I spend my days talking to homeowners about improving the efficiency of their houses. Everyone has heard about “fabric first,” but I thought I would address a few misconceptions.
Your house is a box; inside, it’s warm, outside, it’s cold. The heat from your house leaks out, partly through drafts and partly through the walls, ceiling, floor and windows. It’s just like a cup of tea; it starts off warm but slowly loses its heat. If you have a tin cup, the tea goes cold quickly. If you wrap the cup in insulation, the tea stays hot longer. In your house, you don’t want it to go cold, so the rate at which it loses heat determines how quickly we have to top it up with the heating system and the cost to do it.
When I explain this to people, they say, “It’s OK, I’ve got 600mm thick solid walls. The insulation is good.” Unfortunately, that’s not true. Thick walls do not necessarily mean good insulation. Let’s explore.
Heat loss is measured in how many Watts of heat leak through the wall if it’s 1 metre square and there is a 1-degree temperature difference from one side of the wall to the other. It’s called a U-value. It’s a useful tool to compare insulation. You can google most U-values. Lower is better. Zero is the dream; 0 = no heat loss.
Solid 600mm thick walls have a U-value of 1.68 W/m² K.
Solid 300mm thick walls have a U-value of 2.78 W/m² K. The heat leaks out of the thinner wall faster – no surprises there.
Since the 1920s, we have built houses with cavity walls, a 100-year-old cavity wall of brick, an air gap and blocks inside, as shown below. It has the same U-value as a 600mm solid wall, 1.6 W/m² K.
Filling the cavity with cavity wall insulation reduces this heat loss to 1.0 W/m² K, nearly halving the losses. That’s why it’s so popular; it makes sense and it’s easy to do.
But it gets better. Celotex insulation is the stuff you buy in sheets covered in aluminium foil.
50mm thick Celotex has a U-value of 0.44 W/m² K. Or, to put that into context, 50mm Celotex lets only 1/4 the heat leak out, compared to a 600mm thick wall, and under half the leak rate of a cavity-filled wall.
The problem is it’s hard to slide sheets of Celotex between the walls in an existing house. The only realistic solution is lining the walls of your house with Celotex. It makes a huge improvement in the insulation and massively reduces the heating costs. It’s a great idea, but it’s not easy to do without disrupting the house decorations. But if you are renovating, it’s a very good idea to do it. And you can use it in the floors and ceilings to similar effect.
Nowadays, houses are built with a cavity filled with Celotex, normally 100mm thick. The total wall U-value is 0.18 W/m² K. That’s nine times better than a 600mm solid wall.
Now you know why old churches and cathedrals are so cold.
How do you work out that “Solid 600mm thick walls have a U-value of 1.68 W/m² K" whilst “Solid 300mm thick walls have a U-value of 2.78 W/m² K"? Surely if 600 mm is 1.68 then 300 mm must be 3.36? i.e. double! (ignoring the surface resistances)
@djh if I remember rightly it’s to do with U and R values being reciprocal. Read this to get the idea
https://www.thegreenage.co.uk/article/thermal-conductivity-r-values-and-u-values-simplified/
Yes, I know about U & R values. That’s exactly why I’m asking!
U = 1.68 W/m² K => R = 0.595 m²K/W
U = 2.78 W/m² K => R = 0.360 m²K/W
0.360 isn’t half of 0.595
@djh apologies it was late and I didn’t really consider your question. It might be down to the effect of the wall covering. Using a U value calculator I got 2.43 and 1.56 with the wall covering automatically allowed for.
@bontwoody
A lot of the reason the u value doesn’t scale with 1/wall thickness is because in-situ insulation also includes air boundary layers. That’s why single glazed windows get a u of 6, rather than some insanely high value just associated with the glass (glass lambda is 1.1, so a 4mm pane of glass alone has a U=275W/m2/degC 🤨 ). Indoors for vertical surfaces the still air boundary layer is generally 8W/degC/m2, outdoors it depends on wind speed, I think 20 is often the assumed default.
@robl Thanks, thats interesting 🙂
My house is a converted barn, built of stone in 1840.
There’s not much scope to insulate it 🙁
The terms ‘high thermal mass’ and ‘fabric first’ occur frequently in documents which accompany planning applications by the larger national construction firms.
They confuse the hell out of Councillors who are members of the Planning Committee.
I know that because I enjoy good communication channels with Councillors in my Local Planning Authority (LPA) and have the privilege of being asked by them about energy matters.
If nothing else this discussion topic must serve to better equip readers to write submissions to the LPA when multiple-dwelling applications are ‘out for public consultation’.
Councillors are eager to receive submissions which explain matters to them, as opposed to residents who complain about ‘non-material matters’ which can’t be taken into account.
An Application for 120 houses in my area was recently withdrawn by the developer because I’d picked up a number of their assertions and explained the technical aspects to the Council. Under the Planning Act my submission becomes a public document, available for anyone to view. Thus it serves as a ‘warning’ to planning agents for other companies considering a speculative application for inefficient houses in the district.
I substantially agree with @SUNandAIR that high thermal mass is only effective it it’s inside the insulation layer.
I think you can now deduce my response to this!
But in practice most householders don’t have such a choice to consider.
The existing building style dictates how they might be able to increase the insulation level.
My 1937 Devon ex-farmhouse has concrete blocks for both leaves of the wall, with a then-standard 50mm cavity.
Of course I have now had that cavity filled… funded by a Local Authority scheme about 16 years ago.
The ongoing renovation work has now progressed to the ground-floor room which is coldest – on the NE corner.
The floor has been removed and is being replaced.
It has new joists with PU solid insulation board between them and will be fitted with (wet) underfloor heating.
Most of the insulation boards come from Seconds and Co in Powys.
