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General: PH barn?

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      CommentAuthorfostertom
    • CommentTimeSep 7th 2026 edited
     
    Part Q barn conversion, steel frame portal, 115m2 2-bed, almost cubic, severe restraint on vertical/headroom, every milimetre counts.

    Hence only 25 PIR over the extg reinf conc slab with 32 timber floating flooring, but downstanding 'coffer dam' of 250 EPS (continuous with wall insul) in slit trench 1200 deep, french drain at bottom, gives floor 0.25U, the BldgRegs Table 4.2 'improved' figure, as the whole is a Material Change of Use, not newbuild. The block of subsoil beneath, inside its 'coffer dam', will somewhat adopt an intermediate temp which will be partly-ground-coupled to the interior via the weak insulation.

    Blockwork ground floor and stairwell partitions built off the slab on insulating strip stuff (so penetrating the weak floor insul hardly matters) add more interior massiveness. As ever-strengthening heatwaves become the norm, overheating at least as important as winter loss, internal massiveness has new importance.

    Roof and walls can have extremely good insulation, airtightness and continuity, copiously through-ventilated (vertical bris-soleil?) to keep cladding cooler, as well as shielding from winter clear-sky cold. Windows thoroughly shaded by brise soleil etc.

    Question is - is PH, no heating system, within reach? Though not the classic PH approach, would PHPP be impressed by insulation elsewhere compensating for the floor, internal massiveness and ground coupling particularly as affecting overheating, walls and roof shading?

    If not, it's a centralised plan, so AAHP with very short ducts, incl AC (why not, as well as the passive overheating measures?) which might do the DHW, and/otherwise night-store, and wet solar - is that still a thing? MVHR of course.
  2.  
    Posted By: fostertomQuestion is - is PH, no heating system, within reach?


    I designed our 130m2 house, using the PHPP, to not need a conventional central heating system. The walls and roof had a U factor of 0.095W/m2K and floor 0.1W/m2K. The airtightness was 0.47ACH. It was heated using an electric towel rail in each of the three bathrooms and 4m2 electric UFH in the kitchen. I installed a Genvex Combi 185L which provided the DHW and the EASHP provided the warm air heating through the integrated MVHR when the weather was very cold. The house was built in the SE of England two miles from the coast.

    I would have thought it would be difficult to compensate for the floor sufficiently to not need a central heating system. A ducted A2A system sounds good.
  3.  
    Will it be single storey or 2 storey?
    Our current project is similar floor area but we struggled to meet PH on a single storey building because it goes on total floor area not volume. So if we had say an extra metre in height, we could have squeezed a second floor in it, with not much more total volume and it would have met the PH standard with the same energy use. It's a bit daft how that works. There's only 2 of us so we don't need a first floor anyway. We have gone for 2 bedrooms and fully wheelchair accessible dimensions so that it's future proof. Retirement bungalow!
  4.  
    We will need some heating, so because we were redoing the floor slab, we super insulated under it and put UFH pipes set in the slab. We have gone for GSHP mainly because we have the field space and were so impressed with the previous one (although this one is a Shoebox model because the heating requirement is so low) and this does the Hot Water as well.
  5.  
    Also might be worth bearing in mind that you could go back in with a Full Planning application if it would be better (especially thermally) to go beyond the works allowed for under Class Q, e.g. externally insulate etc. this has been justified before by aiming for PH standards.
    I do this a lot in my line of work.
  6.  
    PH was a response to a 1990s problem: very high-carbon heating.

    In the 2030s and beyond the problems are different - embodied carbon from highly processed materials; unknown trajectory of summer heating; excess renewable energy at some times and deficit others

    Maybe start with those problems and design a solution, doesn't have to be PH? Reusing existing steel and RC slab is good. Jumping through hoops of thick polystyrene to comply with insulation targets is maybe not the 2030s+ response when low carbon heat+cooling tech is widely available now?



    Did you find a way to 3D model the insulation value of the polystyrene skirt? Intuitively the 'aspect ratio' of a 1.2m deep skirt around a 7(?) m wide slab feels a bit shallow, espc at corners, but maybe you found some sums about this?

