Low energy new build (and intro)

[Jeremy]

Hi,

I've been pointed towards this forum by Paul (Paulh_Boats) as I'm in the process of buying a plot and building a low energy house.  My intention is to build to PassivHaus standards (or better) to minimise the energy input needed.  The plot is in a rural location in the Wye Valley, so the only fuels available for heating (if needed) and DHW are electricity, wood, coal or oil.  Burning fuels of any kind is out for me, so that means using solar and electricity as the available energy sources.

The plot is presenting some interesting challenges, as although fairly large (around 1/3rd acre) the buildable area is heavily restricted by the terrain and buried archaeology.  The good news is that we only need a house of around 100 - 120m², which will fit the plot OK.

When it comes to heat sources for DHW and supplementary heating we're restricted by the archaeology.  A horizontal or slinky GSHP system is out, because we're not allowed to dig down more than about 1/2m in the garden.  A vertical borehole GSHP system might be OK, if we pick a spot where there are no buried ruins, but will be expensive.  I know there is water down at around 30m from a well nearby and from other local borehole records, so I may be able to use a wet borehole GSHP system, depending on cost.

I'm planning on using solar for primary DHW, which should work OK as the rear roof pitch faces SSW and gets a fairly clear view of the sky all through the day.  I'd like to try and make the solar DHW system mains electricity independent, by powering the pump and controls from a battery backed solar panel.  Supplementary DHW may use something like an Ecocent system, although I'm still undecided and I may just use the supplementary heating heat pump for top up DHW on cloudy days.  Being a PassivHaus it'll use MVHR and I'm currently torn between fitting underfloor supplementary heating (probably to the ground floor only) or providing supplementary heat via the air vent system that the MVHR uses (I'm trying to fit low level MVHR vent outlets in the habitable rooms, with high level vent inlets in the hot/humid rooms, as used in North American warm air systems).  At the moment, and despite the poor COP figures, an ASHP seems the best compromise.  It would be nice to have an integrated MVHR, DHW and supplementary heating system, but from what I've been able to research so far such systems seem rare and less than optimal in performance.

The house itself will be post and beam, with the timber structure partially visible internally, clad in insulated panels.  All service ducts, conduits, ventilation ducts etc will be built into these panels where possible.  Externally the walls will be finished in a mix of larch or cedar cladding and cream lime render (planners permitting.........).  Windows will be triple glazed, probably wood with coated alloy exteriors for low maintenance.  The foundations present a challenge both because of the archaeology and because the build area is partially made up ground, held back by a retaining wall on one side.  Currently I'm looking at using helical screw piles, which have the (limited) blessing of English Heritage and may therefore satisfy the archaeologists/planners.  The alternative is a steel reinforced concrete slab, something I'd like to avoid if possible because of the high embodied energy.

I've already done a fair bit of research and reading up on various systems, read the EST report on heat pump COP and also read a lot of posts on this forum and the Green Building Forum.  If anyone has any ideas on supplementary DHW/heating options, given the constraints of the site (and my not unlimited budget!) I would be very interested to hear them.

Jeremy

[djh]

Welcome to the forum. It sounds an interesting project and your thinking appears sane A couple of random comments:

(I'm trying to fit low level MVHR vent outlets in the habitable rooms, with high level vent inlets in the hot/humid rooms, as used in North American warm air systems)

I'm not sure I understand you here. It's normal, and building regs require, extract from humid rooms, rather than inlets. So normally fresh-air inlets go to the habitable rooms. If you don't fit a post-heater in the MVHR you will need to be careful about causing cool drafts. Note that the warm-air systems use a lot of recirculated air, so their design is significantly different.

The house itself will be post and beam, with the timber structure partially visible internally, clad in insulated panels.  All service ducts, conduits, ventilation ducts etc will be built into these panels where possible.

I don't quite understand where the insulated panels go. Inside, outside or between the posts? Do you have a diagram?

[Richard Owen]

How nice to have another Wye Valley member.

We've be greening Yew Tree Cottage (see here if you want to wade through the dross) for ever. Ours is a challenging site, but nowhere near as challenging as yours.

I've also just completed a greenish refurbishment of the house next door, with rather excellent results (if I may say so myself.)

We're in GL15 if you wanted to pop along for a coffee and compare notes.

