Cost benefit analysis of thermally broken front door

[Adsibob]

I’m trying to work out whether it is worth spending extra on a thermally broken steel double glazed crittal style front door with side lights and top lights into our 1930s semi that we are in the process of upgrading. The alternative is to get the exact same door, with the same glass units, but the steel frame will be shallower as it won’t have a thermal break. Aesthetically, both alternatives look identical, except when the door is open the thermally broken one is about 40mm thicker.

The dimensions of the opening which im glazing is about 1925 wide and 2400 high. Of that, 925 by 2025 is the door and the rest is fixed. It is almost entirely of double glazed glass, except for a steel kick plate occupying the bottom two fifths of the door.

 

By the time we are done with it, the house will be very well insulated. Not passive haus standard, not close, but pretty good for a house that was built almost 100 years ago. We are putting quite a bit of insulation in, throughout the house, thermally broken double glazing which is A+ rated, whatever that means, insulating the loft, under the floors, insulating externally, putting in MVHR etc. 
 

Whether we get it thermally broken or not, the door comes with built in draft excluders and is pretty airtight. Given about 80% of it is double glazed glass, the thermal break/no break impacts only 20% of the area.
 

The difference in cost is staggering. Something like £2.5k! I will never make that back will I, or is a non thermally broken door and window this big really going to make that much difference?

[Oz07]

No chance. Forget that. 

[Conor]

Find out the overall U value of both unit options and take it from there. I think you'll be unpleasantly surprised by the thermal performance of the unbroken unit. Have you looked at aluminium or composite doors? They'll generally perform much better.

[Adsibob]

2 hours ago, Conor said:

Find out the overall U value of both unit options and take it from there. I think you'll be unpleasantly surprised by the thermal performance of the unbroken unit. Have you looked at aluminium or composite doors? They'll generally perform much better.

Thanks @Conor. I’ve asked for the u value comparison, and am waiting to hear. They seem to only have the values for the glass to hand.  I looked at aluminium, but the frames are far too thick to look anything like Crittall. RK doors for example can’t do anything thinner than 75mm. Although MetTherm have a beautiful thermally broken option in Aluminium that has only 26mm thick frames, they won’t sell them as a front door. No idea why, as they offer pas24. 

[Bitpipe]

I'd say not to bother.

 

We got a passive RK door for our passive new build so it made some sense not to compromise on that when the rest of the house was built to such a high standard but in your case I think the advantages will be negligible.

[Conor]

1 hour ago, Adsibob said:

Thanks @Conor. I’ve asked for the u value comparison, and am waiting to hear. They seem to only have the values for the glass to hand.  I looked at aluminium, but the frames are far too thick to look anything like Crittall. RK doors for example can’t do anything thinner than 75mm. Although MetTherm have a beautiful thermally broken option in Aluminium that has only 26mm thick frames, they won’t sell them as a front door. No idea why, as they offer pas24. 

I'm not surprised, as I'm guessing the overall performance of the door will be quite poor. I'm guessing in around 1.8 or even worse. Make the decision then, and from somebody that lived in a similar house, there's nothing worse than a cold feeling entrance hall!

[joe90]

As most of you will know I am not into stats etc, my front door was made fir me by a locally company who built all the windows and conservatory, I specced “insulated” and they built an oak door filled with insulation. No idea about values. But I have an enclosed porch to act as a barrier, airlock, and this makes a massive difference. Likewise my back doors (bifolds) open into a double glazed conservatory so again a barrier, airlock. I don’t like the idea of a single door to the pit side world as they will never be insulated as well as the walls.