My understanding of achieving good insulation/ air tightness etc is not so much great additional cost but attention to detail. Yes more insulation does cost more money but a relatively small amount. Good doors and windows are costly but necessary. I would much rather spend time and money on basic insulation/airtightness details that lots of bolt on gooodies that have a very long payback period.

I dont agree that low energy measures add 20% to the build cost.

I will try to guess at the relative proportions of say insulation MHRV, air tightness and thermal bridging later.

Well, kWh/m²/yr is fine when comparing space heating demand, which is what I'm trying to get a handle on. Clearly DHW and electricity are highly dependant on lifestyle and occupancy, space heating is also but to a lower extent.

Let's work backwards - the starting point is new-build at passive house standard (space heating demand = or < 15 kWh/m²/yr). Now let's relax the specifications so that space heating demand is between 25 and 30kWh/yr. Where can the specifications be relaxed most cost-effectively to hit the lower target in practise? Where is the trade-off between the 5 energy efficiency measures that need to be applied (fabric u-values, high-spec windows, low air-permeability, minimal thermal bridging and heat-recovery on the ventilation)?

I'm currently building to a spec of around 22kWhr/a so I might be able to outline what measures I would need to take to get to certified passive house levels.

The one complication is that I am building in the south of Sweden and the spec I am currently building to would in fact get certification if built in many parts of the UK. There is a house very similar to mine from the same kit supplier which is a little outside Dublin that was going for certification when last I heard.

Any way the biggest expense for me in lowering the heat demand would be getting better windows. I have pretty decent 3G Nordan windows and to significantly improve the spec would cost somewhere in the region of £10,000 (5% of my budget).

Improving the spec of the HRV unit to a PHI certified Paul unit would have cost an extra £2,500 and would reduce the calculated ventilation losses by almost 1,000kWhr/a, a reduction of about 4kWhr/m²/a, if I were really focused on getting certification I would have gone for it but as my budget is tight and the payback on the upgrade was 20years+ I decided against it.

As I am building timber frame I really don't know how you can otherwise "down spec" the airtightness.

We have used an insulated raft foundation, which are pretty much the norm here although ours has a higher spec than standard and incorporates a thermal break between the main slab and the ring beam which should significantly reduce any cold bridge in that area. In total our groundwork cost about £30K which included site clearance, supply and fit of UFH pipework and preparing a parking area at the front of the house. I'd guess down specing the foundations could reduce costs by maybe £10,000? Not sure what any resulting increase in heat demand would amount to.

As for other cold bridges our TF is a twin wall construction which I think is very effective at reducing cold bridges in the exterior walls. The only area where we have relatively significant cold bridges is at eaves height from the trusses and joists, I've no idea what the costs would be in eliminating these.

As for fabric u-values with the type of TF I have, reducing the quantity of insulation would result in very little in the way of cost savings halving the amount of insulation would give a huge increase in heat demand for a saving in materials of about £3,000.

Recently at the University of Reading a contractor offered a no cost option for 3g windows with European quality ironmongery and seals over the dg upvc specified so I put windows as zero.

Thermal bridging mitigation does have a cost but I dont like the idea of expressing the results of this in terms of percentages of savings, better expressed as percentage of total build.

Insulation is cheap and should be built in I would 1% of the build cost extra would take you past PH so long as you use inorganic quilt or EPS

MHRV becomes a necessity as soon as you go to any reasonably airtight building so is this not zero too?

Air tightness is a must, there is a cost but the cost is far greater if you dont have it!

Left hand column is high spec build on the right the relative additional cost above a majority build

Windows. Say 6% % uplift over chronic PVC 200%

Thermal bridging. 0.3% % uplift over insulated cavity closers 100%.

Insulation. 1.5% % uplift over building regs which require insufficient. 300%

MHRV. 5% % uplift over fans and tv's (I don't agree these are sufficient though). 400%

Air tightness 0.5% % uplift over a traditional colander 500%

Take out the windows and put in my zero cost option ones (that you wont let me have and it is a very different picture)

Also take a look at how many pvc windows are getting replaced after 10 to 20 years, a too high proportion that should be added into the real cost.

Posted By: barneyTo go from Part L to say Passivehaus equivalent would incurr a permium of around 15% in my experience - a small proportion of that in design cost, at least 50% in glass, 25% in insulatation 10% in detailing and 15% in green bling

That's great Barney - thank you very much! That's exactly what I was looking for!

How do those figures compare with other's experiences? Is glass always the "big ticket" item on the list?