They may be under-sized, have a damaged edge, or surface ‘bumps’ due to faults in manufacturing.
That means they’re half-price 😀
I bought a lorry-load in the various thicknesses I wanted, and they sit on pallets outside under tarpaulins until I’m ready for them.
Wall Insulation. There is an opportunity to add internal insulation to the East Wall, where a large fireplace is being removed.
As I already have a stack of 30mm insulation board (aluminium both sides), that’s what I’ll use.
Yes, I could also do the same for the North wall, but it has a large bay window.
I don’t want to spend ages making new window reveals for minimal gain, so I’ll leave that as-is.
So my contribution here is more ‘hands on’ than the theoretical proposals from Graham and Sun & Air.
But that might be useful to @Beazy
I’ll keep taking photos of progress.
Please ask for more detail on anything you see.
Great post @Graham Hendra
We’re about to start renovations at home and looking to install an ASHP along with Solar PV, battery and MVHR. We will also be insulating our house as we live in a 1930s detached house that feels like it’s similar to the churches you refer to! On insulation, we were planning to insulate externally to avoid reducing the room sizes, and it means we should also insulate the floor voids between floors that we wouldn’t be able to do if we were doing internal insulation. Do you have a view (or evidence) of whether external insulation is superior or inferior to internal?
In the days when I grew up it was expected that most science ‘knowledge’ should be conveyed to pupils/students by the use of hands-on practical experimentation.
Oh – happy days! 😀
One task was to evaluate the best hand-washing strategy using soap to remove bacteria.
We started by brain-storming the likely variables
etc
Experimentation was done, hand-swabs taken and cultures grown.
Alas, there was very little variation between any combination of these variables.
Finally the professor suggested that we’d missed out a particular variable…
… the number of times that a washing procedure was undertaken.
And that transpired to be the most important factor of all.
Irrespective of all the other variables, it was the number of times that the hands were washed which greatly outweighed anything else.
Why?
Suppose any hand-washing cycle was around 90% effective….
thus 10% of the bacteria were still present.
Repeating that cycle would then remove 90% of the bacteria which had remained, resulting in just 1% left on the hands.
Repeat the hand washing a third time, and we have only 0.1% of bacteria remaining.
Watch a surgical team preparing for an operation and you’ll see them putting this into practice.
The same is true for the task of keeping heat within the home.
Heat is the movement of particles.
The atoms and sub-atomic particles oscillate or vibrate.
Cool everything down to Absolute Zero (-273°C) and all movement stops.
The three categories of heat have different properties:
Conduction is the movement of atoms within a material.
In a copper pipe, an atom receives a ‘nudge’ and passes this on by bouncing against the adjacent copper atoms.
Convection is the movement of hot air. The atoms are further apart than those in cold air, which means warm air is less dense.
The lighter air rises until it hits the ceiling and then moves across it, losing energy to the plasterboard.
Radiation is energy being transferred as photons.
Unlike the visible light spectrum, you can’t see this heat with your eyes.
It’s in the Far Infra-Red (FIR) region, with wavelengths of 3–100 μm.
The best approach to home insulation is to use several layers through which the heat energy must travel before reaching the outside air.
Having ‘light’ aerated concrete blocks for the inner-leaf of the wall is a good start, but that’s just one layer.
If you have a 1930s house (as I do), I then fill the cavity to prevent the movement of air.
That’s the second layer of defence.
But both of those measures are more effective at reducing heat transfer by conduction and convection.
What about radiation?
The holes within the aerated block and cavity insulation are enormously greater in size than photons, which will pass through unimpeded.
The PIR (Celotex) insulation board which Graham mentioned at the start of this topic also incorporate a thin layer of aluminium foil.
That adds a defence against heat radiation.
In my own renovation project I’ve been identifying cold walls and adding PIR rigid board panels, held in place by a wooden framework.
The (scrap-) wood battens are deeper than the (2nd hand) insulation boards.
They don’t get pushed back against the plastered wall behind them because the rear aluminium foil layer would then be in contact.
I don’t want to allow heat to be conducted from the foil to the plaster.
So the few millimeters of gap prevents that.
The timber battens are then covered in (heavy) cementitious boards.
Fibre-cement boards are made by STS and Fermacell.
These boards have high thermal-mass which acts as a warm layer.
The wall surface retains heat which is being radiated from the underfloor heating.
Once again, there is a 3mm gap between the back of the fibre-cement board and the PIR insulation board.
And you can probably now guess my answer.
There is negligible difference between these approaches.
Each one is merely adding one more layer of defence.
What you’re actually looking for is a multi-level solution.
External wall insulation (EWI) is generally thought to be better than Internal wall insulation (IWI) because it keeeps the thermal mass of the walls warmer and it reduces the potential problems of condensation within the wall structure. My walls have but a single layer of insulation; it is a certified passivhaus.
Nick – I have installed Solar PV, storage battery and MVHR, all using a DIY approach in my 1937 detached house.
If you raise any of these issues in fresh topics, please tag me.
As a self-builder who is also involved with future energy strategy, I don’t get to read all topics here on the Forum.
But I would want to reply to you with photos, diagrams and lists of potential suppliers if you raised these subjects.
Equally, please post relevant photos of your house as it currently exists. 🙂
@Transparent thanks for the note. I’m new to this site so not sure if direct messaging is allowed or facilitated but it would be great to pick your brains on what you’ve done.
Yes, DM is available, although the line-formatting is naff. 😥
Simply click on my name in blue.
You’ll arrive at the info page for me.
Click on the envelope at the right hand side to open the message editing window.
However, we do prefer that discussions are open on the forum.
The idea is that:
Some of our input will depend on your own situation.
Be prepared to share with us your approximate location.
We can then respond in ways which reflect
@Transparent understood, thanks very much. I’ll create a new forum accordingly