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    • CommentAuthordelprado
    • CommentTimeDec 4th 2017 edited
     
    U values:

    Wall 0.5
    Ceiling 0.2
    Triple glazing 0.8
    No meaningful thermal bridging.
    Plaster air tight
    We will have a mechanical ventilation system put in.

    If you calculate the heat losses, then in theory there is still a heating demand up there, but can I get away with it? The whole downstairs will have UFH.
    • CommentAuthortony
    • CommentTimeDec 4th 2017
     
    In my view you need to be using much lower u values like 0.1 for walls and ceilings

    If downstairs was at 21 then you might find upstairs is OK but I reckon it it likely to be 15 or 16 when it is cold outside.

    Will ufh be on 24/7?
    • CommentAuthorRobL
    • CommentTimeDec 4th 2017
     
    Our 4 bed house:

    U values
    Walls 0.6
    Ceiling 0.1
    2G 1.2
    mvhr, air tight for UK, not for here tho!

    We have rads upstairs & downstairs. The upstairs one are on low TRV settings, downstairs full. It is colder upstairs than downstairs, except for my daughters room/study.
    I think that you could do it, but that you wouldn't use a room as a study.
    • CommentAuthordelprado
    • CommentTimeDec 4th 2017 edited
     
    Hi Rob where in the country are you based? What construction is the house?

    What are floor U values? Mine are solid with a 0.2

    Tony I cannot improve the walls any further
    • CommentAuthorRobL
    • CommentTimeDec 4th 2017
     
    Hi delprado

    Cambridge
    Filled cavity walls, breezeblock inner.
    Solid floor, perimeter wall insulation around. Not sure the effective u value, but warm enough to walk on barefoot.
    If I improved something else, it would be EWI down to the perim insulation. Oneday.
    •  
      CommentAuthordjh
    • CommentTimeDec 4th 2017
     
    Posted By: delpradoIf you calculate the heat losses, then in theory there is still a heating demand up there, but can I get away with it? The whole downstairs will have UFH.

    I doubt you will get away with it. The upstairs typically has bigger heat losses than downstairs, because it has the walls and the roof to lose heat through. So I think you will certainly want some heat in bath/shower rooms. Depending how warm you like your bedroom, you may want heat there as well. Certainly you will need it on occasion if people are ill, though plug-in heaters could be used for such situations.

    What is the calculated need for heat in the upstairs rooms?
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 4th 2017
     
    Just do a room by room heat loss analysis, going to be a useful thing to know anyway and don't take to long, especially if you can collect some real data.
    • CommentAuthordelprado
    • CommentTimeDec 4th 2017
     
    I reckon the bedrooms are about 250w max each. But can’t I just upspec the downstairs underfloor and rely on that heat being moved about my mhvr?
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 4th 2017 edited
     
    250W at what temperature differences and the mass flow rates from the MVHR?
    This is why it is worth spending a few hours knocking up a spreadsheet.
  1.  
    We just calculated about the same heat demand. I think 2 people and a lightbulb will provide about that much heat, (switch to LEDs in the summer) but just to be sure I bought a plugin electric panel heater (a good one, £100+).

    I couldn't get wet heating plumbed for that much.
    •  
      CommentAuthordjh
    • CommentTimeDec 4th 2017
     
    Posted By: delpradoI reckon the bedrooms are about 250w max each.

    That sounds remarkably good, but as ST asks, what temperature differences?

    Is the 0.8 an installed Uw number? That sounds exceptional, given the 0.5 value for the wall. I'd expect the psi value to push it up a bit.

    But can’t I just upspec the downstairs underfloor and rely on that heat being moved about my mhvr?

    No
    • CommentAuthordelprado
    • CommentTime7 days ago
     
    Thanks all.

    Yes, the window is green building store's ultra. It will be installed using iso chemnies bloco one expanding tape creating a perfect seal.

    If your house is airtight, surely all you need is an appropriate load for the whole house, by adding up all the heat losses, and then providing that, wherever it may be, and the ventilation moves it about evenly?