[dhaslam]

You could probably get away with just  direct off peak electricity for  heating and to supplement hot water.     A  small electric coil in the  ventilation system is a good idea and should be thermostatically controlled with a sensor on the air intake.   It would be just used to  bring the inlet air temperature  from about 18C up to say 22C in cold weather. This would only need  a few hundred watts.   You could use electric underfloor heating in  bathrooms.  The main thing is that insulation doesn't get you any heat it just keeps what you have  so rooms that are not south facing or don't have any other heat source do need heat.    PV panels should be able to offset most of your annual electricity use.  It is nice to have a stove in a living room.   Surprisingly a stove won't raise the air temperature all that much because the  heat is absorbed by the walls just as in a poorly  insulated house.

[Jeremy]

Welcome to the forum. It sounds an interesting project and your thinking appears sane A couple of random comments:

(I'm trying to fit low level MVHR vent outlets in the habitable rooms, with high level vent inlets in the hot/humid rooms, as used in North American warm air systems)

I'm not sure I understand you here. It's normal, and building regs require, extract from humid rooms, rather than inlets. So normally fresh-air inlets go to the habitable rooms. If you don't fit a post-heater in the MVHR you will need to be careful about causing cool drafts. Note that the warm-air systems use a lot of recirculated air, so their design is significantly different.

The house itself will be post and beam, with the timber structure partially visible internally, clad in insulated panels.  All service ducts, conduits, ventilation ducts etc will be built into these panels where possible.

I don't quite understand where the insulated panels go. Inside, outside or between the posts? Do you have a diagram?

Most MVHR systems I've seen here in the UK seem to use high level inlets and outlets, which seems counter-intuitive to me.  Warm air heating systems use low level outlets to get better mixing in the habitable rooms so why not do the same with MVHR?  My guess is it may be just a pragmatic choice based on ease of retrofitting systems, as I can't see any logical reason for returning warmed air to the top of a room otherwise.

The insulated panels go outside the frame, both for ease of installation and better sealing.  It is hard to get a good, reliable, seal when using infill panels as the frame will move a fair bit over the first couple of years post-construction as the timber dries to the interior relative humidity.  There are sealing systems that can deal with this movement, but the labour and cost involved makes an external panel system look better from our perspective.

Jeremy

[MarkB]

Wow, so many things to comment on.

Hi,

I've been pointed towards this forum by Paul (Paulh_Boats) as I'm in the process of buying a plot and building a low energy house.

Welcome.

My intention is to build to PassivHaus standards (or better) to minimise the energy input needed.  The plot is in a rural location in the Wye Valley, so the only fuels available for heating (if needed) and DHW are electricity, wood, coal or oil. Burning fuels of any kind is out for me, so that means using solar and electricity as the available energy sources.

Excellent target, but I'm slightly confused about the burning fuels bit.

You seem to be suggesting the use of mains electricity and a heat pump later, so you're quite happy for others to burn fuel on your behalf, but not burn it yourself?

It's highly unlikely that you'll be able to provide all your own energy on site from solar. Trying to do this brings a whole pile of problems unless you are very lucky with your plot (any hydro potential?)

I'm planning on using solar for primary DHW, which should work OK as the rear roof pitch faces SSW and gets a fairly clear view of the sky all through the day.  I'd like to try and make the solar DHW system mains electricity independent, by powering the pump and controls from a battery backed solar panel.

Unless you're planning to go off-grid, I'm not sure why you would go to the hassle of having a battery backed system. If there is heat to capture with the solar then PV will almost certainly be able to generate enough electricity. If there isn't enough electricity coming out of the PV to power the pump then there probably won't be much heat to capture from the thermal system. Trying to capture the minimal heat when there is enough in the panels but not enough light to power a pump almost certainly is so far down the diminishig returns curve as to not be worthwhile.

By all means use PV to power the pump, but consider carefully if it is worthwhile going to a battery backed system.

Being a PassivHaus it'll use MVHR and I'm currently torn between fitting underfloor supplementary heating (probably to the ground floor only) or providing supplementary heat via the air vent system that the MVHR uses (I'm trying to fit low level MVHR vent outlets in the habitable rooms, with high level vent inlets in the hot/humid rooms, as used in North American warm air systems).  At the moment, and despite the poor COP figures, an ASHP seems the best compromise.  It would be nice to have an integrated MVHR, DHW and supplementary heating system, but from what I've been able to research so far such systems seem rare and less than optimal in performance.

It's good that you have accepted a supplementary heat system is probably needed. Many people confuse PassivHaus with zero heating - this isn't the case and some heating will be required in the coldest of weathers.

There is no easy system for this.

