Heat Loss through Double-Glazed Windows

[windowproblem2025]

Hello,

I only joined the community yesterday, so apologies in advance for any misunderstanding on Forum Theme.

My house was built circa 1975 - and had the old-style chimneys. It doesn't probably have sufficient cavity insulation etc. as it does in all modern buildings.

I suspect I am having major heat loss through my Double-Glazed Windows. Although, they are structurally OK, from Thermal Imaging they seem like the biggest spots of heat loss.

To clarify, I have attached the thermal images for consideration. My questions for the community are:

1. Considering the Thermal Images are taken inside the property, Do I have major issues with Windows?
2. Do I need to replace with more modern Catchment Windows (Without having to go through Council Paperwork)?
 

Regards, 

[mjc55]

Not an expert but would probably help to take photos from the outside.

[RobLe]

It’s most likely that your DG windows are U=2.7W/m2/degC, or thereabouts - that’s what you get from two panes of glass with a 16mm air gap between, as happened until around 20 years ago.  It’s possible to measure the U value of a window.  Now there’s a metal low emission film applied, a warm edge spacer, and ‘heavier than air’ gas inside the cavity.

The obvious thing to check first  is if you e had your cavity filled yet?  You can generally tell from the brick pattern outside.  I say this as the wall looks cold too.

[windowproblem2025]

1 hour ago, RobLe said:

It’s most likely that your DG windows are U=2.7W/m2/degC, or thereabouts - that’s what you get from two panes of glass with a 16mm air gap between, as happened until around 20 years ago.  It’s possible to measure the U value of a window.  Now there’s a metal low emission film applied, a warm edge spacer, and ‘heavier than air’ gas inside the cavity.

The obvious thing to check first  is if you e had your cavity filled yet?  You can generally tell from the brick pattern outside.  I say this as the wall looks cold too.

Thank you - I am thinking at this stage, what is my best investment options. Loft Insulation only provides roof support. But the walls (as you said and I had suspicion about) are the biggest cultprit.
Would replacing Windows (some of them) to Triple Glazed be a better choice to have heat loss (U value improvement) prevention?

FYI - I have attached some external wall pictures too - Taken from outside (which someone else also asked about).They are both taken during evening hours, and the radiators inside were switched off an hour before. 

[Redbeard]

Crudely, as long as you can see through it the insulation value is not that good. A really good triple-glazed window may get you a whole-unit U value (i.e. everything; not just the glazed unit) of as low as 0.7W/m2K. Assuming your cavity has no fill and is 'typical' for 1975 (circa 50mm) you'd get 0.55W/m2K or a little below with blown-in insulation. (I'd recommend graphite expanded polystyrene, not mineral wool). Your area of wall will (unless your house is very atypical) be far greater than your glazed area, so the right order is walls 1st, windows and doors later (unless your roof insulation is the original (?circa 50mm) in which case carefully lay 300mm on top: U value around 0.14-0.15W/m2K, and do that before everything else).

 

What you do with doors and windows also depends on the state of the whole units. If the frames are not original they may have a reasonable standard of draught-stripping, in which case your main 'let-down' is the glazing spec, and when it comes to glazing compared to other fabric elements it's a question of 'less bad' more than 'better' (The 'holes in the fabric' will always fare worse than most other thermal elements even with 'bells and whistles').

 

[windowproblem2025]

4 minutes ago, Redbeard said:

Crudely, as long as you can see through it the insulation value is not that good. A really good triple-glazed window may get you a whole-unit U value (i.e. everything; not just the glazed unit) of as low as 0.7W/m2K. Assuming your cavity has no fill and is 'typical' for 1975 (circa 50mm) you'd get 0.55W/m2K or a little below with blown-in insulation. (I'd recommend graphite expanded polystyrene, not mineral wool). Your area of wall will (unless your house is very atypical) be far greater than your glazed area, so the right order is walls 1st, windows and doors later (unless your roof insulation is the original (?circa 50mm) in which case carefully lay 300mm on top: U value around 0.14-0.15W/m2K, and do that before everything else).

 

What you do with doors and windows also depends on the state of the whole units. If the frames are not original they may have a reasonable standard of draught-stripping, in which case your main 'let-down' is the glazing spec, and when it comes to glazing compared to other fabric elements it's a question of 'less bad' more than 'better' (The 'holes in the fabric' will always fare worse than most other thermal elements even with 'bells and whistles').

 

Thanks again. Yes my loft insulation and roof work is original. So from what you are recommending, I understand this.

1. Re-insulate the roof with 300mm - Fibre Glass or Mineral Wool?
2. Re-insulate the walls - the most needed one.
3. If Thermal profile doesn't improve - move to Windows one at a time.

Also for no. 2 - How do I detect missing insulation? My understanding is that if the walls doesn't look colder from the outside/warmer from the inside - this means they're not well-insulated. But I am unsure if that's a good way to verify. Is there anything you could recommend before I call the tradespeople?

