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    • CommentAuthorKenny_M
    • CommentTimeMay 24th 2018
     
    So I remember years ago reading that "thermal mass is a good thing, because the heat of the day is soaked up by the thermal mass and regulated out as it gets colder" etc. I remember thinking, I live in Scotland, what heat of the day!! It was obviously written by someone living on the southern edges of the UK, if not somewhere like Arizona.

    Thinking about this again today I found this page https://sustainabilityworkshop.autodesk.com/buildings/thermal-mass and the assertion that in hot or cold climates thermal mass might be a bad thing. It made me wonder about mass and thermal conductivity and the impact of internal/external insulation in a typical modern occupancy pattern.

    My house has 600mm stone walls, a lot of wood/lime plaster etc, so a fair amount of mass. When the heating is off, say from around 10pm to late afternoon the following day, that mass will be equalising to the temperature outside. Even with some external insulation installed, thats only slowing the transfer of heat, so eventually the walls will cool to the temp outside. Before I get home the heating system then has to come on and heat up that mass before the house will become comfortable, which would normally mean having the heating come on a few hours before getting home. With external insulation, the whole wall has to be heated first before the insulation will have any effect, but with internal insulation the rate of heat transfer would presumably be slowed as soon as it as begun to pass through the plasterboard in front of it?

    In such a scenario, is there an argument to say that a wall with a lower u value, could actually be more effective/cheaper to heat? It just seems that the u-value model seems only to consider a steady state. Maybe this is why people seem to notice a benefit from things like sempatap, that on paper don't look very effective.

    I just wondered if there had been research or knowledge out there on this, or maybe there are things I haven't considered.
    •  
      CommentAuthordjh
    • CommentTimeMay 24th 2018
     
    Posted By: Kenny_Ma typical modern occupancy pattern

    There's no such thing.

    In such a scenario, is there an argument to say that a wall with a lower u value, could actually be more effective/cheaper to heat?

    Walls with lower U-value are better insulated, and thus are generally cheaper to heat. Is that what you meant?

    If you have a lot of thermal mass inside the insulation then it is generally better to keep the inside at a roughly constant temperature, rather than switch the heating on and off. If you have insulation inside the thermal mass then it makes sense to only heat the building for the time you are there.

    There's a lot of research on this topic and there's been a lot of discussion on this forum, among many other places.

    As well as the insulation and 'thermal mass' (actually volumetric heat capacity) you also need to think about thermal admittance and suchlike and 'decrement delay'.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 24th 2018
     
    Posted By: djhactually volumetric heat capacity
    Or specific heat capacity as that uses mass as the measure, not volume.

    Start by looking up thermal inertia, the formula is pretty simple.
    I think you will find that in the UK's relatively mild climate, the mass of the building makes little difference.
    One of the problems is that it is very hard to compare 'like for like' or, as in your case, how a 600mm wall of Rockwool would perform.
    •  
      CommentAuthordjh
    • CommentTimeMay 24th 2018
     
    Posted By: SteamyTeaOr specific heat capacity as that uses mass as the measure, not volume.

    And as has been discussed in various places over the years, volume is a far more useful basis than mass for this property.

    If you type thermal inertia into google, you'll find that it's a subheading in the volumetric heat capacity article in wikipedia. It's more of a material property than a useful way to view wall structures.

    Try https://www.cibsejournal.com/cpd/modules/2013-01/

    The wall structure makes quite a big difference to the best way to heat a house. 600 mm of stone behaves quite differently to a timber frame stuffed with rockwool and a sheet of plasterboard on the inside.
    • CommentAuthordelprado
    • CommentTimeMay 25th 2018
     
    Comfort also plays a part doesnt it? I understand that when you have IWI you feel "radiant" heat "bouncing" back off it
    • CommentAuthorSigaldry
    • CommentTimeMay 25th 2018
     
    From http://www.bre.co.uk/filelibrary/Briefing%20papers/116885-Overheating-Guidance-v3.pdf

    "The ability of materials with a high thermal mass can be exploited to minimise diurnal swings. An example of a very high thermal mass building would be an old stone church. The temperature of the internal space changes marginally on a daily basis and almost not at all diurnally. In dwellings, a high thermal mass can help prevent overheating if the heat stored during the day can be effectively rejected at night. This therefore requires continuous high levels of night-time ventilation, which in some locations may not be practical or desirable (NHBC, 2013).

