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General: Heatwave advice - "keep your windows closed"

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  1.  
    • CommentAuthorlineweight
    • CommentTimeJul 20th 2022
     
    In the last few days there's been quite widespread advice to (counter-intuitively) keep your windows closed during the day (and open them at night when it's cooler).

    I understand the basic principle and for sure it is good advice in many cases, but it doesn't necessarily make sense in all buildings, does it?

    For example, presumably anywhere with limited thermal mass indoors (eg internally insulated buildings) will see much less benefit from this strategy than buildings with a lot of thermal mass (and especially exernally-insulated solid wall buildings).

    In may cases it will rely largely on stopping a volume of cool-ish air from (a) being replaced by warmer air from outside and (b) being warmed up by heat energy making its way through walls and windows . Without very good insulation and some amount of thermal mass, surely it's only a matter of time before it reaches a similar temperature to the outside air. Isn't it likely that that amount of time might only be a few hours in many UK buildings? In which case, keeping the windows closed beyond mid-morning or lunchtime might become counter-productive.

    The other issue (and the reason I chose to keep certain windows open during the last couple of days) is that you lose the benefit of changing the air (for stuffiness) and if there's a breeze the benefit of moving air (for cooling).

    I was interested to see if there was anything I could look up to compare the cooling effect (on a human) of, let's say, stationary air at 30 degrees versus moving air at 35 degrees.

    I can find various "wind chill" calculators but they are all aimed at examining effects at much lower temperatures.
    • CommentAuthortony
    • CommentTimeJul 20th 2022
     
    I recommend shading the outside of windows, shutters, dust sheets, cardboard anything to stop the sun shining in. Live in North facing rooms
    •  
      CommentAuthordjh
    • CommentTimeJul 20th 2022 edited
     
    Posted By: lineweightIn the last few days there's been quite widespread advice to (counter-intuitively) keep your windows closed during the day (and open them at night when it's cooler).

    I understand the basic principle and for sure it is good advice in many cases, but it doesn't necessarily make sense in all buildings, does it?
    Well the basic principle is close the windows when it's warmer outside than in, and open the windows, and doors where sensible, when it's cooler outside. I think the basic principle makes sense in all buildings, but not the over-simplified widespread advice you state.

    For example, presumably anywhere with limited thermal mass indoors (eg internally insulated buildings) will see much less benefit from this strategy than buildings with a lot of thermal mass (and especially exernally-insulated solid wall buildings).
    Indeed so, and this is one of the major advantages of external insulation versus internal insulation. I wonder how the recent poster who was deliberately creating a very light thermal mass (for ease of heating) would have got on?

    In may cases it will rely largely on stopping a volume of cool-ish air from (a) being replaced by warmer air from outside and (b) being warmed up by heat energy making its way through walls and windows . Without very good insulation and some amount of thermal mass, surely it's only a matter of time before it reaches a similar temperature to the outside air. Isn't it likely that that amount of time might only be a few hours in many UK buildings? In which case, keeping the windows closed beyond mid-morning or lunchtime might become counter-productive.
    Indeed the strategy will generally only work for short periods or with guaranteed very cold nights. Most buildings have time constants longer than a few hours, since they're mostly built from brick or blocks, but late afternoon and early evening is going to be the most difficult time for sure.

    The other issue (and the reason I chose to keep certain windows open during the last couple of days) is that you lose the benefit of changing the air (for stuffiness) and if there's a breeze the benefit of moving air (for cooling).
    The answer to changing the air is called ventilation, and MVHR in particular. But that can be turned down for quite a few hours before the air begins to get stale.

    Traditional buildings in tropical areas often rely on massive ventilation, because they use lightweight materials of necessity and can't guarantee anything like an airtight construction. But as well as encouraging drafts, building in extra shading features is another common feature.

    Solar gain through windows is quite an important factor, so external shading is a very good idea. For our biggest window, I bought some suction cups and used them to attach a sheet of garden fleece I happened to have around to the outside of the window. It worked well - the temperature got to about 26-27°C
    • CommentAuthorlineweight
    • CommentTimeJul 20th 2022 edited
     
    Posted By: djhIndeed so, and this is one of the major advantages of external insulation versus internal insulation. I wonder how the recent poster who was deliberately creating a very light thermal mass (for ease of heating) would have got on?


    I myself am in an internally insulated flat (solid brick walls).

    This means I have little thermal mass to take advantage of, to cool down overnight and then act as a buffer to heat during the day. However, it also means I'm protected from the heat radiated internally by those heated-up solid walls in the evening and overnight (something I've previously been very conscious of, living in similar but uninsulated solid brick houses).