Underfloor heating can be problematic because it is usually couple with high thermal mass slab which will take a long time to heat up and without very careful control will lead to overheating and wasted energy. It's also quite expensive.

Heating through the MVHR is potentially feasible, and has been used widely in PassivHaus but not always satisfactorily. The problem is that the extract air is taken from the humid areas, e.g. bathroooms, where you need heat the most, and the supply air goes to the living rooms that you don't generally walk around without clothes in... So the heat is suppied to the wrong place. You also need insulated ducting to ensure the hot air gets to the end of the ducts and you need to make sure that there are no dead spots in the air flow that won't get enough heat.

I have heard several comments of PassivHaus' in the UK where duct heating isn't working as well as expected.

I think we've probably settled on the use of low temperature radiators. These are cheap, can be distributed to the coldest parts and have low inertia so are easily controlled.

The house itself will be post and beam, with the timber structure partially visible internally, clad in insulated panels.  All service ducts, conduits, ventilation ducts etc will be built into these panels where possible.  Externally the walls will be finished in a mix of larch or cedar cladding and cream lime render (planners permitting.........).  Windows will be triple glazed, probably wood with coated alloy exteriors for low maintenance.  The foundations present a challenge both because of the archaeology and because the build area is partially made up ground, held back by a retaining wall on one side.  Currently I'm looking at using helical screw piles, which have the (limited) blessing of English Heritage and may therefore satisfy the archaeologists/planners.  The alternative is a steel reinforced concrete slab, something I'd like to avoid if possible because of the high embodied energy.

Be careful of the potential thermal bridging issues of using piles. It will be impossible to thermally isolate them from the main structure of the house and they will be a weak point. While a concrete slab isn't the most environmentally obvious choice you can put the insulation under the slab and implement a continuous insulation layer aroudn the structure. It makes the thermal calculations much simpler. (Also, whil;e not great, in-situ concrete isn't quite as bad as many think - see http://www.greenspec.co.uk/embodied-energy.php for relative emodied energy content of different materials.)

I  hope this helps.

[Jeremy]

How nice to have another Wye Valley member.

We've be greening Yew Tree Cottage (see here if you want to wade through the dross) for ever. Ours is a challenging site, but nowhere near as challenging as yours.

I've also just completed a greenish refurbishment of the house next door, with rather excellent results (if I may say so myself.)

We're in GL15 if you wanted to pop along for a coffee and compare notes.

Thanks Richard, Paul has passed me your contact details already and I may give you a call to pick your brains.  I've read your interesting thread, your house is maybe two or three miles from the plot we're buying, just across the other side of the river/border.  My next problem to overcome is finding either a local main contractor or a project manager, as we currently live a 2 hour drive away from the site.  First I need to deal with some issues over "indistinct" boundaries and rights of way, then we should be all systems go (depending on the speed with which the planners approve reserved matters...............).

Jeremy

[Jeremy]

You could probably get away with just  direct off peak electricity for  heating and to supplement hot water.     A  small electric coil in the  ventilation system is a good idea and should be thermostatically controlled with a sensor on the air intake.   It would be just used to  bring the inlet air temperature  from about 18C up to say 22C in cold weather. This would only need  a few hundred watts.   You could use electric underfloor heating in  bathrooms.  The main thing is that insulation doesn't get you any heat it just keeps what you have  so rooms that are not south facing or don't have any other heat source do need heat.    PV panels should be able to offset most of your annual electricity use.  It is nice to have a stove in a living room.   Surprisingly a stove won't raise the air temperature all that much because the  heat is absorbed by the walls just as in a poorly  insulated house.

Thanks, I have been considering the idea of a simple electric supplementary heating/DHW system, but on balance would rather go for a heat pump that at least gives a modicum of gain over direct electric heating.  I'm slightly put off a grid tie PV system because ethically I think the present incentive system is wrong.  Maybe I'm just an oddball, but having a system that relies on a heavy government subsidy to make it cost effective just seems wrong to me. 

The MVHR will be designed to redistribute heat around the house, moving the warm air from those rooms with high solar gain to those that don't.  The house will be designed to make the most of the solar gain on the SSW facing side and minimise losses on the cooler NNE facing side.  It will have a low thermal inertia, which is one reason that I think supplementary heating may be needed, for example, to quickly get the interior up to temperature after a period of being away.  We lived in a very high thermal inertia house in Cornwall for some years and found it to generally be a nuisance.  It was great at staying cool in summer, but took several days with the heating on full blast to get it back to temperature after a period of it being unoccupied.  We also lived in a Norwegian built timber framed house in Scotland for some years, which was the opposite.  It was highly insulated and had a low thermal inertia.  Despite having oil central heating it was cheap to heat and best of all warmed up within an hour or so of turning the heating on.  This convinced me that high levels of insulation coupled with a low thermal inertia was the sort of house we preferred to live in.