    Just to give background on calculations, for a bedroom, I have calculated two external walls, ceiling and window. I have discounted the interior walls which I assume is right?
    • CommentAuthortony
    • CommentTime7 days ago
     
    Yes,

    Re distributing heat, it is very unlikely that your ventilation system will be capable of distributing the amount of heat needed to keep your bedrooms sufficiently warm, it will move air but not enough heat. It will be like trying to fill a bucket with a load of holes in it, water will not get to the top ie the rooms will not be warm enough.
    •  
      CommentAuthorSteamyTea
    • CommentTime7 days ago
     
    I think if your MVHR can cause a temperature difference 5°C above the ambient bedroom temperature, and it has a flow rate of 25 litre/second, then you can shift 156 W.
    At 2°C above ambient it is 63W.
    To get 250W you need a flow rate of 100 litres/second with a temp difference of +2°C.
    Be a bit windy under the vent.
    • CommentAuthordelprado
    • CommentTime7 days ago edited
     
    Thanks everyone.

    What about if I stick 1.3kW somewhere on the landing. Would that help?
  2.  
    But the air coming out of a MHRV inlet is Colder than the house internal ambient, not Warmer, or did I misunderstand?

    Because the MHRV exchanger is less than 100% efficient at recovering heat from the internal air to heat the incoming air. So MHRV is overall a way of cooling a room, same as any ventilation is, the 2nd Law and all that.
    • CommentAuthorTimSmall
    • CommentTime7 days ago
     
    I have:

    Walls 0.12 to to 0.45 (will be 0.11 to 0.17 when finished)
    Roof 0.07 to 0.1
    Windows 0.7 to 1.8 (will probably upgrade the remaining 1.8 ones in future to circa 1.0)

    I have rads in all rooms except GF and bathrooms (which has UFH).

    All have a design temperature of 35 degrees (for -5 outside temp with all measures finished) and run directly from a gas boiler with no mix down (they currently run at 44 max). All are currently needed, and I think they'll still be needed when all is finished, but the upper floor one doesn't get much use.

    IIRC the total heat demand with all measures finished is about 2.5 kW, which should be OK for a small heat pump in the future (I've considered hacking in a second hand mini-split with an added flat plate exchanger, but will stick with the gas boiler for the time being).
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    Posted By: delpradoYes, the window is green building store's ultra. It will be installed using iso chemnies bloco one expanding tape creating a perfect seal.

    A nice window as far as I can tell. We're not asking about the airtightness - we're giving you the benefit of any doubt and assuming you make it perfectly airtight. The question is about how the edge of the window is insulated to reduce the thermal bridge. Are you using SAP or PHPP rules to estimate the thermal bridge?

    If your house is airtight, surely all you need is an appropriate load for the whole house, by adding up all the heat losses, and then providing that, wherever it may be, and the ventilation moves it about evenly?

    No, it doesn't work like that completely. Rooms on the south side get warmer than those on the north side; those upstairs and those downstairs have different characteristics for reasons already stated. Ventilation air, and conduction, can move some heat around but not that much.

    Just to give background on calculations, for a bedroom, I have calculated two external walls, ceiling and window. I have discounted the interior walls which I assume is right?

    Yes, that sounds a reasonable basis. I used the same basis but ignoring the window and a 3 m x 4 m x 2.5 m box for the room and didn't get quite as encouraging numbers as you. Again, what temperatures are you assuming?

    Remember you need to keep your house warm when it is cold, not just when it is average winter. You can always turn heating down but life is miserable if it's not warm enough.

    What about if I stick a 1300w somewhere on the landing. Would that help?

    Possibly. I currently have 800 W on my upstairs landing overnight (E7) and my post-heater heats the interfloor void. I also have 800/1200 W downstairs on E7. It works in my house. But my insulation is considerably better than yours.

    But when you say a 1300 W, what do you mean? I think you would be better off planning for heating in the bathrooms and bedrooms. What external wall temperature are you expecting, for example? With say -10°C outside.

    Posted By: WillInAberdeenBut the air coming out of a MHRV inlet is Colder than the house internal ambient, not Warmer, or did I misunderstand?

    True unless the MVHR has a post-heater fitted.
    • CommentAuthordelprado
    • CommentTime6 days ago
     
    Thanks. I think I am restricted as to how I install the bay windows, since they need to be where they are in order to be loaded with the timber stud above them.

    In any event, it is not clear to me the benefit of seating the windows further in when your insulation is in the cavity. In the following image I understand the top sketch is best practice, but mine will be like the second picture. I figured as long as it was partly in the insulation layer there is no thermal bridging to the outside.

