permit. Any chance of posting the image somewhere else, or posting it directly on here?

As you observe, one major problem with cold bridges is condensation. Breathable IWI won't make much difference since the condensation will still form on its surface. Without seeing the plan it isn't easy to know what to suggest.

Thanks Peter,

The garage annexe has a sloping flat roof which only overhangs at the 'eaves' by 60mm. I was under the impression new, longer joists would be required to accommodate the EWI. Relaying new joists is probably beyond my DIY skills, however, would bolting on short joist extensions work? Or I could simply lay new, overhanging decking (of a sufficient thickness) on top of the existing rafters?

The other complication is that the far Eastern side of the garage ----> is right on the boundary and adding EWI there would mean losing rear access. So the garage itself must remain cold. Any EWI on that aspect of the bedroom would have to be added inside the garage and cut to fit the existing joists. Do you think that would work?

Lastly, how long can EWI remain on an exterior wall exposed to weather before being rendered?

Many thanks for your advice and help

<blockquote><cite>Posted By: Peter_in_Hungary</cite>Are you planning to fill the extension cavity? If so then EWI the extension - this can be done part DIY with the rendering or final decorative coat professionally done depending upon DIY skills.

Think there might be fire proofing regulations, if insulation is fitted inside a garage? ISTR it has to be covered with fireproof board ('resist surface spread of flame' or some such).

Garages are usually cold-ish because the garage doors front/back are not insulated airtight. However it is protected from frost and cold winds, so isnt as cold as outside. So the insulation in the dividing wall doesn't have to be as thick as the outside walls.

The longest cold bridges are where the wall meets the ceiling and floor. From the description of insulation fitted inside the ceiling and floating on the floor, it would be much better to do IWI on the walls and join it directly to the floor insulation. Can't see how this could be done well with EWI?

Edit to add: for a cold flat roof, it's important that whatever is done at the eaves maintains a good flow of ventilation between the insulation and the deck.

Also the render finish by the approach to the front door has to be flawless as you will see it every day and curse if it's not right (don't ask ).

Posted By: WillInAberdeen

The longest cold bridges are where the wall meets the ceiling and floor. From the description of insulation fitted inside the ceiling and floating on the floor, it would be much better to do IWI on the walls and join it directly to the floor insulation. Can't see how this could be done well with EWI?

Edit to add: for a cold flat roof, it's important that whatever is done at the eaves maintains a good flow of ventilation between the insulation and the deck.

Thanks Will, I have ventilation both ends in the cold roof already. In summary do you think the IWI plan could work? or would EWI be much preferable?

Posted By: maxsmThanks Peter, the joists run E-w so on the Northern aspect would I use 90 degree brackets to add extensions?

Where additional roof is required beyond and parallel to the rafters, a construction called a 'gable ladder' is usually used. There's lots of descriptions etc on the intertubes.

Posted By: djh

Posted By: maxsmThanks Peter, the joists run E-w so on the Northern aspect would I use 90 degree brackets to add extensions?

Where additional roof is required beyond and parallel to the rafters, a construction called a 'gable ladder' is usually used. There's lots of descriptions etc on the intertubes.

Very useful, thanks