Posted By: Tullichthe only other acceptable way to avoid the issue, is to use the equivalent quantity of insulation outboard of the timber frame as is contained within it

Absolutely - that's the answer.

Otherwise, the outer board, be it OSB or soft board, is inevitably exactly where the zone of condensation begins, and maybe spreads inwards. That may happen at least momentarily in any cool climate - the key question is how long it will stay wet before re-drying. 'Next summer' seems to be good enough. But in some climates/conditions it may never fully re-dry before next winter, then the board gets steadily wetter and wetter year by year. This can be seen graphically in WUFI simulations. Do we trust that bituminous softboard outer sheathing, recommended as the answer, is really immune forever to decay in that case?

The cure is to put substantial non-organic insulation outboard of the studwork and its outer sheathing, then any condensation happens in non-organic non-structural zone, and OSB outer sheathing is fine.

Anyway, studwork with insulation between is prime thermal bridge - really poor practice. A substantial layer of insulation across either the inner or outer face of the sheathed studwork easily cures the bridging. Best on the outboard face, for reason given above.

I'm guessing not as it doesn't meet the "inorganic" criteria?

In Swedish TF's do they not just use an external membrane in place of external racking resistant boards, and have an internal air tightness / racking make up as described above?

Posted By: TullichIf using an airtightness layer on the internal side of the timber frame (OSB or equivalent) would this not also serve to provide racking restraint? Could the exterior structural sheathing board then be substituted for an insulation board (woodfibre from Pavatex, Steico, etc) instead?

I don't see why not, but I would ask Pro Clima (via Ecological Building Systems) to confirm it.

PS. Here's the rest of the text of that section that you quoted above:

"If the OSB boards are to be installed
externally they have alternatively to be
insulated additionally on the external
side. In this way the dew point is
abated. The thickness of insulation apart
from the OSB boards should be as high
as the insulation between the posts.

"Concrete masonry and quarrystone
masonry should be insulated externally.
An internal insulation should only be an
additional insulation in connection with
an external insulation."

The documents are available from https://proclima.com/service/downloads
Thanks for drawing attention to them.

Thanks Tom and djh.

To give a little more background to the build....

I'm determined to only use materials available from local builders merchants. I'm in the Scottish Highlands and don't want to risk delays in supply or to pay extortionate delivery costs. I've settled then on stick timber and Rockwool, which are both self build friendly materials for a hands on construction.

I costed blown cellulose at the outset, but per 100mm thickness it's £19/m2 @ 60kg/m3 density. Rockwool is less than £8 for the equivalent thickness.

Aside from materials the other constraint is the need to get the build to wind and watertight as quickly and cost effectively as possible. I'd hoped to avoid any genuine external insulation and to rely instead on the stud/counterstud frame approach to increase available depth for insulation.

Hi Tullich,

my first thoughts are that wanting to get weather tight quickly, will probably mean the OSB skin on the on the outside, as we traditionally do with timber frames here in the highlands.

That lets you insulate with non-waterproof insulation in the dry from the inside, but of course doesn't stop you adding a layer of rigid board over the outside, which I've done on some occasions, including over the roof, for the very good reasons pointed out above.

Consider how you'll demonstrate to building control as regards vapour control, for your building warrant. I'm not giving any direction on this, by making the comment, but if you're not using a proprietary wall build up, which has been tested, and it sounds like you're going for stick build, then the responsibility will be on you to demonstrate no (or virtually no) interstitial condensation. Good design, and use of cond modelling software would do this.

Totally agree with Tom.

If you put enough insulation outboard, then the timer is always on the warm side of any condensation. If it did by some remote chance get wet, it would then dry.

Also agree that the easiest way is to make a tea cosy of OSB and then insulate over. I would though use Icynene insulation between the studs as that helps with the air tightness and yet lets vapour out.

I built with a metal frame from U-Roof http://www.u-roof.com, created a tea cosy of Insulation and then sprayed Icynene inside. Worked perfectly! The metal frame has it's own challenges when finishing inside, but the roof is self racking so less requirement for purlins etc.