    In relation to thermal bridging in general around the window area I dont see how I will have any, since ISO chemies expanding tape is air tight, water tight and insulating?

    To run through my calculations, I used the steamy tea calculator and did as follows.

    I calculating using a 21 degree temp difference, which is aiming for 18 degrees in the bedroom, and down to minus 3.

    Insulation in the walls is 70mm eps, blown in. The assumed u value of this is about 0.5. I am adding about 50mm insulating render to the outside but I am not factoring this in the calculation as its mostly to kill rogue thermal bridging from cavity ties etc.

    Loft insulation is 250mm woodfibre pavatex batts, u value about 0.2, probably a bit less.

    Therefore in the bedroom in question:

    External wall 1 - 4.6m x 2.5m (11sqm) - 115w
    External wall 2 (area around large bay) - 2sqm - 21w
    Window - 2.3m v 1.2m - 2.8sqm - 47w
    Ceiling - 17sqm - 77w

    TOTAL before thermal bridge and ventilation losses = 260W

    Does anyone have a sensible estimate I can use for ventilation loss, assuming the house is plaster airtight, and foamed/taped around joists and service penetrations? I was thinking about saying 0.6, with a 96% efficient zehnder unit.

    My understanding for the ventilation loss equation you use is multiply the room volume (m³) by the air change rate by the temperature difference by 0.34W/m³°K.

    So for the room in question and assuming 0.6 air changes, I get:

    4.6 (wall length) x 3.7 (other wall length) x 2.5 (height) x 0.6 (air changes) x 21 (temp difference) 0.34 = 182w

    So for fabric loss plus ventilation loss we have 260 + 182 = 442W

    What about thermal bridging?
    • CommentAuthordelprado
    • CommentTime6 days ago
     
    Forgot to link to proposed window install mentioned in my post:

    https://imgur.com/a/l1Aki
    •  
      CommentAuthordjh
    • CommentTime6 days ago
     
    Posted By: delpradoI figured as long as it was partly in the insulation layer there is no thermal bridging to the outside.

    In relation to thermal bridging in general around the window area I dont see how I will have any, since ISO chemies expanding tape is air tight, water tight and insulating?

    Sketch 2 is certainly a lot better than having the frame entirely in line with the block or brick work. But it still provides a reduced length path for heat to flow from the inside whythe into the frame, so sketch 1 is better and sketch 2 will still have a positive psi value to be added in.

    Even better is to have some insulation over the outside (or inside) of the edge of the frame. That can result in a negative psi value.

    I calculating using a 21 degree temp difference, which is aiming for 18 degrees in the bedroom, and down to minus 3.

    It's usual to use -10°C for the outside temperature. Remember it sometimes gets even colder than that and you want to design your heating to cope.

    Your insulation U-values sound reasonably conservative, though I haven't checked.

    External wall 1 - 4.6m x 2.5m (11sqm) - 115w

    4.6 x 2.5 = 11.5
    4.6 x 2.5 x 0.5 x 21 = 120.75 W

    or using my delta T: 4.6 x 2.5 x 0.5 x 21 = 161 W

    I was thinking about saying 0.6, with a 96% efficient zehnder unit.

    0.6 is fairly high. Personally I would work with something like 0.3. OTOH, 96% is way too high for the MVHR efficiency. Firstly, Zehnder don't do a unit with that Heat re­cov­ery rate AFAIK (see https://database.passivehouse.com/en/components/list/ventilation_small) and secondly, it won't operate at its specified rate anyway because of losses through the intake and exhaust ductwork.

    My understanding for the ventilation loss equation you use is multiply the room volume (m³) by the air change rate by the temperature difference by 0.34W/m³°K.

    Dunno, sorry.

    What about thermal bridging?

    If your dimensions are internal dimensions, then there's a geometric bridge at every edge to be taken into account.

    But it seems clear you'll want some heating. It depend how exactly you want to provide that but typically you will oversize the emitter by a significant factor anyway.
    • CommentAuthordelprado
    • CommentTime6 days ago
     
    Thanks so much DJH.

