The way to think of insulation and airtightness is not 'how much fuel (fossil or renewable) will this thickness/expenditure save me, therefore what's the payback time or Return on Investment, assuming various future fuel-price rises?', and worrying about diminishing returns.

For a start, that thinking ignores the additional 'payback' consideration of capital property value accelerating increasingly ahead of the market, as buyers/renters (quite soon now) get wise and start discounting off their offer price the 8 yr future cost of heating this building against that one.

That kind of 'payback' will shortly put the traditional running-cost savings 'payback' in the shade, and will become the no-brainer conventional-wisdom incentive-for-all that will suddenly kickstart popular demand for serious insulation/airtightness etc, leave Building Regs etc trailing in the dust, and rapidly devalue the investment of unfortunate people who have recently spent their all on a 'state-of-the-art' new sticks-and-cardboard estate house, which is incapable of the thermal upgrading that everyone will be demanding. It's really time to be wholeheartedly the first of the new, not the last of the dinosaurs.

Apart from all that, 'how much fuel will this thickness/expenditure save me?' thinking is fundamentally obsolete. We should now be thinking 'how much thickness/expenditure do I need, as part of a design to get this building off fuel-burning (fossil or renewable) altogether, for heating?'

It's about reaching that threshold where all that plumbing and gubbins can be ditched completely,
in a building that relies entirely on the plentiful free solar heat that's available during the heating season,
gathered by the arrangement of the building itself (not wet or PV solar panels, windmills, heat pumps etc) in a way that doesn't cause solar overheating,
and stored in a way that gives nearer 10 days' storage, than the 1.5-2 days that is all that present Passivhaus-type techniques can offer.

It's Passivhaus's deficient duration of storage that still creates need for a back-up heating system, not any deficiency of insulation/airtightness. No further improvement of insulation/airtightness will finally get rid of the heating system; only prolongation of solar heat storage duration. Given that, you can relax a little bit on superhuman standards of insulation/airtightness, and big SE/SW glass area is actually a necessary part of the design.

Posted By: sinnerboy
The sensitivity analysis tender specified varying improved U Values for assessment

Walls 0.21, 0.15 and 0.12 W/m2K.
Cold Roof 0.16 , 0.14
Warm Roof ( ceiling slopes with rafter) 0.16
Ground floor - 0.21 , 0.15

So aa44 the day is approaching fast , I believe , when there won't be a choice of 0.15 vs 0.12 for the external wall U Value

I can't wait to see the 'robust' details!

Posted By: fostertomIt's Passivhaus's deficient duration of storage that still creates need for a back-up heating system, not any deficiency of insulation/airtightness.

I generally agree with that but would point out that there are still large savings that could be made by the use of insulated shutters. Windows lose a lot of heat even though they also [solar] gain a lot. When I played around with PHPP, shutters easily halved the heat demand.

Posted By: sinnerboyIn Ireland we are expecting a revised Part L this year . The aim is to bring min b regs up "to passiv standards"

AFAIK in UK the goal of 'passiv' standard is going to be reached by lazy stages. If Ireland's 2010 uprate gets there in one jump, UK's will go one third the way in 2010, two thirds in 2013 and the full Code 6 'aspiration' in 2016. Semi-independent Wales intends to get there by 2013 - anyone know about Scotland and NI?

So in Ireland,

Posted By: sinnerboypretty soon min b regs requirements will move the debate on

but in UK

Posted By: fostertompopular demand for serious insulation/airtightness etc (will) leave Building Regs etc trailing in the dust

<blockquote><cite>Posted By: fostertom</cite> a building that relies entirely on the plentiful free solar heat that's available during the heating season,
</blockquote>

Tom, I quite agree with your general thrust, but would point out that this winter there were long periods of very cold and very cloudy weather with precious little solar energy available for collection wehther by active or passive means. My hot water (solar thermal only, running in 'guinea-pig' mode :-) remained decided chilly for pretty-much all of january, with only a few days of feeble warmth. Now that's not a normal winter in recent times, and the year before there was only one period of 5 days (and one of 4 days) without significant heating all winter. But the UK does have an unfortunate climate that produces cold and cleggy at least as often as cold and bright, and I'm not entirely convinced that even with 10-day storage and super-insulation we can live entirely on solar. And is 10-day passive storage actually practical?

Certinaly we should try and get as close as is practical but the terrific UK mismatch between solar availability and heating requirement in mid-winter is a real limitation.

Tom

Put some numbers to it, think you will find that the insulation become rather thick.

(Try a U-value of 1.16x10^-5 W/m^2K for a 10 K temp difference)

Tom

To storing solar energy as heat for 10 days. I come up with needing ridiculous amounts of insulation (@deltaT=10) and and that is before I take into account phase shifts in time and varying inputs.

@Viking House

1. That's pretty much the setup I am looking at.

2. No, but I will!

3. Yes, there will be a MHRV system in the house.

Thanks for your interest.
AA

I am planning to use one of the specialist ICF ledger connectors so that I can attach a ledger board to the inside wall.

The different manufacturers have different ways of getting the extra thickness. Some (e.g. Beco, Logix and Polarwall) have extra thick blocks. Some (e.g. QuadLock and Nudura) have methods of attaching extra foam to the inner face of the outer leaf. Some (e.g. Amvic) attach it to the exterior. Personally I like the idea of attaching it to the inner face of the outer leaf because that still leaves the attachment points at the outside of the block. Where the suppliers use thicker blocks you can sometimes end up with the fixing points buried in the middle of the block. Where I have been given specific pricing for the extra thickness, it works out dearer than just buying extra boards and sticking them on so that is something to think about.

Posted By: Mike George

Posted By: djh"The above chart and table relates specifically to heat losses through a wall via conduction. Ie, exactly what is calculated using the u-value."

I don't believe that definition of U-value is correct is it? The U-value is the total heat transmission coefficient, as measured with a guarded hot box, not just that for conduction. EN ISO 8990 if my googling is good. Low-emissivity coatings on glazing affects its U-value, for example.

That's a fair comment, but it is the thermal conductivity [k or lambda value] which is measured via a hot plate. The u-value being subsequently calculated using said k-value.

Well, I didn't mention hot plates but hot boxes. I think there's a lot of confusion because of approximations made in the building industry where small factors are just ignored. My understanding is that the U-value is the total transmission coefficient and it can be measured directly with a guarded hot box. Specifically that means that it is NOT derived directly from lambda, which is the thermal conductivity because:
(a) the U-value includes the thermal resistance of the surface air layers, which lambda clearly does not
(b) the U-value includes radiative and convective transfer, which lambda does not

An approximation to the U-value can also be calculated, as you say. I don't know much about hot plates. I see that they state they measure conductivity but I'm not sure how they exclude convection within the material? Radiation appears to be excluded by physical contact between the test material and the apparatus.

I am not sure to what extent emissivity affects the lab tests and this has been an area of some debate There is a link somewhere to a NPL report which discusses this [see multifoil thread]. There has also recently been an attempt [same NPL report into multifoil] to incorporate the affect of air infiltration / ventilation losses though such losses are certainly not factored in generally.

Indeed, but infiltration/ventilation is not the same as convection and convection is part of the U-value but not the lambda.

As you say, the numbers are quite close however you do the sums, which is why the approximations are used and confusion can consequently arise about the definition.