    Any foundations issues with deep trench of squishy polystyrene? Support ok for the edge of the slab, what foundation does the steel frame have?
    •  
      CommentAuthordjh
    • CommentTimeSep 8th 2026
     
    Posted By: Dominic CooneyIt's a bit daft how that works.
    More floor space is generally useful, so I'm not sure why you think it's daft?
    •  
      CommentAuthorfostertom
    • CommentTimeSep 8th 2026 edited
     
    Thanks gents, v gd. It's 2-storey and almost cubic, so optimum in that respect, tho the smaller a PH gets - only 115m2 - the tougher the challenge.

    Will I like your first line - I'll use that! + next 2 paras. Saying PH, I really mean 'not needing a heating system' (but if we have to, an AAHP) and making use of PHPP. Whether to pursue that or not, with the weak floor insulation, is really my question.

    Peter, could you add to "difficult to compensate for the floor sufficiently to not need a central heating system" - you're poss right, but can you say exactly how/why?

    Making extensive use of my friend ChatGPT, surprised to discover that downstand perimeter insulation doesn't require modelling in Therm, but is straightforwardly calculated according to BS EN ISO 13370, best known as origin of the P/A calc that's incorporated in Bldg Regs, but also extending to perimeter downstand insulation, indeed as far as having only that, without any under-slab insulation. Algebra being all greek to me, I can't verify the formulae that ChatGPT offers, but trusting so far that it's right, we've thoroughly explored the several variables and what I've outlined above seems optimal.

    Yes, deep slit filled with squish, especially as it's in the 2m wide flat bottom outboard of the barn, of an excavation from 0m to 4m high that was cut as flat site for the barn - will need Engineer, maybe cellular propping of the trench and/or substituting 600mm 'thick' Leca or in fact a wedge of Leca from say 1.2m wide to 150 wide at bottom. And yes, not undermining slab or col bases.
  9.  
    Posted By: djhwhy you think it's daft?


    because a building of a given volume could have 30kWh/sqm/annum at single storey but 15kWh/sqm/annum at 2 storey, but the same energy use for the building volume. I think it misrepresents the energy use of the building.
    •  
      CommentAuthordjh
    • CommentTimeSep 8th 2026
     
    Posted By: Dominic Cooney
    Posted By: djhwhy you think it's daft?

    because a building of a given volume could have 30kWh/sqm/annum at single storey but 15kWh/sqm/annum at 2 storey, but the same energy use for the building volume. I think it misrepresents the energy use of the building.
    But the floor area is what is important. You can't make effective use space above 2.5m except for special cases, so PHPP doesn't count it.
  11.  
    In this case, the daft bit is that Dominic only needs and wants the one floor that the existing building provides, so more floor space is not useful in this case. He could go to major expense to get pp to raise the roof a metre and add an unnecessary second floor that is never used, and PHPP would say that halves the kWh per m2 so now he can have a PH plaque, but that would be daft!


    No metric is ever perfect and the kWh/m2 metric rewards bigger buildings (>100m2) with more floors. Good in a city for blocks of flats, but bad if it leads to oversized rural retirement living, or barn conversions that spill into huge extensions. Whereas the BR 'equivalent building' approach doesn't encourage efficient building shapes, so can't win them all.

    ETA an EPC will be issued based on energy cost per m2 floor area, with the same daftness.
  12.  
    @Tom, is the building inspector going to accept ChatGPT's design calcs that you can't verify? It's a wild new world for professional designer people!

    Does ChatGPT have PI insurance in case there's a problem later in the project, or will the AI-powered lawyers still come looking for you?!!
  13.  
    We are at about 3.5m max internal room height down the centreline where the ridge is. We followed the 15 degree roof pitch with the ceilings. Internal height at the eaves is 8 foot (2440mm) one sheet of OSB.
  14.  
    PHPP says that it will be a PHi Low Energy Building, but not a fully certified Passive House. We are happy with that. The software also assessed overheating and that is okay, so we are glad that we got it put through the software.
  15.  
    Posted By: fostertomPeter, could you add to "difficult to compensate for the floor sufficiently to not need a central heating system" - you're poss right, but can you say exactly how/why?


