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Can wooden doors be as energy efficient as uPVC?

Posted by Tim Pullen on 27 June 2011 at 10:05 am

Q: I need to fit a new back door and want to make it as energy efficient as possible, but my carpenter seems a bit ignorant about what is the best way to go about this. I want to install a wooden door, not uPVC. Should I fit a sill? Or will it rot? It will sit on concrete. Or is one of those brush draught proofers as effective. What would you recommend?

A: There is a lot of talk about the comparative efficiencies of different types of doors. Timber has a relatively poor thermal efficiency compared to uPVC, but at least it is not uPVC. Timber with an insulated core is a compromise and people get very exercised about what thickness is best and the relative merits of each.

A typical external door is broadly 1.7 sq m in area. A uPVC door with a U-value of 1.8 will emit about 70W per hour of heat in the coldest months. A solid hardwood door with a U-value of 3.0 will emit 115W. It is not clear, to me at least, that it is worth putting up with all the negative aspects of uPVC for that relatively small advantage.

The door will need some assistance to stop rain and drafts getting in underneath and a sill or a rainwater-bar set in the concrete will be needed. There is no reason to suppose a sill will rot if it is properly installed.

Similarly, fitting brush draft excluders will help enormously. Even the very best timber doors are likely to move over time and a draft excluder will prevent it becoming a problem.

Photo by Laura Smart

About the author: Tim Pullen is eco-editor for Homebuilding & Renovating magazine, author of Simply Sustainable Homes and founder of sustainable property consultancy WeatherWorks.

If you have a question about anything in the above blog, please ask it in the comments section below.

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Comments

3 comments - read them below or add one

Feanor

FeanorComment left on: 2 April 2017 at 8:10 pm

Does anyone know why uPVC doors are less energy efficient than double glazed windows? Modern UPVC windows, if I remember correctly, have to have a u-value of no more than 1.3; doors, 1.8. It seems mad to me that glass insulates better than a solid material. I get that the argon gas in a double glazed window is very effective, but surely a layer of celotex or similar inside a door would be much more effective?

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anne miller

anne millerComment left on: 21 July 2011 at 8:02 pm

Interesting discussion. 

About 5 years agod we had a modern wooden back door installed (no way were we going to use PVC!).  Our builders installed it with excellent rubber seals set into the frame, which work really well to stop draughts.  Initially the main draught came thought the keyhole, but adding a simple old fashioned keyhole cover worked well to stop that. 

It opens straight into a well insulated kitchen so the thermal performance is quite important to us.... 

The surface of the door is however noticeably colder surface than the walls, so we're about to install a thick thermally lined curtain that we can draw to add an extra layer of insulation to the door in the winter.... I just need to get that sewing machine out to adjust them to fit!

Having spent much of my career as an engineer, doing heat transfer calculations, I don't think painting the door white would do anything to reduce the losses.. partly because the emmisivity of paint is't a lot different to wood, and partly because its all at relatively low temperature so radiative losses aren't anything like as important as conductive ones.  Hence interlined door curtain should be a much better solution.  

The only problem will be that I suspect we'll get condensation on the cold surface of the door behind the curtain, and we'll have to see if thats a problem.


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mj

mjComment left on: 27 June 2011 at 11:14 am

Please don't use 'x Watts per hour'; this is incorrect as Watts is a measure of power, not energy. What internal/external temperatures have been assumed to arrive at a heat loss of 70W through the door in the coldest months?

Would the colour of the interior surface of the wooden door have any impact I wonder on it's performance by, for example, reflecting some heat back inside if it is painted white?

I suppose there are a couple of other questions too:
1) uPVC is thermally more efficient, perhaps, but what about its environmental impact during manufacture and disposal compared with a wooden alternative.
2) Wood will need to be treated periodically with paints/resins - what is the cost and environmental impact of this during its lifetime?

I think, overall, the difference of 45 Watts (approx. 1kWh/day) as described in the article can be lived with if other adjustments are made such as using energy more efficiently in other areas of domestic life, or maybe driving a few miles less each year. A modern 1.1 petrol engine will use about 1kWh/mile, so reducing the number of miles travelled will have a huge impact.

 A very interesting topic, however!!



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