A stove is out - I couldn't tolerate the high air throughput it would need and the house wouldn't then come close to meeting PassivHaus standards (unless it was a sealed, externally vented, unit, perhaps).  Also, having lived with a big woodburner for years, I really don't want to go back to having to deal with stacks of seasoned firewood and the clean up all the ash and dust that burning solid fuels creates.

Jeremy

[Jeremy]

Excellent target, but I'm slightly confused about the burning fuels bit.

You seem to be suggesting the use of mains electricity and a heat pump later, so you're quite happy for others to burn fuel on your behalf, but not burn it yourself?

It's highly unlikely that you'll be able to provide all your own energy on site from solar. Trying to do this brings a whole pile of problems unless you are very lucky with your plot (any hydro potential?)

Thanks for the welcome.  The burning of fuels thing comes down to a few factors.  PassivHaus means the building will be airtight, so any form of conventional internal fuel burning heating appliance is out.  There's no gas supply anywhere near, so my fuel burning choices come down to oil, LPG or an externally fitted solid fuel system (perhaps a pelletised wood system).  The house footprint is limited and defined by the planners and archaeologists, so I don't have space next to the house to build a heating installation.  The house heating and DHW requirement will be modest, so an oil or LPG installation is unlikely to run efficiently and would have a high running cost.  All told, this restricts me to using either direct electricity or electricity driving a heat pump to get better efficiency and reduced cost.  Using an electric system also has the potential to be easier to control, depending on things like thermal time constants.

It's a fact that power generators can burn fuel more efficiently than domestic users as a rule, even accounting for transmission losses, so ethically it could be argued that using electricity is probably more environmentally sound than burning oil or gas.  Add in that I could choose to use a renewable electricity provider and that lowers the overall carbon footprint to near-zero.

I'd love to use hydro, but the nearby fast flowing stream is about 50m away from the boundary of the plot.  If the site extended down to the stream then it'd make the ideal GSHP input, too.

Unless you're planning to go off-grid, I'm not sure why you would go to the hassle of having a battery backed system. If there is heat to capture with the solar then PV will almost certainly be able to generate enough electricity. If there isn't enough electricity coming out of the PV to power the pump then there probably won't be much heat to capture from the thermal system. Trying to capture the minimal heat when there is enough in the panels but not enough light to power a pump almost certainly is so far down the diminishig returns curve as to not be worthwhile.

By all means use PV to power the pump, but consider carefully if it is worthwhile going to a battery backed system.

I'm an inveterate tinkerer (a retired scientist) and have been playing with efficient electric systems for years.  I detest being reliant on the grid, even where I live now it annoys me intensely when the power goes off and we have no heating or hot water.  The modest electrical load that the solar pumping and control system will use is ideally suited for a small PV and battery back up system, in fact one about half the size I have powering my solar power river boat would do the job a treat.  The additional capital cost would be modest, and would probably be offset by the freedom from running cost within 5 to 10 years.  For me, not needing the grid to be able to have a shower would be the real bonus.

It's good that you have accepted a supplementary heat system is probably needed. Many people confuse PassivHaus with zero heating - this isn't the case and some heating will be required in the coldest of weathers.

There is no easy system for this.

Underfloor heating can be problematic because it is usually couple with high thermal mass slab which will take a long time to heat up and without very careful control will lead to overheating and wasted energy. It's also quite expensive.

Heating through the MVHR is potentially feasible, and has been used widely in PassivHaus but not always satisfactorily. The problem is that the extract air is taken from the humid areas, e.g. bathroooms, where you need heat the most, and the supply air goes to the living rooms that you don't generally walk around without clothes in... So the heat is suppied to the wrong place. You also need insulated ducting to ensure the hot air gets to the end of the ducts and you need to make sure that there are no dead spots in the air flow that won't get enough heat.

I have heard several comments of PassivHaus' in the UK where duct heating isn't working as well as expected.

I think we've probably settled on the use of low temperature radiators. These are cheap, can be distributed to the coldest parts and have low inertia so are easily controlled.

