It seems odd to me that your additional layers would make the situation of your existing sarking boards worse as they should now be much warmer. OK, the breather membrane won't be as vapour open as the original build up but still …

Ed - the additional layers didn't make thinks worse. As original with the VCL there is condensation on the inside at the point of the rafters (= mould) and without the VCL the sarking boards get condensation up to 48% moisture content ( all at 65% RH internally)
With the additional layers and with the VCL there is no condensation anywhere. Without the VCL there is condensation on the sarking boards when the internal RH is 65% giving a 12.9% moisture content. Taking out the breather membrane lowers the moisture content of the sarking from 12.9% to 11.9%

Will - If there is no sarking layer and no VCL then condensation starts under the breather membrane above 50% RH internally. If the breather membrane is removed then there no condensation anywhere up to 85% RH internally.

So - With a VCL there are no problems (no surprise there!)
without a VCL there are problems at the sarking layer above 65% RH internally.

The breather membrane can make things worse in marginal situations (but not IMO enough to warrant leaving it out)

With a VCL there are no problems, without a VCL there are always problems at the sarking layer but less with the additional layers.
The question is what level of damage would be acceptable to the VCL before interstitial condensation problems occur at the sarking (What level of damage or poor workmanship to people accept to the VCL in the ordinary course of building?)

FWIW http://www.dpcalc.org/ gives estimates of mould risk and other problems.

Posted By: Ed DaviesIs there mould on your sarking boards now, do you know?

I can't see the sarking boards now - or ever unless I trash the roof as any mould will appear on the inside of the boards which are not accessible, however as the building has not been occupied in the winter months since the loft conversion I would not expect to see any problems.

Posted By: WillInAberdeenI think that's a good tool for u-values, but wasn't convinced about doing condensation stuff with it. Previous thread:

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16112" rel="nofollow" >http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16112

IMO a good tool but I view such tools as an indicator to be used with caution, i.e. if it says there is no problem - then there may not be, if it says there is a problem then there probably is.!
Do any of the predictive software modeling tools (free or otherwise) come with a guarantee?

The base question remains
How much damage or bad workmanship to a VCL is acceptable before replacement or remedial action is needed?

In the previous thread I linked to, we discussed an oversimplification in the BS EN method, that indicates condensation, where in fact there cannot be.

If you think about the existing sarking after you have added the proposed insulation, in winter the existing sarking will be considerably warmer than the proposed breather membrane. Now imagine that the sarking is wet. The proposed breather membrane acts like a cold 'dehumidifier' element and attracts water vapour from the sarking, drying it out. So long as the membrane layer is more breathable than the combined (T&G, VCL and sarking) layers, more vapour will escape the construction than can enter it.

You want the biggest vapour resistance to be on the inside.

When the T&G was fitted, with 1000s of nails, presumably the VCL became well holed..? If you worry that will let more vapour through than can escape through the sarking, then you could drill some 1" holes in the sarking before fitting the new insulation.

If you examine the existing sarking now (midday in mid June) you'll probably find it is too hot to touch! and hence bone dry.

Do you think the existing wool still fills the cavity below the sarking, or will it have slumped, based on the workmanship you see?

Posted By: WillInAberdeenWhen the T&G was fitted, with 1000s of nails

but presumably (= theoretically...) the T&G should be airtight - the primary point of interest

therefore moisture from inside the room should not be able to escape into the roof structure becos no air to carry it.

So IMO, the fist point of interest should be the quaility of the ceiling (and not the hypothetical issue of the quality of the vapour barrier and/or sarking layer above it...).

The second point of interest - which then becomes primary - should be the quality of installation of the to-be-installed rainscreen, to prevent external moisture (precipitation...) (via the slates etc.) pooling on the barrier, then getting firstly onto the new insulation and secondarily into the roof structure.

So yes, PiH - obviously go for a state-of-the art installation of the new rainscreen - no deliberate rips, holes, sagging, bagging etc. ! and a proper roofing job.

gg

Posted By: Peter_in_HungaryI would rather not take down the T@G or remove the sarking boards due to the work and expense involved

The practical solution to this potential issue would be to use battens and cross-battens under your slates with a
vented eaves & ridge detail to create a min 25mm clear air path. The normal advice however is that ventilation is only needed if you use a close-fitting roof finish such as interlocking roof tiles.

My understandings in this are based on multi cases I ran when I had 8wks free access to WUFI - but using fairly mild English (Atlantic climate) weather data sets. It may be different in Atlantic but harsher Scotland, and much more so in Continental Hungary.

WUFI told me that

1) the 'biggest vapour resistance to be on the inside' old conventional wisdom/rule of thumb had no effect, whether I made that 'resistance' or 'resistivity' (a big difference, never clarified in the old wisdom).

2) with constructions fully 'breatheable' (water-vapour-open) right through, even if only moderately permeable, like EPS and OSB, 1) applied;
and interstitial condensation only happened in the outermost zone,
but was OK because it soon enough dried out with a bit of sun on an approx weekly scale, or failing that, certainly come summer - it never built up year by year to become permanently wet, and mould/rot dies in the dry period, if it even gets started.

3) adding a VCL made matters far worse!

The 'ideal' construction that this leads to is, from inside to out:
plasterboard and skim;
on 95 or 145 studs or rafters;
11mm OSB3 sheathing ootside the studs/rafters;
blown-in cellulose fibre full-filling the inter-stud/rafter space;
150 or 100 EPS fixed EWI-style over the sheathing;
down-slope battens fixed together in pairs over the ridge, to prevent sliding down the slope;
relective breather underslating felt draped over the battens, shiny side down;
battens and slates/tiles/shingles;
or instead of the downslope battens etc, acrylic render on the EPS.

Note, no timber in the sandwich, outboard of the sheathing, where interstitial condensation can occur;
the OSB, gapfilling glued and screwed and all joins nogged, is the airtight layer; even if not a super air barrier, in tandem with the blown-in cellulose, make a overall adequate, and robust, airtight buildup.

Peter's proposed builup looks like this, except that he's putting thermal-bridging rafters in the outer 150mm - no need, I'd say. Tho WUFI might give a different answer for Hungary.

Peter, hope your project goes well, if the sarking has 5mm gaps between boards then it is completely air- and vapour- permeable. (Air circulates round the boards and equalises the moisture both sides, that's what the gaps are there for). The 'problem' is something to do with a problem in the calculation, not in the construction. Good luck however you proceed!

Posted By: Ed Davies

Posted By: WillInAberdeenThe 'problem' is something to do with a problem in the calculation, not in the construction.

I think that, too. A resistance/resistivity muddle or something like that, perhaps?

But I don't know enough about the subject to sensibly differentiate between calculation and construction so the tendency would be to err on the safe side.
However if a 5mm gap makes the sarking boards effectively vapour open (which surprises me that 5mm is enough) then perhaps a re-think