    In relation to the window fitting, the plan was for an insulated reveal internally (the windows outer frame will look freaky prior to installation as the they will be 40mm oversized to allow for this), and for the insulated plaster to run 40mm on top of the frame outside too for the same effect. I will post a diagram in a second as this has prompted another improvement on that basic design.

    In relation tot he heat stuff, is there a calculator somewhere which will tell me what 250mm of woodfibre does in a loft space and 70mm eps in a clay brick cavity wall etc?

    Im assuming with insulating external render my only thermal bridge will be foundations. But then I dont understand whether relative thermal bridges count, because clearly they should, or is it that they are so small they dont make a massive difference? Ie some areas are warmer than others basically - is the issue only when the difference is massive (aside from the colder areas losing more heat adding to the general heat loss?)
    •  
      CommentAuthordjh
    • CommentTime6 days ago
     
    Posted By: delpradoIn relation tot he heat stuff, is there a calculator somewhere which will tell me what 250mm of woodfibre does in a loft space and 70mm eps in a clay brick cavity wall etc?

    Well there are several U-value calculators online that have been mentioned here before, but I don't remember them. It's fairly easy to work out from first principles anyway. The main thing is to take account of the 'timber fraction' of the rafters.

    Im assuming with insulating external render my only thermal bridge will be foundations. But then I dont understand whether relative thermal bridges count, because clearly they should, or is it that they are so small they dont make a massive difference? Ie some areas are warmer than others basically - is the issue only when the difference is massive (aside from the colder areas losing more heat adding to the general heat loss?)

    You calculate U-values in a 1-dimensional fashion by adding up the resistance of the separate layers, including the appropriate fraction of different materials in each layer. Then the U-value is the reciprocal of the resistance.

    That methodology ignores the fact that heat doesn't flow one-dimensionally, but actually goes wherever is 'easiest', so tends to average out. But the only way to know for sure is to do full calculations - i.e. a model. Areas with lower resistance (i.e. higher U-value) will have a lower internal temperature and so be prone to condensation. This is always true of geometric features like corners.

    That's why such calculations are always approximations and need a margin including to eliminate risks.
    • CommentAuthoradwindrum
    • CommentTime6 days ago
     
    I probably have similar calcs to your build delprado and have no heating upstairs except towel rads in bathroom and a flue passing through the hall.

    When -1 outside the bedrooms hit 16 degrees while downstairs is about 21 degrees 24/7 with ufh. We like it as a) kids (3 teen and near teen girls!) don't spend all evenings in their rooms and b) we all like a cooler room for sleeping in. Bathrooms benefit from some extra heat though for coziness.

    We have a distinct thermocline as you go up the stairs.

    MVHR must move some heat as it will be mixing all average temps especially as extract vents tend to be in warmer rooms (kitchen and bathrooms).
    • CommentAuthordelprado
    • CommentTime5 days ago
     
    Can you share what the construction is adwin?
    • CommentAuthoradwindrum
    • CommentTime5 days ago
     
    Timber frame with 240mm rockwool in walls, 300mm rockwool in roof, 300mm cellotex in raft foundation. Mix of double and triple glazing.
    • CommentAuthordelprado
    • CommentTime4 days ago
     
    Thank you. Your walls will be tonnes better than mine, although the house in general sounds like it has much lower thermal mass
  3.  
    I have are that our roof was specced at 0.14 and our walls mostly 0.11. Windows are 3g passive.

    Shape of the building is not optimal - we're a small footprint but over 3 floors so lots of exterior wall for the interior space.

    I installed underfloor everywhere but turned it off on most of the upper floors last year. This year I put it back on. When we had a cold spell the rooms were dropping to about 16C which is a little bit cool to be comfortable. We're not putting much heat into those rooms at all but I'm glad we are able to.
    • CommentAuthorgravelld
    • CommentTime4 days ago
     
    Surprised your rooms dropped to 16C at all with that type of spec, is the house occupied?
  4.  
    These are the spare bedrooms on the third floor so the rooms were unoccupied. Nearly all outside walls. Doors were closed and a 100mm of acoustic insulation between floors so fairly well insulated from the rest of the house.

    We had people staying for 2 months and when they were using the room it was comfortable but when other visitors were coming in that time the second room was feeling a little cool.

    I was similarly surprised - I don't remember it happening last winter when I'd got the UFH turned off up there.
   
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