    A low thermal mass building would be a timber framed building with lightweight floors, ceilings and internal walls. The internal structure assumes the temperature of the air very quickly and so has minimal impact in limiting any change in temperature."

    The above is in relation to summer overheating, but ties across to heating - as mentioned above, lower thermal mass tends to heat up more quickly, but also cool down more quickly (subject to the more significant effect of insulation of course) - this is more useful for properties requiring a quicker responsiveness; A higher thermal mass property (with good levels of insulation), will suit a continual release lower temperature heating regime more, but would be less responsive.
    • CommentAuthorKenny_M
    • CommentTimeMay 25th 2018
     
    Posted By: djh
    a typical modern occupancy pattern

    There's no such thing.

    What I mean by this is that a lot of what is said about old houses and how they behave is based on a very out of date model of occupancy where there was someone there all day with a fire or range going constantly. For lots of people now they leave for work early in the morning and come back in the evening, so quick heat up times are potentially more important than overall u value.

    Posted By: djhWalls with lower U-value are better insulated, and thus are generally cheaper to heat. Is that what you meant?


    Sorry, I worded that very badly. What I meant was the opposite. Could a wall with a higher u value actually result in less energy use to heat the occupants to comfort levels because heating the room up quickly, but for shorter periods of time.

    If I take an extreme example to demonstrate what I am getting at. Say my house has 100mm of EPS on the outside skin. If I was away for a month in January with no heat in the house and then tried to heat the house it would take a long time to heat the mass in those walls before the heat reaches the external insulation. If there was 50mm of EPS on the inside I would expect the room to heat relatively quickly, as it would take longer for the heat energy to reach the walls.
    • CommentAuthorKenny_M
    • CommentTimeMay 25th 2018
     
    Steamy, djh, Sigaldry, thanks for the terminology. Looked up some of these terms and found a link discussing decrement delay and thermal buffering http://www.greenspec.co.uk/building-design/decrement-delay/ and they give an example where a caravan, and a stone building might have similar u values, but on a sunny day the caravan heats up much more quickly. This is the reverse of my point I suppose, because in Scotland its pretty rare that you are going inside to get away from the heat.

    This seems to verify, with the correct terminology and scientific background that I am lacking, that the u value is really only fully relevant to a steady state scenario, where the material through which the heat transfer is occurring has already reached equilibrium. I can understand why the u value is pervasive though, as it would get pretty complicated explaining the rest to a customer, or a government minister! ;)

    If I am understanding this correctly it sounds like if I want a quick heat up to comfortable room temperature, then its really only the thermal conductivity and specific heat capacity of the interior surface that is the primary concern. If I want to stabilise the interior temperature then density becomes important.
    • CommentAuthorgyrogear
    • CommentTimeMay 25th 2018 edited
     
    Posted By: Kenny_MEven with some external insulation installed, thats only slowing the transfer of heat, so eventually the walls will cool to the temp outside.


    Yes, but it all depends on what you mean by "eventually".

    For example, I had a winter holiday of 10 days over xmas-new year: no heating on in the house, no occupancy.
    On leaving the house was at 20°C, on returning it was at 16°C. The average external temperature over that period was probably 11 or 12°C (mild winter). Just with 50mm EPS external wall insulation.

    I put this down to the windows, and high mass.
    At a guess, "eventually" in my case might be, say, 3 months... or not at all, even.

    One way to determine the length of the eventuality might be to actually instrument the house with data loggers etc.

    gg
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 25th 2018
     
    Posted By: gyrogearOne way to determine the length of the eventuality might be to actually instrument the house with data loggers etc
    Always a good idea and very cheap to do now.
    I am knocking up simple 6 sensors loggers for a few quid.
    • CommentAuthorgyrogear
    • CommentTimeMay 25th 2018
     
    Posted By: SteamyTeaI am knocking up simple 6 sensors loggers for a few quid.


