Green Building Forum - Eaves Details: new build with EWI

Eaves Details: new build with EWI

Bruno — Sun, 14 Sep 2014 21:57:38 +0100

Hi all,

I'm new here and, although this topic has been covered in previous threads, I can't find suggested eaves details online that fit my planned spec, and I wonder if anyone can point me in the right direction, because I don't want to reinvent the wheel if a consensus solution has already evolved.

My setup is as follows:

Detached house, with rooms in the roof
Aircrete single-skin 215mm block walls, with 200mm EPS EWI
Hollowcore concrete floors
Airtightness target: <1
Roof to be insulated over-rafters (to meet EWI), probably with an inter-rafter contribution too
Proficient airborne soundproofing - since we're on a (minor) flightpath
Roof U-value target: 0.11
No requirement for a significant eaves overhang
Roof finish will be clay tile
Hipped roof, with a central table-top flatroof (with a good fall designed in and a top-quality membrane)
[Other misc details, in case it helps: square footprint, bay windows to front (with hips over), two dormers, and several south-facing rooflights]

To recap, I'd like to find any commendable eaves details that complement the above spec. (They must exist)!

If I have any specific related questions, they would be:

1) Should we cut the rafters short at the wall plate or carry them through, and accept the thermal bridge? (And does anyone have any quantitative suggestions as to what difference this 'actually' makes, thermally)?
2) If we stop the rafters at the wall plate, do any roof boards exist that allow direct fitting of fascias and gutters? (I'm hesitant to pursue a solution involving cantilevered mini-rafters and such).
3) Traditional rafters or engineered timber?

All comments gratefully received!

Eaves Details: new build with EWI

tony — Mon, 15 Sep 2014 13:19:28 +0100

I would carry rafters through and over sail blockwork by 300mm to allow 100 soffit after your EWI.

You could use 50x75 sprockets and fix tour fascia to that, I used cedar for fascias, doesn't need treating or painting and does not rot.

Traditional, but you will need an engineers design for the roof, I would go hollow concrete floor again

Eaves Details: new build with EWI

Bruno — Mon, 15 Sep 2014 21:30:40 +0100

Thanks very much Tony

Any recommended strategies to limit fascia depth (courtesy of the ERI)? Are tapered rafters something that have been (or could be) used successfully?

Eaves Details: new build with EWI

tony — Mon, 15 Sep 2014 22:39:26 +0100

Yes or better cut the underside off level to suit a 100 wide soffit with a 120 or 145mm fascia, join the wall and roof insulation seamlessly.

Eaves Details: new build with EWI

davidfreeborough — Tue, 16 Sep 2014 13:13:47 +0100

If its a warm roof then you'll probably have counter battens between the over rafter insulation and the tile battens. These can be extended out over the EWI with an inverse taper (i.e. getting thicker towards the end) to form false rafter feet. This way the rafters can be cut flush at the wall plate, you won't need to cut the EWI around the rafters & the roof will appear very thin. It's also a lot easier to make windtight.

You can fit a fascia & sloping soffit to the false rafter feet or form traditional open eaves with a section of sloping soffit/sarking board on top of the false rafter feet at the overhang. Ventilate the batten cavity with a continuous section of over-fascia vent either on top of the fascia or on top of the sloping soffit/sarking board where it will be hidden by the gutter.

David

Eaves Details: new build with EWI

owlman — Tue, 16 Sep 2014 13:23:02 +0100

Posted By: Bruno,.................No requirement for a significant eaves overhang.

What is your rationale for this? Is it simply you don't like it, or is it site constraints that prevent it.

Eaves Details: new build with EWI

Bruno — Tue, 16 Sep 2014 20:29:19 +0100

Thank you David. Great feedback. I like the inverse taper on the counter battens idea and am now running with that.

I've made no decision yet on which insulation product/s to use. From what little information I've provided (airtightness target: 1, maybe even 0.6, MVHR, good soundproofing, U-value target: 0.11), is there anything that makes you think there is a 'standout candidate' for the roof insulation for this build? Let's start with the 'money no object' solution please (and I'll work swiftly backwards from there)!

Hi Owlman, site constraints I think is the answer to your question. [Aesthetically, I'd plump for an overhang and exposed rafters. I included that comment to indicate that (a) I might be willing to forgo such an overhang if that meant being able to achieve a superior result, in terms of overall integrity and thermal performance, for example, and I guess I was also thinking: (b) even though the suggested spec might hint that we have Passivhaus pretensions, there are no vast swathes of south facing glass that might require the eaves to perform a significant summer shading function].

Eaves Details: new build with EWI

tony — Tue, 16 Sep 2014 21:24:33 +0100

Money no object then go aerogel or vacuum insulated panels, I would use best bang for the buck, inorganic quilt, fg, eco or rockwool.

Eaves Details: new build with EWI

davidfreeborough — Wed, 17 Sep 2014 10:59:27 +0100

With insulation you're normally paying for the thermal conductivity (lambda or k). In other words you pay more to achieve a given thermal resistance in a thinner overall construction depth or a higher thermal resistance in a given overall construction depth. So the key question is: how deep are your rafters?

If you have 300-400mm deep I beam rafters then you have enough depth to use relatively cheap mineral wool.

If you have more standard 220mm deep solid timber rafters then you could go for 220mm low cost mineral wool between the rafters and 200mm low cost EPS over the top.

I haven't done the calculations, but if you have 150mm deep rafters then you might need to use graphite loaded (platinum) EPS, polyurethane or polyisocyanurate.

If you end up needing phenolic or aerogel for anything other than localised cold bridges then you probably need to re-think your construction buildups!

David

Eaves Details: new build with EWI

alistairm — Fri, 19 Sep 2014 12:07:24 +0100

We are currently building a new dwelling with EWI. We have no overhang with the attic trusses and have carried the insulation straight up outside of timber frame to meet roof. We have then fixed a wall plate along top of wall and fixed timber triangles to carry the sarking and slates. The external block wall will be built up and have a block on the flat to close the cavity which will also create the fascia for afixing the gutter. The block on flat will also support the overhang.

Eaves Details: new build with EWI

davidfreeborough — Fri, 19 Sep 2014 12:50:58 +0100

I'm a bit confused. Could you draw a picture?

I think you are saying that the attic trusses sit on a wall plate which sits on the top of the masonry wall. I assume the timber triangles are fixed to the outside face of the attic trusses to allow the sarking & slates to project over the EWI.

So what does the block on flat sit on? Where is the cavity which it is closing? Does it project out over the EWI? Is it not a cold bridge from (warm) masonry to (cold) outside?

David

Eaves Details: new build with EWI

alistairm — Fri, 19 Sep 2014 13:46:16 +0100

Its a timber frame house.

140mm kit clad with 50mm insulation 50mm cavity then 100mm blockwork rendered.

Block on flat closes the cavity.

Nothing penetratres the EWI except fixings.

Picture is during construction, roof to be loaded before external leaf is built.

Eaves Details: new build with EWI

tony — Fri, 19 Sep 2014 14:52:39 +0100

Not a good idea to close the cavity! introduces a thermal bridge.