I've been mulling over a detailing query on the current roof design, and there are two questions that keep coming back - do I need an eaves tray, and where is the VCL in the design...

Attached shows the buildup from the ICF walls through to the roof. These are stub ICF walls at 1.4m high, and then the rafter creates a partial sloping ceiling. To get a decent u-value the rafter section below the ceiling is partially filled with 140mm of Celotex, and then underlined with 50mm over all the rafters, with tape on the joints. Above ceiling height is 300mm of Crown wool as a standard construction detail.

My questions are these:

Does the inner EPS of the ICF wall create the VCL, and is it sufficient to tape onto it and use the foil backing of the insulation to create the sloping VCL. If so, what happens at the slope to ceiling join marked red on the drawing..? Can I just tape onto the edge noggin..? If so, where is the VCL above the ceiling..?

The original build up had mineral wool in between the rafters and required an over eaves vent tray (although the spec also showed breathable membrane....) and this was designed in to stop the crown wool bunching over the eaves - this has now moved up the rafter to the ceiling height, so should the tray also move..? IS there a requirement to have 2 vent trays, or none at all given this is supposedly breather membrane...?

I've now taken this off the architect to resolve as the u-Values he was providing were getting steadily further away from the targets, and I don't have an endless pot of money to burn to keep this warm !

Cheers

Peter

Hi Nick

Understand the celotex being PIR - the ICF is EPS hence the query about what happens at the junction between the two.

If I use a membrane such as visqueen, are you suggesting it goes the full wrap over the sloping wall and ceiling..? And do the modern VCLs need to be counter battened ...?

Can you give me an example on an intelligent membrane as it's not something I've come across before

Cheers

Peter

On the eves, I used a non vented plastic tray and it is great. Provides a much better link into the gutter than just relying on the felt/membrane.

As an aside, are they open eves or fascia & soffit? If you have PIR over the rafters, what are you doing at the bottom of the rafters and above the point the insulation stops? Nailing 50x50 on?

Are you using rafter brackets to hold the gutter?

Posted By: PeterWThe original build up had mineral wool in between the rafters and required an over eaves vent tray (although the spec also showed breathable membrane....) and this was designed in to stop the crown wool bunching over the eaves - this has now moved up the rafter to the ceiling height, so should the tray also move..? IS there a requirement to have 2 vent trays, or none at all given this is supposedly breather membrane...?

The main purpose of the ventilation gap is to ventilate the cold loft above the flat ceiling. Using a breather membrane would allow you to avoid ventilating the rafter space in the sloping section &, if the breather membrane BBA certificate allows it, could potentially allow you to avoid ventilating the cold loft space too. The latter would likely require a continuous VCL or at least what is typically called a "well-sealed" ceiling. Doing this would increase the thickness of insulation that could be accommodated in the sloping ceiling & reduce wind washing of the mineral wool insulation in the loft.

The main purpose of the corrugated vent trays typically installed at the ceiling/rafter junction is to stop mineral wool insulation blocking the ventilation channel. However, they can have a secondary benefit of reducing wind washing of mineral wool insulation. So the upper vent tray is a good idea; the lower one is now surplus to requirements. Before removing it check that it wasn't being used to provide an insect screen.

David

Have you considered fully filling the rafter space with Earthwool? My personal opinion would be that it has a much better chance of performing well in reality than cutting Celotex accurately to fit between the rafters and will involve less labour.