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.

My house is a converted barn, built of stone in 1840.

There's not much scope to insulate it 🙁

@djh @bontwoody 

this is very interesting - so can we take this discussion further?

I’ve always thought mass is very useful in internal walls as it becomes a store which radiates slowly back into the living space…. Thereby reducing fluctuations in room temps say during an overnight setback. 

whereas an external wall of high mass construction is a constant drain on the latent heat of a living space. I am assuming!!! Creating down drafts at the sides and corners of a room. 

So would internal PIR insulation or external PIR cladding be better for a 600mm external walled building? 

does insulated external mass help in any way to the comfort of the occupants?

@djh @bontwoody @grahamh

this is very interesting - so can we take this discussion further?

I’ve always thought mass is very useful in internal walls as it becomes a store which radiates slowly back into the living space…. Thereby reducing fluctuations in room temps say during an overnight setback. 

Whereas an external wall of high mass construction is a constant drain on the latent heat of a living space.
So, I am assuming!!! Creating down drafts at the sides and corners of a room. 

So would internal PIR insulation or external PIR cladding be better for a 600mm external walled building? 

Does Externally-insulated external mass help in any way to the comfort of the occupants?

while internally insulated walls will lower the internal mass of the living space and so not helping so much to store heat?

These are, I think important nuances which could be better understood.

Of course accuracy of quoted data makes for confidence and peace of mind.

Posted by: @sunandair

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Does Externally-insulated external mass help in any way to the comfort of the occupants?

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.

Posted by: @sunandair

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So would internal PIR insulation or external PIR cladding be better for a 600mm external walled building? 

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.

@sunandair So my take on it would be that having the walls thermal mass inside the insulation layer, would act as a thermal store and be ideal for 24/7 heat pump installations. However if external thermal insulation wasnt feasible, it would still be worth insulating the inside of the walls, just the keep the heat inside even if no thermal store effect.

For non heat pump uses like a traditional boiler with lots of offs and ons in a day, it would probably be better to keep the insulation inside the walls, for speedier heat ups when the boiler was turned on.

@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 🙂