    I've got sensors embedded in the brickwork (in the inner face) which means I can see what temperature the brickwork (or more accurately the air immediately adjacent) reaches through the day. Yesterday a NE-facing wall approached its peak of around 34 degrees by early afternoon - a SE facing wall reached the same temperature around the same time but continued to climb to 37 degrees by 6pm and didn't start declining until after 10pm.

    In fact yesterday (following the previous day) none of my sensors recorded minimum temperatures of less than 28 degrees. The minimums were mostly around 9am or so. So, without the internal insulation, if yesterday morning I'd closed all the windows and managed to trap in a volume of air at a bit under 30 degrees, then my walls' thermal mass would be working against me rather than for me, pretty much from the beginning.

    Makes me wonder if a typical UK 9" solid brick wall really has enough thermal mass to buffer things all that usefully in these kinds of conditions.

    Presumably in countries where traditional buildings use thermal mass to buffer high temperatures, they either have high daytime + low night-time temperatures, or the thermal mass is sufficient that it can buffer on a much longer cycle than 24hrs.

    I've seen a lot of people commenting that eg "in Spain we just close the windows in the day and open them at night and it works fine". But buildings elsewhere aren't necessarily built with the same climate expectations as in the UK. I'm guessing they often have much more substantial thermal mass. And of course things like external shutters. Deliberately shaded internal courtyards and so on.
    • CommentAuthorCliff Pope
    • CommentTimeJul 20th 2022
     
    Traditional farmhouse in west Wales. Solid stone walls, no insulation.

    We just leave the windows open all the time, and the doors too during the day.
    It's pleasantly cool indoors, with a nice breeze through the house. They are small windows, set quite deeply in the walls, so little solar warming, apart from the attic floor which is stifling from the hot slates
  6.  
    If you are trying to buffer day/night cycles, then you only need a few centimetres of "thermal mass" - tiles, plaster, etc. Cycles cannot soak any deeper into masonry in that time frame.

    If you are trying to delay the midday heat getting through the wall so it gets through in the evening, after heating through windows has stopped, then you need 10s of cm thickness of thermal mass. Insulation is better though.

    If you are trying to buffer multi-day cycles or even interseasonal cycles, that's when the 2-foot thick stone walls come in.

    There are decrement calculators around on the web where you can play with this.
    • CommentAuthorlineweight
    • CommentTimeJul 20th 2022 edited
     
    Posted By: WillInAberdeenIf you are trying to buffer day/night cycles, then you only need a couple of centimetres of "thermal mass" - tiles, plaster, etc. Cycles cannot soak any deeper into masonry in that time frame.


    I might be misunderstanding what you are saying, or misinterpreting my observations, because that doesn't seem to square with what I see when I look at the temperature at the inner face of my 22cm thick brick walls, which peaks a few hours after the general outdoor temperature peaks.

    I've taken that to mean that the peak in temperature makes its way through the wall in that sort of timescale but is that not actually what I'm seeing?
  8.  
    No sorry:

    If heat is soaking into a masonry surface during the day, being stored, and reversing direction to come out of the same surface again at night, then it can only penetrate a few cm deep. If it is a week-long heatwave it can penetrate deeper, and if it is a whole summer it can go deeper still.

    If daytime heat is going in through one side of a wall and coming out the other side, without reversing direction, it can get through any thickness of wall. The peak will come out the other side delayed a bit later, (the 'decrement delay') depending on the thermal mass and the insulation value of the wall. Tens of cm of masonry will delay the peak by a few hours, as you measured. Internal insulation also helps delay the peak and make it smaller.

    If it helps at all, it's like a "RC" electronic circuit, which delays the phase of a wave more or less, depending on the cycle frequency (daily/seasonally), the resistance (insulation) and the capacitance (thermal mass).

    Eg "Considering thermal mass on a *daily* cycle basis, the most effective depth of the material is the first 50 mm. Between 50 and 100 mm, efficiency further diminishes and beyond 100 mm the mass effect is largely inconsequential."
    https://www.greenspec.co.uk/building-design/thermal-mass/
  9.  
    Dont know what the problem has been. Have no opening windows only MVHR maximum temperature inside downstairs 22C upstairs 24.5. Old farmhouse originally built approx 1650.
    •  
      CommentAuthorfostertom
    • CommentTimeJul 20th 2022 edited
     
    Will, glad you made the distinction between bi-directional (reversing) heat flow, and uni-directional.
    Posted By: WillInAberdeenIf daytime heat is going in through one side of a wall and coming out the other side, without reversing direction, it can get through any thickness of wall.
    True, but I'm surprised at
    Posted By: WillInAberdeenThe peak will come out the other side delayed a bit later, (the 'decrement delay') depending on the thermal mass and the insulation value of the wall.
    Just "a bit" later?
    Posted By: WillInAberdeenTens of cm of masonry will delay the peak by a few hours, as you measured.
    When I last had occasion to calc it, it seemed that 500mm of masonry would give a week's delay as well as considerable flattening of any peak or trough in external heat input, which together served as a week's heat storage in the sense of bridging between input peaks e.g. clear-sky days in winter.

