U-value vs SHGC & double glazing

Dymonite, I had assumed the effect was due to the fact that some heat transfer occurs through radiation as the heat being conducted by the glass also causes the glass to radiate. Cut that radiation and you cut the heat transfer rate. I must admit though I was surprised that it could be significant. For example only 3% of external radiant heat is absorbed and subsequently re-radiated inside -- even if you cut it all, it's peanuts. I'd say it's tinted glass where it comes into its own as that tint would absorb a lot of heat. That's the radiant case though, not necessarily sure what proportion of conducted heat ends up leaving the glass as radiation rather than through convection.

But the thermal conductivity equation (U) states that energy flow has a linear relationship to temperature. However, radiant energy flow is a power relationship.

I would have thought the opposite. Conduction is linearly proportional to temperature difference (across the window), while radiant energy is virtually the same regardless of the inside or outside temperature.

Radiant energy is proportional to the surface temperature of the emitting body/bodies. The irradiance (radiant energy received) is related to the distance and angle in relationship to the radiant source.

During the day, solar radiant energy will dominate. At night time it will be from the background heat of the surrounding objects.

Dymonite, technically that is true what you said earlier about a non-linear relationship, but it is nearly linear.

Assuming a black body, radiation loss from a surface goes as -T_surf^4 and radiation gain from the environment goes as T_env^4. The net is T_env^4 - T_surf^4. Yes this is a quartic but it's not simply (T_env-T_surf)^4! Try it out with a plot in Excel or something. It's nearly linear.

As a Taylor series:

(DeltaT+T0)^4 - T0^4 = DeltaT^4 + 4 DeltaT^3 T0 + 6 DeltaT^2 T0^2 + 4 DeltaT T0^3

So you can see that the last term, the linear one in DeltaT, has a much bigger coefficient than the other ones.

Edit to add: in a practical sense surely these U-values must derive from a standard test of some sort. And as there is no way to measure conduction in isolation from radiation that doesn't alter a bunch of other things about the meaurement (short of actually measuring the radiation and subtracting it out in the analysis), I'd say it would be likely to be included in the result.

I view U values/R values for building materials with a great deal of skepticism.

The behaviour of a building component is different depending on what heating mechanism is operating. How does heat reflective paint get an R value? It can keep a building cool but by no means keep it warm.

The quoted U values for building materials is a bit of a fudge because it is a composite value for a variety of heat transfer mechanisms.

The value might be have been calculated under standard conditions but this means little in a real world situation. I would be interested to know the methods used for the net energy transfer through a building material.

For instance a standard thermal conductivity test involves two hot plates sandwiching the test material. The energy input is measured to hold a constant temperature differential. However, the measurement of radiant energy transfer involves a completely different technique and is not dependent on air movement as it is for convective flows.

For instance if I were to calculate the U value of foil under a radiant load I would obtain a fairly respectable value. However if calculated U value on the using energy transfer via conduction or convection it would be fairly dismal. Hence the nonsensical statement of winter and summer U or R values.

Similarly batts under a radiant load or subject to wind movement would seem to under-perform.

Yep, the problem is that DeltaT when you are talking about solar radiation is about 6000 degrees . But it is still surely useful to include non-solar radiative heat transfer in U values because this will also occur in practice as your house radiates through the window and the outdoor environs radiate in the other direction. Then add to this a separate treatment of solar radiation, which is where SHGC comes in for windows. Perhaps they should be quoting SHGC for RFLs to make them sound better... it would be pretty close to zero!

That would sound reasonable as the magnitude of this effect would be more on par with convective transfer. Best to conceptualise U value as a 'night time' or 'overcast' parameter and SHGC as a 'daytime' one.




Emissivity would be the most correct term.

The AWA article above seemed to suggest it really is a double glazed application. I reckon that if you are going to have window coverings anyway, the advantage of the low-e is somewhat mitigated. If I had a choice to upgrade I would always go with double glazed.

But in a temperate zone with reasonable insolation and good passive design I probably can get enough solar heat storage during the daytime to offset any losses I might experience with single glazed windows during the night.

I you mean storage in the building fabric I can't see it... even if you did have enough storage, it would deplete to zero as the day/night wore on without any control. Used to have night storage bank - same thing basically, hot when you don't need it, not hot enough when you do. On average it's great...

Radiant gain from solar energy during the day is 10x convective heat loss at night. Why wouldn't this be enough?

Thermal mass materials can store quite a lot of heating energy without reaching uncomfortable levels. It's why underground houses in Coober Pedy can moderate temperate extremes regardless of season.

10x solar energy in during the day versus losses at night is not enough. Firstly, winter days are short. You probably only get effectively 6 hours of equivalent "full" sunlight. Therefore you need 3x the input just to match the window loss. However, the windows are the only source of solar energy and needs to counteract it's night time losses and the night time losses of all the walls and ceilings. So the house doesn't heat up enough. Now if you can half the U value of the windows, you start getting a net heat gain.

In summary, double glazing is the way to go.

But what about radiant heat losses through the night... aren't they huge? They were in my home... (Albury then) - ffffreezing.

Regardless of all theories.
A poor frame and single glazing is a cold surface which causes discomfort.
It feels like sitting beside an open frigde and even with heavy drapes the cold sneaks up on you
Everyone around here who has changed to double glazed PVCu or timber windows from single glazed wood or aluminium (without having changed anything else in the house!!) is going on about how much warmer the house is
Theory is one thing, the real physical experience is the other

You are right, there is no such thing as a free lunch. You will eventually need supplemental heating. My question is how much heating energy will be offset by double glazing. Apart from the dead of winter there is a much less requirement for heating (if at all) with a passive solar design. Therefore the main advantages with insulated glazing is for a fraction of the year which becomes even less once you add window coverings.

On the other hand, if you wish to completely prevent heat loss then even triple glazing won't achieve this. Not even if you were to decide to have no windows in your house! Regardless of design, it is impossible to achieve a zero energy dwelling by passive means alone. You will always need some kind of additional heating input.

The question is not whether insulation works but how far much you can save in heatings costs for each incremental improvment you make to your building envelope.

True if you were to snuggle up close to an uncovered window on a winter night or actually leant against the pane Most people don't this. If you stay within reasonable limits of the external envelope the ambient air temperature is nowhere like that experienced near the window itself. The effect is lessened even more with a reasonably fitted window covering. Moreover, there is enough radiant heat from the interior structure to provide adequate comfort. Part of the discomfort near a sunless window is that there is no significant radiant heat approaching from that angle. That is negated the further away you move from it.

Your body, with its metabolism, provides a heat input, as does any electrical or gas appliance you use. I seem to remember reading about houses that can stay warm enough merely with those heat sources.

I used to argue with my wife about that when we were stuck in a rental with only electric heating. She would go round turning off unused lights and so-on to "save energy" and would not listen when I told her it would merely cause her to need to turn the heater up to use an extra amount of power exactly equal the amount no longer being burned by the lights . Still, those amounts are miniscule compared to the total amount used on heating.

Incidental heat from other domestic appliances can be significant but I am not aware that this can provide the only source of heating energy in extreme climates. Again it is a matter of proportions. Just turning on a stove top or oven might be quite adequate for a mild climate.

The opposite would be true in densely populated structures such as office buildings. Human activity, lighting and computer stations will require systems to actively prevent the building from overheating.