No wood, nothing to rot!

https://www.borpin.co.uk/2012/09/07/the-u-roof-frame-is-up/

Really appreciate your input so far, and the links suggested. Thanks.

I'm familiar with the REMOTE wall concept, and the excellent video series produced by the Cold Climate Housing Research Centre. I've often wondered why nobody produces a t & g profiled sheet (as seen on external woodfibre boards etc) which could be installed in one thickness to save layering sheets and the additional labour costs?

With EPS100 being the default material for EWI the issue is availability of materials locally again. Any number of retailers will sell me EPS70, but EPS100 is a special order item and the delivery costs are eyewatering. Is EPS70 a safe (albeit lower density) substitute?

Icynene's not a self install option in much the same way as cellulose isn't, your steel frame looks amazimg and terrifying in equal measures though borpin!

If I were to bite the bullet on an externally air tightened timber frame option, the next step would be to figure out how this can be integrated with a suspended timber floor. I'm already feeling a little bit like i'm selling out by considering EPS options, I really don't want to pour a slab....

EPS under a raft

http://www.isoquick.co.uk

Posted By: TullichIs EPS70 a safe (albeit lower density) substitute?

I think EPS70 is ok - I'm using it at the moment for my EWI. The data sheets for my adhesive and render system states EPS70 is also OK.

If you do use EPS as the rot-proof outer insulation layer of the timber frame ensure you have a higher density sound attenuating insulation layer inboard - wool batts, cellulose or spray foam.

So, things have moved on a fair bit since April....

Against all the advice i've recieved from local builders, other self builders, my draughtsman, etc i've decided after a lot of reading and head scratching to go with the REMOTE wall model as discussed here previously. This decision is mainly based on buildability as this is a hands on self build. I understand the construction type and feel I can achieve it. My planning permission's been granted, so things are heading in the right direction.

The wall make up now looking like:
12.5mm platerboard, taped and filled
145mm timber frame
140mm Rockwool in stud bays
OSB3 racking layer, screwed and glued
EWI type and thickness tbc (see below)
Weather barrier
Batten
Larch cladding

There are two design issues i'm still wrestling with: floor construction and which type and thickness of foam material to spec for the ewi aspect of the walls. So, I have a couple more questions....

Firstly, has anybody here successfully self installed a non proprietary EPS insulated raft type foundation? Sticking to the principle of "only building with materials off the local builders yard", I can source EPS sheets but not the edge profiles. I don't want to spring for the full Kore or similar package - it's serious money that I don't have.

Secondly, i've read as much as I can find around the topic of which type external insulation to use outside of the timber frame in a REMOTE type system. The standard approach of layering EPS sheets with staggered board joints make sense. However, I have a note somewhere to the effect that 38mm of EPS = 23mm of PIR? I've been thinking about using 120mm of PIR therefore in lieu of 200mm of EPS. Would it be feasible to then use just one layer, with the edges intentionally gapped, say 5mm, and foam filled after the fact? I know PIR loses to EPS in respect of breathability but does that matter in this example? Would 140mm Rockwool Flexi in stud bays internally and 120mm PIR externally cut it on a condensation risk analysis?

Ha! I guess i've got more to figure out still than I thought!

Hi Tulloch, very happy for you to ignore my comments, if you're already past this point, with supporting calcs

Stating the obvious, but you do need to nail all this down for your building warrant, and get a struct engineer to sign off the design with an SER. You'll need interstitial condensation calcs for all the structural elements too. This insulation info will also be required by the SAP calculation guys...apologies if you're aware of all this.

I've done several timber frames with the insulation on the external OSB face, so no big deal really.

I don't see a vapour control layer between pboard and the frame? Have you considered a service cavity under the pboard, so you don't puncture the VCL, or have services squashed into the insulation, or have to drill out all the studs and dwangs?

You seem to be talking about very large thicknesses of the sheet insulation externally.