    Posted By: lineweightI look at the temperature at the inner face of my 22cm thick brick walls, which peaks a few hours after the general outdoor temperature peaks
    could be yesterday's peak coming through i.e. 27hr delay.
    • CommentAuthorlineweight
    • CommentTimeJul 20th 2022
     
    Posted By: fostertomcould be yesterday's peak coming through i.e. 27hr delay.


    Hm, but the relative heights of the approx 10pm peaks over the past three days seem rather correlated with the maximum external air temperatures of the days immediately preceding them rather than >24hrs previous.
      Screenshot 2022-07-20 at 23.33.30.jpg
  12.  
    Playing with the CIBSE calculator, it suggests that an uninsulated 9" brick wall has a decrement delay of 5 hours, and a decrement factor of 0.6 (IE if the inside surface temperature change night-to-day of the brick, is 60% of the outside surface temperature change). That looks pretty much like LW's graph (the thermocouple is at the inside surface of the brick?)

    If we add 200mm of EPS IWI, the delay at the internal surface of the room is 9hrs, and the decrement 0.25, so the IWI is very helpful for damping the day-night swing.

    Got to remember those numbers depend on the frequency of the cycle (edit see below) - if we are talking about a day-night cycle, a brick wall will delay that a few hours. If we are talking about a summer-winter cycle it will delay it several days, but not decrement it by very much. Which were you thinking of, Tom?

    The steady state component of the temperature difference is what causes net heat to flow (frequency=0). The capacitance of the wall has no effect on that component, it's only the resistance that matters - the good old U value!
    •  
      CommentAuthorfostertom
    • CommentTimeJul 21st 2022
     
    That sounds v gd fresh way of understanding it - if only I remembered by A level electronics!

    I guess the frequency I was thinking of is the variable frequency of clear, strong-sun days in winter - an average frequency (should say period) of say 7 or 10 days? Any idea what delay for that, with 500mm masonry? Seems counter-intuitive that decrement would be 'not very much'.

    period 1 day = frequency 1, sqrt = 1
    period 365 days = frequency .0027, sqrt = .0052
    period 7 days = frequency .14, sqrt = .38

    So how do we get from that to delay = 5hrs, 'several days', and ??? respectively?
  14.  
    (This was an edit to my last post, but starting a new post for clarity)

    Once the resistance (capacitance? impedance?) of the wall is sufficient, then the heat transfer through windows and airchanges become the dominant effects. You can add resistance (shading, draught proofing) but windows and ventilators have no thermal capacitance (thermal mass), so no delay.

    You can also add internal capacitance, such as masonry internal walls, floors etc. As mentioned, you only need a few cm thickness of masonry to tune this capacitance to absorb a day/night frequency, but much more if you want to filter out multiday or even seasonal cycles.

    Simplified calculations you find on the web, such as the CIBSE thermal admittance method, only deal with one frequency (day/night) and ignore the internal thermal capacity and the windows. If you want a more general method then maybe there is some paid software somewhere, otherwise start from first principles with the Fourier equation! I tried that once, will see if I can find it. Or maybe use the electronic resistance/capacitance/impedance analogy and see if there's some free circuit software that does what you want?

    There was a confusing over simplification in my last post sorry about square roots. The depth that the heat sinks into a surface is sqrt(2a/w) where a= diffusivity =(conductivity/density.heatcapacity). So summer/winter heat penetrates sqrt(365) = 19x deeper than day/night.
    We can't directly relate the sqrt(w) to the phase delay, just generally high frequencies are delayed more than low frequencies. Took this out of previous post.
    •  
      CommentAuthordjh
    • CommentTimeJul 21st 2022
     
    Posted By: WillInAberdeenPlaying with the CIBSE calculator
    Do you have a link to it, please. My google-foo is failing today.
  16.  
    • CommentAuthorlineweight
    • CommentTimeJul 21st 2022
     
    Posted By: WillInAberdeen

    Eg "Considering thermal mass on a *daily* cycle basis, the most effective depth of the material is the first 50 mm. Between 50 and 100 mm, efficiency further diminishes and beyond 100 mm the mass effect is largely inconsequential."
    https://www.greenspec.co.uk/building-design/thermal-mass/" rel="nofollow" >https://www.greenspec.co.uk/building-design/thermal-mass/


    Thanks for that link which in turn references this paper (now going on for 20 years old):

    https://pure.strath.ac.uk/ws/portalfiles/portal/80494486/strathprints006598.pdf

    There's a section of this which I think is relevant to my original point/question in this thread.