    I might just be interested in some of those :smile:

    gg
    •  
      CommentAuthordjh
    • CommentTimeMay 25th 2018
     
    Posted By: Kenny_MIf I am understanding this correctly it sounds like if I want a quick heat up to comfortable room temperature, then its really only the thermal conductivity and specific heat capacity of the interior surface that is the primary concern. If I want to stabilise the interior temperature then density becomes important.

    That's a pretty good article, I think, although I just skimmed it. I don't think it makes sense to separate factors in the way that you have; what matters are the overall properties of the materials. But you're correct that the interior materials matter much more if you're going to vary the internal temperature, whereas if the internal temperature is stable then the materials further out in the wall also becomne relevant.
    • CommentAuthortony
    • CommentTimeMay 26th 2018
     
    We seem to be talking about living in insulated boxes.

    For me this minimises incidental solar gains so does not work.

    I like maximum solar gain during the winter and shade windows during the summer to stop over heating (mass reduces temperature swings)

    I have been shading my windows for the last month -- some friends are still running their heating!
    • CommentAuthorKenny_M
    • CommentTimeMay 26th 2018
     
    Just to clarify where I am coming from on this. We have a modern extension which is south-ish facing, and is the main cooking/eating/sitting area. I plan over the next year to take a 'normal' approach and insulate externally, triple glaze, and this room would benefit from the thermal mass in the interior to regulate the temp and take advantage of solar gain.

    My thoughts in this thread are related to the old part of the house. I can't ever externally insulate the north/front face due to conservation area rules, and while the sides might get approved if I could make a strong enough case, it would be tricky and expensive. That's made me think about how those rooms are used.

    For example there is a small room at the front of the house, with one external wall with a window facing north, and the other facing east but in the shadow of the house next door. There is little to no solar gain there. I have projector in there and 2 or 3 evenings a week at most I might go in there to watch a movie. To make that room comfortable I would need to leave the heating on reasonably high all the time, because it takes a long time for the surfaces to heat up.

    Even if I was allowed to do it I am not convinced external insulation would benefit me over internal in this scenario, because I would still have to heat the mass in the walls, compared to internal where the heat up time for the internal surface should be much shorter. In a scenario like this I am suggesting that perhaps internally insulated, with say a wall u value of 0.7, might be more energy efficient, than a u value of say 0.3, because its energy required to heat the room for comfort maybe 3 times a week for 3 hours at a time, rather than a steady state scenario.

    I also think the impact of solar gain is over stated, perhaps because UK models sometimes presume a location in the southern half of England, not so much sun here during the six month winter! :cool:
    •  
      CommentAuthordjh
    • CommentTimeMay 26th 2018
     
    Posted By: Kenny_MFor example there is a small room at the front of the house, with one external wall with a window facing north, and the other facing east but in the shadow of the house next door. There is little to no solar gain there. I have projector in there and 2 or 3 evenings a week at most I might go in there to watch a movie. To make that room comfortable I would need to leave the heating on reasonably high all the time, because it takes a long time for the surfaces to heat up.

    That's quite similar to the position in an old church. Two parts to the answer. First, don't think about heating the room, think about heating the occupants, usually by radiant heaters or perhaps by forced air. Secondly, apply internal insulation and in particular a surface finish with low admittance, so it feels warmer faster.
    • CommentAuthorgoodevans
    • CommentTimeMay 27th 2018
     
    Posted By: SteamyTeaI am knocking up simple 6 sensors loggers for a few quid.
    Me to. I want to track temp and RH through several points of my roof makeup I may be able to give comfort to others - my OSB sheathing has only 37% of the insulation value on the outside. but I havent got time to research the hardware/software required for a cheep monitoring solution.
    • CommentAuthorKenny_M
    • CommentTimeMay 27th 2018
     
    Posted By: goodevansusually by radiant heaters or perhaps by forced air


    I am planning to fit a stove in this particular room, at the opposite side from the projector screen, so that I get the radiant heat from the stove.
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