Like you, I think that a lot of people don't understand the PassivHaus concept and somehow believe that no supplementary heat is needed.  The key issue for me will be getting the heat distribution system right, something that seems to frequently cause problems with low energy heating.  As I mentioned originally, the ducting will be inside the super-insulated wall panels, to minimise losses.  We don't want radiators (only because they tend to get in the way) and the underfloor heating won't be in a high thermal mass slab because there isn't a slab, so it's thermal inertia will be modest.  The heating pipework (if we opt for an underfloor system) will be embedded in the highly insulated suspended floor, fitted between the timber perimeter ring beam that rests on the pile caps.  There will be a ventilated under croft to allow the underfloor area to breathe.

Be careful of the potential thermal bridging issues of using piles. It will be impossible to thermally isolate them from the main structure of the house and they will be a weak point. While a concrete slab isn't the most environmentally obvious choice you can put the insulation under the slab and implement a continuous insulation layer aroudn the structure. It makes the thermal calculations much simpler. (Also, whil;e not great, in-situ concrete isn't quite as bad as many think - see http://www.greenspec.co.uk/embodied-energy.php for relative emodied energy content of different materials.)

I  hope this helps.

Not a significant problem, I think, as the insulated floor sits above the ground and the piles are only 60mm diameter steel tubes anyway.  There is a small thermal bridge from the pile cap plates to the perimeter timber ring beam, but this ring beam only thermally connects to the interior via the timber post ends.  Given that these are thick pieces of wood, the thermal conductivity and limited conductive area are low enough for bridging here not to be a problem (I hope!).

Jeremy

[djh]

Most MVHR systems I've seen here in the UK seem to use high level inlets and outlets, which seems counter-intuitive to me.  Warm air heating systems use low level outlets to get better mixing in the habitable rooms so why not do the same with MVHR?  My guess is it may be just a pragmatic choice based on ease of retrofitting systems, as I can't see any logical reason for returning warmed air to the top of a room otherwise.

Bear in mind that the incoming air in an MVHR system is still *colder* than the air in the room, unless there is a heater. Passive heat exchange is not 100% efficient. So unheated MVHR systems get better mixing by using high-level room inlets.

The insulated panels go outside the frame, both for ease of installation and better sealing.  It is hard to get a good, reliable, seal when using infill panels as the frame will move a fair bit over the first couple of years post-construction as the timber dries to the interior relative humidity.  There are sealing systems that can deal with this movement, but the labour and cost involved makes an external panel system look better from our perspective.

Right, that makes sense.

the underfloor heating won't be in a high thermal mass slab because there isn't a slab, so it's thermal inertia will be modest.  The heating pipework (if we opt for an underfloor system) will be embedded in the highly insulated suspended floor, fitted between the timber perimeter ring beam that rests on the pile caps.  There will be a ventilated under croft to allow the underfloor area to breathe.

the insulated floor sits above the ground and the piles are only 60mm diameter steel tubes anyway.  There is a small thermal bridge from the pile cap plates to the perimeter timber ring beam, but this ring beam only thermally connects to the interior via the timber post ends.  Given that these are thick pieces of wood, the thermal conductivity and limited conductive area are low enough for bridging here not to be a problem (I hope!).

Bear in mind that the time constant of the heating system depends on how well insulated the house is as well as the thermal mass of the slab. So a fast response in a normal house may well turn out to be moderate or even slow response in a passivhaus.

One point to consider with a timber ring beam is the height of the finished floor above ground level. The timber needs a clearance above ground and the ring beam will be quite deep, so providing level access can be tricky. It could also make any ridge height limitations more difficult. Otherwise, I like screw piles. Do make sure to do the thermal bridge calculations early, as this area seems to be under-researched, so there's scope for good or bad solutions. For the modelling, you will need Therm or something better.

[MarkB]

By all means use PV to power the pump, but consider carefully if it is worthwhile going to a battery backed system.

I'm an inveterate tinkerer (a retired scientist) and have been playing with efficient electric systems for years.  I detest being reliant on the grid, even where I live now it annoys me intensely when the power goes off and we have no heating or hot water.  The modest electrical load that the solar pumping and control system will use is ideally suited for a small PV and battery back up system, in fact one about half the size I have powering my solar power river boat would do the job a treat.  The additional capital cost would be modest, and would probably be offset by the freedom from running cost within 5 to 10 years.  For me, not needing the grid to be able to have a shower would be the real bonus.

Well, the heat will be pumped from the solar into some kind of cylinder/store during the day, so you only need a pump when loading the cylinder/store, which should be adequately by a direct solar panel without batteries. You shouldn't need any power to extract heat from the cylinder/store (unless you're going for a thermal store with an external plate heat exchanger).