    From my own limited experience, I've only designed and built one house, I find it difficult to understand how improving the other areas of insulation and airtightness in the building can get you near to a heat loss value which would allow the building to be heated without a central system. I ran a large number of iterations of PHPP before I found a set of data which would allow the building to be built for a reasonable cost and maintained at 23C. I was under some pressure to ensure that 23C was achievable, or the project wouldn't have continued. I couldn't have managed that with an overall floor U value of 0.25W/m2K. This was done sixteen years ago so maybe software and materials have improved and it may now be possible.
  16.  
    Are you averse to redoing the floor slab?
    If the floor area is only 57.5sqm (the ground floor of your 2-storey 115sqm) it's not that big, and you can dig out to gain a bit of internal height. Crush the old concrete and use it as hardcore in the base of the newly excavated floor (as we did) and also useful if you need any hardcore elsewhere, without having to buy in virgin material. We dug out level with the top of the steel foundation cubes and managed to get 300mm graphite EPS down under the new slab at 115 - 120mm
  17.  
    Or is your building 115sqm x 2 floors so 230sqm floor area of house?
  18.  
    Another thought - does the existing barn floor slab have a DPM, and is it laid sloping to drain?


    I had understood that Part Q is strictly for 'conversion' of existing buildings and there are limits how much of the fabric can be demolished and replaced, is this enforced?
    •  
      CommentAuthordjh
    • CommentTimeSep 8th 2026
     
    Posted By: Dominic CooneyWe are at about 3.5m max internal room height down the centreline where the ridge is. We followed the 15 degree roof pitch with the ceilings. Internal height at the eaves is 8 foot (2440mm) one sheet of OSB.
    That doesn't sound anything like a maybe-two-floors situation. So I still don't understand your 'daft' comment.
    •  
      CommentAuthordjh
    • CommentTimeSep 8th 2026
     
    Posted By: PeterStarckI find it difficult to understand how improving the other areas of insulation and airtightness in the building can get you near to a heat loss value which would allow the building to be heated without a central system.
    FWIW, that's exactly what we did. We have a lot of insulation under the floor (400 mm of grey EPS) and in the roof (450 mm of Warmcel) and that compensates for the marginal straw bale walls. And it's airtight.
  21.  
    Posted By: djhFWIW, that's exactly what we did. We have a lot of insulation under the floor (400 mm of grey EPS) and in the roof (450 mm of Warmcel) and that compensates for the marginal straw bale walls. And it's airtight.


    It's a shame Tom can only have 25mm PIR under the floor otherwise it would be more straightforward. We had 300mm Peripor under the floor which is one reason why we didn't have to have central heating.
    •  
      CommentAuthorfostertom
    • CommentTimeSep 8th 2026 edited
     
    Thanks gents. I wrote quite a bit, commenting, before Dave's response to Peter next above - but somehow lost it. Then Dave's message of hope, prob all I need before tomorrow's make-or-break meet.

    Any other last desperate measures to beat Peter's conundrum? Before 10am please!

    Here I go - given heatwave-willingness for unprecedented comprehensive calculated effective brise soleil, hence unusually large southern glazing free of overheating penalty, what effect would that boost to winter intermittent solar gain have, seeing that AFAIK PHPP mainly relies only on reveal depth, overhangs and limits to southern window size, rather than full brise soleil? Could that extra intermittent gain more than overcome, over time, the accompanying winter loss?
    •  
      CommentAuthordjh
    • CommentTimeSep 8th 2026
     
    Posted By: fostertomThen Dave's message of hope,
    I did say marginal straw bales (U=0.116), not off-by-a-mile 25mm PIR (U=0.25)! Have you considered aerogel? Or VIPs? Or don't care that much about PH?
  24.  
    Posted By: djhSo I still don't understand your 'daft' comment.


    WiA summed it up quite well. A small increase in height would allow a second floor, then surprise surprise you suddenly meet the PH standard on floor area for the building, because of the second floor, even though the volume has barely increased, and the energy use of the whole building has barely increased.
    •  
      CommentAuthorfostertom
    • CommentTimeSep 8th 2026
     
    Posted By: djhdon't care that much about PH?
    More that I'd like to save the cost of a heating system.
    Any response to my last para above? Failing super-insulation, instead boost winter solar gain, without summer overheating consequence. Comment, anyone?
    •  
      CommentAuthordjh
    • CommentTimeSep 8th 2026 edited
     
    Posted By: Dominic Cooney
    Posted By: djhSo I still don't understand your 'daft' comment.