What's the lambda value of the rockwool between studs? If you improve this, say with a frametherm 35 or even 32, you will be able to reduce the external board thickness. Why? well it makes life a bit trickier fixing your battens through the sheet insulation into the timber frame, and also issues with future "sag" due to the vert loading of the timber cladding. Compressive strength of the chosen insulation boards also comes into play. It's all do-able in various ways, but some are a bit more tricky than others.

Just as examples:

frametherm 32 between studs, with 50mm PUr board = Uval 0.15 W/m2K

rockwool/other 44 between studs, with 100mm EPS boar = Uval 0.16 W/m2K

compressive str of EPS70 (usual, easily avail) is half that of PUR boards (kingspan type stuff)


Ground Bearing Slab...

I see you're building a raft, rather than strip foundations with typical block underbuild. I usually stick to the strip type, not that that means anything. I've done lots of slabs with EPS70 underneath. In one case, it was cheaper to add a 3rd layer of EPS70, rather than build up the solum level with crushed stone. The edge detail can be dealt with in various successful ways, but afraid I can't help with details for full raft under foundations. "Viking House" of this forum is the king of raft slabs from what I've seen. Has your struct engineer confirmed the bearing strength of your soil conditions for this or any other type of foundation?

What do you think about my idea for the EWI install?

Intentionally gapped PIR boards with joints spray foamed afterwards?

It reduces the overall thickness of the EWI, avoids the need to install two layers, and from what GP has said above has a better compressive strength to install battens and timber cladding over afterwards.

Is the lack of breathability an issue?

If using 140mm Rockwool flexi in the stud bays, what thickness would I need over the frame to hit 0.1 W/mk?

Lastly, bearing in mind this thread started out with a quote from Pro Clima about condensation risk, would any of this pass?

over here EPS is the cheapest form of slab insulation and whilst for a given U value it is thicker than others if you are doing a new build I would not have thought that a couple of inches extra thickness would matter. If you are doing a timber frame with OSB boards I would have thought that the result would (should) be flat enough to put up the slab insulation without the need to leave gaps to be foam filled later.

If you don't build off the slab but use strip foundations then depending upon the required depth you can insulate down to the bottom of the foundation wall and avoid the need to insulate the floor slab but if you need to go that deep an insulated slab might cheaper / easier.

For your thoughts about the DIY slab edge insulation a problem could be the internal part of the edge profile. Usually the slab edge is thicker than the center so an internal downstand is needed, this will need to be supported to take the strain of the concrete pour without moving. One way around this is to pace the internal downstand EPS in place, brace it on the outside edge and then back fill with a weak concrete mix to stop the concrete pour displacing the EPS

The plan for the outside edge of the slab as a post fit up to the EWI sounds OK, although you could pre-install the EPS inside the shuttering and pour into an ESP box supported on 3 sides

Posted By: TullichThen lay out flat EPS sheets to the slab size, install a timber shutter at the perimeter as per a conventional slab, pour the slab and install a vertical EPS element afterwards to bridge the sub slab EPS to the EWI.

It might work. I don't have the knowledge to judge. Bear in mind that the outer edge of the slab is a lot deeper than a normal floor slab. Ours is 400 mm, though I think 200-300 mm is more normal. Hence the loads on the shuttering are greater than for a normal floor slab. Apart from that, I think the main issue might be any gap between the EPS and the shuttering, perhaps due to movement of the latter, will result in concrete filling the gap.

I looked at a suspended timber floor on piles as an alternative, but that seemed a lot more complicated and probably more expensive, plus gave issues with the height of the floor above ground. Passive slab seems very simple by comparison.

djh
Your last sentence sums it up for me. A passive slab seems simple compared to any alternative. I started out planning to build off of a series of pillars and a ringbeam, with a cassette type floor over the top. This later became iJoists over a stem wall which still needed a concrete solum and didn't cost out well, and so I finally arrived at the raft/slab idea. The main difficulty with which is finding someone to engineer it quickly so I can get my warrant application in. SE's work anything but quickly for good reason, but the weather window for building up here is narrowing daily....