Of course, if you're doing it for fun and you can DIY a system then go for it.

As I mentioned originally, the ducting will be inside the super-insulated wall panels, to minimise losses.

With the exception of the main external supply and extract ducts, all of the ducting should be fully inside the thermal envelope if at all possible. If you intend to put some of the ducting partway through the insulation then you will need to be careful to model the additional losses from the ducting to the outside. This may not be trivial to do in PHPP.

I'd push for all the internal ducting to be fully inside the thermal envelope. It will perform better and be much simpler to model.

[Jeremy]

Bear in mind that the incoming air in an MVHR system is still *colder* than the air in the room, unless there is a heater. Passive heat exchange is not 100% efficient. So unheated MVHR systems get better mixing by using high-level room inlets.

That makes sense to me.  I had wondered why the use of high level vents was so prevalent.  Given that installing low level ducting will be potentially harder (and have greater duct loss, assuming a roof space mounted MVHR unit) then it may swing things in favour of independent MVHR and supplementary heating.  This then makes underfloor look somewhat easier to do and probable easier in terms of finding a suitable heat source.

Bear in mind that the time constant of the heating system depends on how well insulated the house is as well as the thermal mass of the slab. So a fast response in a normal house may well turn out to be moderate or even slow response in a passivhaus.

I have a suspicion that this is the thing that tends to cause more heating related problems (in terms of even heat distribution) with low thermal inertia houses.  We had significant control problems with the low thermal inertia Norwegian house we had in Scotland, and I ended up with three different control zones, all independently programmed, to get a good, economic and evenly heated house.

One point to consider with a timber ring beam is the height of the finished floor above ground level. The timber needs a clearance above ground and the ring beam will be quite deep, so providing level access can be tricky. It could also make any ridge height limitations more difficult. Otherwise, I like screw piles. Do make sure to do the thermal bridge calculations early, as this area seems to be under-researched, so there's scope for good or bad solutions. For the modelling, you will need Therm or something better.

The current plan is to scrape the site down to the proscribed depth (set by buried archaeology) over the area of the house footprint, then pile, install underground services (insulated where they rise to the floor), lay a membrane, lay a thin layer of coarse chippings (primarily for vermin control).  The result will be that internal floor level should be around 200mm above the ground level at the back of the house and about 400mm above ground level at the front of the house (the plot slopes slightly).  Ridge height will remain within the 8m maximum above the datum (set by the front edge of the house footprint on the ground, pre-excavation).

All this may go for a ball of chalk if we can't use screw piles for some reason, which would mean looking again at the reinforced slab.

The thermal calcs look pretty straightforward, as the pile CSAs are known, all the material thermal conductivities are known and the areas of thermal contact are known.  Ten minutes with Excel should give me a good enough answer to know whether or not there's a potential problem with bridging into the pile caps.

Jeremy

[djh]

The thermal calcs look pretty straightforward, as the pile CSAs are known, all the material thermal conductivities are known and the areas of thermal contact are known.  Ten minutes with Excel should give me a good enough answer to know whether or not there's a potential problem with bridging into the pile caps.

There will be a psi-value. You need a numerical answer to plug into PHPP, and typically there's 2D flow. You can see some examples in the Passivhaus details book, and also on the AECB forum, IIRC.

[Jeremy]

There will be a psi-value. You need a numerical answer to plug into PHPP, and typically there's 2D flow. You can see some examples in the Passivhaus details book, and also on the AECB forum, IIRC.

Yes, but it's all pretty basic physics and maths, none of it is particularly complex.  I can understand why PHPP and other tools have been developed, to allow a simplified approach for those who'd rather not work from first principles, but the core physics behind heat conduction and radiation aren't that complex, it's only when dealing with the intricacies of the convection component in mixed, gently unstable flow that things get to be really fun to model.  The worst case will be conductive loss from cold wind ("wind chill"), where convective losses are tiny enough to be pretty much ignored, at least in these simple models used in the building industry.

Jeremy

[djh]

I don't want to keep labouring a point, since you obviously know what you're doing, but you said you wanted to build to the Passivhaus standard. That requires you to use PHPP. The complication comes with the boundary conditions and the particular conventions and approximations that have been chosen. So, yes, it's back-of-an-envelope calculation but it's their envelope and its been calibrated against reality for over a decade.

One strange aspect of this particular detail is that there's no common definition for the material property that makes an appropriate figure of merit to choose the substance(s) the pile structure is made from.