    WiA summed it up quite well. A small increase in height would allow a second floor, then surprise surprise you suddenly meet the PH standard on floor area for the building, because of the second floor, even though the volume has barely increased, and the energy use of the whole building has barely increased.
    Our ground floor has 2.5 m ceilings everywhere and the first floor has min 2 m eaves and well over 3 m in the centre. You seem to be suggesting that you can convert 2.5 m into two floors by barely extending the height? That doesn't sound realistic to me. And again, floor space is what has economic and practical value. Extra volume is just for show. So I don't see why you'd think it's unreasonable for a building standard to value one more than the other.

    Edit to add: the other option is to keep the exterior roof where it is, but bring the insulation and ceilings down to a single floor as you desire. But yes, it's harder to make a small PH than a larger one; that is perhaps a valid criticism.
  27.  
    Posted By: WillInAberdeenI had understood that Part Q is strictly for 'conversion' of existing buildings and there are limits how much of the fabric can be demolished and replaced, is this enforced?


    Our usual approach is to apply for Full Planning permission, after the Class Q consent is secured, so that all of the required works are covered. This can include more work than would normally be allowed under Class Q on its own.
    There are other advantages too.
    • CommentAuthorMike1
    • CommentTimeSep 8th 2026 edited
     
    Posted By: WillInAberdeenPH was a response to a 1990s problem: very high-carbon heating.
    It's certainly one of the ways it's been promoted over the past couple of decades, but it developed out of an earlier emergency - the oil + balance of payments crises that hit Sweden in the 1970s.

    The 1975 Swedish Building Code toughened the requirements for building energy efficiency and airtightness, and there was renewed interest in super-insulation. In 1986 Wolfgang Feist went to to meet Sweden's top building research expert - Professor Bo Adamson - to learn how their developments might be adapted for Central Europe, which had also been hit by the oil shock. Adamson convinced Feist that it was theoretically possible to build passively heated homes. Feist went on to prove the idea and to make it reliably reproducible via the PHPP.
    •  
      CommentAuthorfostertom
    • CommentTimeSep 8th 2026 edited
     
    To restate my question (I don't know the answer in principle):

    Do south facing PH grade windows in the heating season absorb more (intermittent) solar heat in total, than they lose (more continually) by conduction/radiation?
    If so, that beneficially alters the building's nett heat loss, doesn't it? But by boosting solar gain instead of concentrating on insulating the losses outa sight, which dominates the normal approach.
    That's because there's a strict limit to how much south window area, otherwise summer overheating.
    But AFAIK, PHPP doesn't really embrace or expect full brise soleil, so concentrates on limiting window area.
    So does the possibility of full brise soleil change things? That is, no direct solar rays whatever, penetrate the glass during the summer season (whose date-boundary should be optimised).
    If so, then unusually large south glazing, with unusual full brise soleil, may compensate surprisingly for some inadequacy in insulation.

    Is there any sense in all the above?
    • CommentAuthorMike1
    • CommentTimeSep 8th 2026 edited
     
    Posted By: fostertomSaying PH, I really mean 'not needing a heating system' (but if we have to, an AAHP) and making use of PHPP. Whether to pursue that or not, with the weak floor insulation, is really my question.
    I would put it through PHPP as it's the best tool for determining what can be done, and you may get close enough to only need a small ducted ASHP. Installing 115m² of 25mm aerogel on the floor is another option to consider I guess, but costly.

    Posted By: fostertomwet solar - is that still a thing?
    Not really; it all seems to be PV now.

    Posted By: fostertomgiven heatwave-willingness for unprecedented comprehensive calculated effective brise soleil, hence unusually large southern glazing free of overheating penalty, what effect would that boost to winter intermittent solar gain have, seeing that AFAIK PHPP mainly relies only on reveal depth, overhangs and limits to southern window size, rather than full brise soleil? Could that extra intermittent gain more than overcome, over time, the accompanying winter loss?
    It might get you below 15 kWh/m²/year, but I suspect you'd need more than 10 W/m² heating to remain warm enough when the sun goes away.

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