Regards, 

[lookseehear]

Not an expert, but I would be a little wary of IR camera results from window panes themselves.

 

Glass will allow IR radiation through, so it's possible you're measuring the temperature of what is outside when measuring from inside, or what's inside when measured from outside which will distort your view of the windows.

 

As has been said before, if your glazing isn't blown and is generally in a good state of repair, you can find out what the u value is roughly and therefore what is to be gained by replacing them.

 

Thermal cameras are better for identifying cold bridging around the windows, unseen draughts and the overall performance of the frames than the window panes themselves.

[windowproblem2025]

@lookseehear Thanks for the reply 

 

Quote

if your glazing isn't blown and is generally in a good state of repair, you can find out what the u value is roughly and therefore what is to be gained by replacing them.

So how do I check if the glazing isn't blown? Because I can see that the windows are not cracked or Glaze Gaskets (Rubber) aren't completely off. 
Sorry for these daft questions, as I never had to be entrenched into house maintenance at this level. 

[RobLe]

Your walls likely don’t have CWI (cavity wall insulation); you can usually see the pattern drilled into them if it’s been done.  Typically 25mm dia holes into morter, every 500mm or so over the entire wall.  Suggest taking a photo of the brickwork (regular, not IR), and we can likely tell.

[Redbeard]

3 hours ago, windowproblem2025 said:

So how do I check if the glazing isn't blown?

 

If it's 'blown' (the seals on the separator bars between the panes have failed) there will be condensation inside the units, between the panes. If not, they're OK. It is very hard (without a special detector) to establish whether a unit has low emissivity glass to at least one pane, but if you can find the date of the windows you may be able to 'reverse engineer' with ref to the Bldg Regs at the time.

[Redbeard]

3 hours ago, windowproblem2025 said:

1. Re-insulate the roof with 300mm - Fibre Glass or Mineral Wool?

 

'Mineral Wool' is usually used as a 'cover-all' term for either Rockwool (or rockwool!) or glass-fibre. I don't like working with either of them so tend to avoid them, but if they don't bother you, use what you can find cheapest. (Note that rockwool is available with a range of lambda values. 0.044W/mK is 'standard'-ish, but I think you can get it down to 0.035 or even lower for rigid or semi-rigid batts (the EWI stuff for example tends to be around 0.034, but you would not use (nor want to pay for) that in your roof void.

[RobLe]

1 hour ago, Redbeard said:

 

If it's 'blown' (the seals on the separator bars between the panes have failed) there will be condensation inside the units, between the panes. If not, they're OK. It is very hard (without a special detector) to establish whether a unit has low emissivity glass to at least one pane, but if you can find the date of the windows you may be able to 'reverse engineer' with ref to the Bldg Regs at the time.

You can figure it out from a thermal conductivity (U value) ratios, based on a known value:

Indoors, in still air, a vertical boundry air layer has a U value of 8W/m^2/degC.  That is from the centre pane glass surface, to the air temperature in the middle of the room.  The U value of the glass represents the heatflow per m^2/degC through the glass, based on the air temperature in the middle of the room and the outdoor air temperature.

 

So, if you measure the values below with the same IR thermometer pointed at a bit of tape stuck on the glass etc:

Tindoor = the indoor air temp

Toutdoor = the outdoor air temp

Tglass = indoor centre pane glass temp

 

Uglass = 8* (Tindoor - Tglass)/(Tindoor - Toutdoor)

 

I can generally get the answer within +/-20% of the expected value when it's cold outside.  That's good enough to tell if the units are just glass, or have a coating on them.

  

 

 

 

 

 

[SteamyTea]

Right, not read everything in detail.

 

Rather than fixate on one components i.e. your windows.  Work out all the external areas i.e. walls, windows, doors, roof/ceiling, floor.

Get an idea of what the U-Values are and then work out the losses.  It does not have to be very accurate, just a good estimate.

 

You will probably find that your roof/ceiling is the largest area, so improving the insulation there with some properly fitted wool will help a lot.

 

3 winters ago I made some very cheap secondary glazing to cover my old timber framed double glazed units.

This is the difference it made.

 

[windowproblem2025]

17 minutes ago, SteamyTea said:

Right, not read everything in detail.

 

Rather than fixate on one components i.e. your windows.  Work out all the external areas i.e. walls, windows, doors, roof/ceiling, floor.

Get an idea of what the U-Values are and then work out the losses.  It does not have to be very accurate, just a good estimate.

 

You will probably find that your roof/ceiling is the largest area, so improving the insulation there with some properly fitted wool will help a lot.

 

3 winters ago I made some very cheap secondary glazing to cover my old timber framed double glazed units.

This is the difference it made.

 

 

 

@SteamyTea Sorry if I misunderstood - So you created a sort of triple glazed Window ?

[SteamyTea]

7 hours ago, windowproblem2025 said:

So you created a sort of triple glazed Window

Yes.

Just some cheap styrene sheet, a frame made from thin moulded timber and stick on foam draught excluder.  As I have wooden window frames I just screwed them in place.