Advantages of DG Windows

[fictitiousemail]

A friend of mine stays in a bungalow about 4-5 years old . I always think it's freezin' when I sit in her living room but due to her ,ahem, " Inbuilt heating system" she doesn't seem to . The room is only heated by one rad which is under the window( looks about 1000 x

400) and she has no full length curtains ,only one Austrian Blind (?) . The room must be losing loads of heat by this arrangement I'd imagine. One day I placed my hand flat against the glass and it was freezing cold. It set me thinking about how much advantage DG actually gives over SG windows or is feeling the glass misleading?

[John Rumm]

You can compare them by looking at the u-values. A single glazed window could be as high as 5, and good DG as low as 1.7.

So for a given size and temperature difference, the best case heat loss of the DG window is about a third of that of the single glazed one. These things are all relative though - if its in a large expanse of solid brick wall for example, you will probably find far more heat is lost through that since its u values could be 2.2

[Roger Mills]

If the bungalow is less than 5 years old, it presumably already has double glazing - and insulated cavity walls, and insulated roof space?

Assuming that the window is double glazed with a UPVC frame, the heat loss from it will be about 62 watts per square metre for an inside temperature of

21 degC and an outside temperature of -3degC.

A single glazed window with a wooden frame would lose almost twice as much heat (120w/M^2). How big is the window?

The glass *will* feel cold - even for a double-glazed window - because heat is flowing out through it, but DG won't feel as cold as SG. Some heavy curtains would certainly help.

Assuming the radiator to be a single panel with fins, its output will be about 700 watts (or 1250 if double with double fins). One would need to know a lot more about the room size, number of outside walls, etc. in order to determine whether that was adequate.

[Owain]

And in this case, a fairly new build, the rest of the property should be fairly well insulated too. I suspect 'she' just doesn't turn the 'stat up.

Owain

[The Natural Philosopher]

ER, maybe for a modern cavity wall with thermalite as the inner leaf. For single 9" brick, make that 6...

the main value in DG is to replace draughty old windows, and to keep window condensation down.

I you want warm, avoid windows anyway. A well insulated wall is U=0.4 or less..

Not even triple glazed can get near that..

[The Natural Philosopher]

my rough calcs on SG plus thermally interlined curtains suggests that if the window is draughtproof, the combo is as good as the (insulated) walls, especially if the curtains trail to the floor. Its the ability of cold air to flow off the window and down behind the curtains that sets up the heat loss. If the curtains effectively prevent that, there is very little convection, and you have a dead space of about 6-9" of air acting as a brilliant insulator. Even wit internal convection in teh window space, the curtains themselves with interlining, are probably a U value of not much more than 1-2. Our rather good curtains when hanging, represent about 8-12mm of 'stuff' - show fabric, interlining, and lining, and effectively more when pleated as is the current style..

Also shutters, if sealed, act as a much better result.

Going down the mediaeval theme,. you would be surprised how good an insulator 3 meters of rubble and stone castle wall is, especially when hung with thick handwoven tapestries, or panelled in wood.

[dennis]

That doesn't make sense, a single glazed one should be much worse.

[pete]

According to an "official" website, surprisingly little. If you go over to

do one of their home energy checks, you can play around with different combinations. One "what if" that I did was to compare the energy bills with several home improvements on my 3-bed[1] semi. The difference between SG and DG was an fual bill saving of £47 / year. That's far below the cost of the interest payments for having it installed - so it would never pay for itself.

[1] laughingly described as such by the estate agents.

[chris French]

In message , Roger Mills writes

The glass will feel cold anyway, because it's a good heat conductor and conducts heat away from your hand. A sheet of glass at room temp will still feel cold to the touch.

Though one ina SG pane will feel colder it really isn't a way to judge the heat loss

[Roger Mills]

What I said was based on the U values built into the Myson Heatloss program, which are: Wooden DG - 2.9 Aluminium DG - 4.3 (or 3.7 for thermal break) UPVC DG - 2.6 Wooden SG - 5.0 Aluminium SG - 6.4 (or 5.8 for TB)

I was comparing wooden SG with UPVC DG - which (according to Myson's figures - 5.0 vs 2.6) is just under twice as bad.

With what do you disagree?

[John Rumm]

Do you have a reference for that?

(all my tables have it much lower)

Depends on if you include solar gain ;-)

[fictitiousemail]

All I know is that her system does not have a room stat ..just TRV's on all except the bathroom which is usually the warmest room ,although it's smaller than the other rooms.

[The Natural Philosopher]

solar gain is a heat input, not a heat loss.

At best it might, on a large picture window room, net you 5W/sq meter of room area in the afternoon.

In winter at night you will be pushing at least ten times that in to heat a well insulated house.

Solar gain is useful, but its a low order effect.

[dennis]

8<

There is 20 mm of air/argon insulation in a double glazed window, none in a single. There is 8 mm glass in a double glazed window vs. 4 mm in single glazed. So a single glazed window is half before you add the much better air to the equation.

[dennis]

IMO if you are going to fit a really crude system using TRVs then they need to be fitted to all rads but supplied with one too few heads. The missing head should follow the wireless stat so that the user can put the stat where it suits them. You get none of these control problems when you fit a few zone valves and stats.

[Roger Mills]

That's alright then! You clearly know better than all the scientifically measured published figures!

Judging by the published figures, the frame contributes quite a lot towards the overall heat loss - but, in your infinite wisdom, you have chosen not to take that into account.

[The Natural Philosopher]

its not just the glass conductivity either.

There are issues with the way convection flows around the pane.

which complicate it a bit beyond simple mathematical models.

[dennis]

Obviously!

If you compare aluminium to UPVc then yes the frame will make a big difference. However you are comparing DG to SG not different frames.

[Roger Mills]

No I'm not! Read what I wrote!

I'm comparing UPVC DG with Wood SG [I don't have figures for UPVC SG - if it exists].

But that's not the point anyway. If - as I suspect - a sizeable proportion of the heat loss is through the frame, the *overall* effect of going from SG to DG will be less than it would if you just consider the glass.

[John Rumm]

Which was my point - by allowing heat to enter it lowers the nett flow out - making the thermal resistance appear larger than it is.

Much higher than that in the summer...

Depends on the circumstance - it can be several hundred watt / m^2 in the summer - when it is often a problem rather than a benefit.