    They are looking at peak temperatures in a simulated room in a "cooling" scenario. They are looking at differences between high & low thermal mass constructions but also using two different cooling strategies which they describe as

    "Two ventilation patterns were investigated, the first
    labelled ‘summer ventilation’ is a constant 4.5ac/h
    which is to represent windows constantly open, the
    second labelled ‘night cooling’ is 4.5ac/h from 6pm
    until 8am and 0.45ac/h during the day between 8am
    and 6pm which represents windows mainly opened
    during the cooler parts of the day. Both of the
    evaluated ventilation schemes are simple and
    designed to represent normal practice by occupants"

    The attached table shows some of the results. I'm sure it's easy to over-extrapolate from this one table but it's interesting to see that for the "low thermal mass" construction, a strategy of only opening the windows at night leads to higher peak temperatures (unless the window is shuttered). This is in contrast to the "high thermal mass" construction where the ventilation strategy of only opening the windows at night seems to produce the better results regardless of whether the window is exposed, shaded or shuttered.
      Screenshot 2022-07-21 at 13.39.55.jpg
    •  
      CommentAuthordjh
    • CommentTimeJul 21st 2022
     
    Posted By: WillInAberdeenGot it from here some time ago https://www.cibsejournal.com/cpd/modules/2013-01/
    Ah thanks. It wasn't my google-foo then >I'd found that page but not noticed the link :(

    Just stumbled uponhttps://www.htflux.com/en/free-calculation-tool-for-thermal-mass-of-building-components-iso-13786/ which looks promising
    I'll look at that too, ta.
    •  
      CommentAuthordjh
    • CommentTimeJul 21st 2022
     
    Posted By: lineweightThere's a section of this which I think is relevant to my original point/question in this thread.
    Yes, that's pretty much what I'd expect. I'd forgotten quite how important it is to have the insulation on the outside. :shamed:
    • CommentAuthorjon
    • CommentTimeAug 26th 2022 edited
     
    Interesting thread:

    Had the chance to do various tests on my house (bungalow with high thermal mass & good insulation at front and exceptional to rear) to see what worked.

    Fans pushing air in or out help, but for night time cycling, the biggest difference found was when loft hatches fully open and also the loft window (non habitable space.. we're just lucky on this one) was fully open. This combined with open small windows seems to set up a through draft effect and resulted in the biggest cooling effect (after adjusting for different start and night-time minimum temperatures). Fans pushing air in at low level help but fans pushing air out don't. Fans at high level don't seem to help at all.

    A very large difference observed by opening a door (with temporary security shutters). However, this reduces the effect in rooms with the small windows open: overall a lower temperature achieved, but not very well distributed.

    In short, internal temperature never exceeded 26 (external shaded up to about 36 on occasions)

    Hope this helps a little
    •  
      CommentAuthordjh
    • CommentTimeAug 26th 2022
     
    Posted By: jonFans pushing air in or out help, but for night time cycling, the biggest difference found was when loft hatches fully open and also the loft window (non habitable space.. we're just lucky on this one) was fully open. This combined with open small windows seems to set up a through draft effect and resulted in the biggest cooling effect
    Agreed. Through ventilation, powered either by thermal gradients (upstairs warmer) or by wind/breeze, is definitely more powerful/useful than any mechanical assist.
    • CommentAuthordereke
    • CommentTimeAug 31st 2022
     
    For external shading I've had good results wrapping £1 emergency survival blankets around velux windows.
    For the next heatwave I've got some doublesided tape and I'm going to stick them on my other windows that have no shading.

    The velux windows I have have internal blinds and without the shading I measured temperature at 70C on a sunny 30C day. With the emergency blanket it dropped down to around 35C.
    • CommentAuthorowlman
    • CommentTimeSep 2nd 2022
     
    Maybe paint your house white with one of the new high reflective paints being researched?
    • CommentAuthorjon
    • CommentTimeSep 2nd 2022 edited
     
    For external shading I've had good results wrapping £1 emergency survival blankets around velux windows

    I used OSB shutters (which are actually stowed roof access panels) and they worked well. However, fitting 40-50mm poly sheets to the glazing inside face works very well if not better (albeit makes almost as dark a room as the OSB). Has advantage that you can, if needed, use same arrangement in winter to cut down heat loss.

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