Insulation / heat transfer question

And related, if any one knows. The opposite case, in summer. Traditional cavity in brickwork allows the outer skin of brick to heat up from direct sun, but heat gain substantially passes up to the loft, yes/no ?. Then someone fills the cavity with insulation, cutting off the vertical convection, leaving conduction to the inside brick skin or not? I cannot find data on this comparison summertime.

Yes - I'd have thought you are far better off having the cavity-effect than an external wall.

But you are losing some energy due to temperature differences. According to (ahem) AI there's a 460W continuous flow from B to A - likely estimate based on a number of assumptions.

In message <10frji3$82bb$ snipped-for-privacy@dont-email.me, at 06:00:35 on Sat, 22 Nov

If it helps, the party wall is about 2m tall, and 14m long; on each floor. The long outside wall is the same and has a plastic front door.

The house is 4m wide, has a bay window at the front, and some patio doors at the rear. All the windows (one per room) have been recently replaced with fairly high spec double glazed units, and most windows have drawn, heavy curtains.

The loft isn't boarded, and has about three inches of insulation. Downstairs is about half solid floors - partly insulated to recent standards, and half a suspended wooden floor which was replaced with substantial oak planking about 20yrs ago.

It’s a two-edged sword…

Some decades ago when we lived in a semi, our immediate neighbours bought a new house off-plan in another town, in a deal which included the builder buying their then current property. It was empty for about four months, as luck would have it this was over winter. The extra load on our heating was noticeable and the party wall was noticeably cooler than normal. I’ve probably got the figures somewhere but not at present to hand.

Yes, you will be losing heat to the attached property, but in return it will be warmer than outside, limiting the loss.

I suppose the bottom line is that there is nothing you can do about it, it’s just one of those things.

Insulate the party wall.

-- “Those who can make you believe absurdities, can make you commit atrocities.”

― Voltaire, Questions sur les Miracles à M. Claparede, Professeur de Théologie à Genève, par un Proposant: Ou Extrait de Diverses Lettres de M. de Voltaire

Both. A is acting as a leaky thermal battery. B is putting heat into A which raises the temperature of A compared with ambient. Some of that heat is leaking away but some of it isn't.

Consider a windy day. Every watt put through an exterior wall is instantly blown away, such that all the walls are kept at a fixed outdoor temperature. Watts put into A are not blown away, so B's side of the party wall is warmer than their other exterior walls.

OTOH the surface area of A is bigger than the surface area of the party wall, which may increase the wind exposure (and thermal conductivity generally). That's why the battery is leaky.

Indeed.

That's bollocks because heat flux is measured in watts per square metre. Without knowing the wall area there's no way to know the number of watts.

(for the record, heat loss of a normal room in 'worst case' UK winter conditions is typically 500W to 2kW so that's low for a whole house)

Theo

I did try to explain (perhaps not that well) that the calculation was made on a number of assumptions - including assumptions about wall area of a semi-detached house.

You didn't say what the assumptions were, just quoted a number. Did you specify any assumptions or did it come up with them? How much prompting did it need?

Well, 1.7*45*6 = 459W so it can multiply.

But, while that's a calculation of heat loss, it's not a 'heat loss calculation'. That's a standardised calculation based on the coldest winter night-time temperature of the average year at that particular location. When I did my calcs I think it was -3C. For that you'd get

In this case, what we need to know is the temperature of the inside of A on the coldest winter night. That depends a lot on the construction. For example, suppose it was a leaky house with an open fire, rattly sash windows, lots of air bricks to provide air for the fire, and the wind whistling under the floor boards. If the north wind blew you might cause the interior of an unheated house to drop closer to the outside temperature.

If this was just a single night it might not matter because things would warm up in the morning, so the thermal mass of the house might shave off the bottom of the fluctuation. But if it was a prolonged cold snap in which the house was 'cold soaked' below freezing (the kind of winter when you get burst pipes) then it wouldn't. Generally the calcs are accounting for the worst case (or the almost worst).

OTOH if it was a well-sealed house with a boiler set on frost prevention mode, you might find the temperature doesn't drop nearly as much. eg the boiler will keep it at 5C no matter what happens outside.

Indeed, solar gain will help. But the moral is you can't tell from a single datapoint.

Theo

None. I just fed it the OP's post, gave it a quick scan and the figures and factors didn't look outrageous. I didn't post all of the assumptions, and it generated multiple scenarios. I just gave a summary.

Is it? I couldn't conclude that - only the delta is given. Which looked like a not untypical autumn assumption.

Yes, very good. But this isn't an external wall.

Yes, I know, thanks. It was only ever a ballpark estimate.

No, I know. My post was only ever a rough guide. It wasn't supposed to be a last word and definitive comprehensive breakdown. Perhaps I should have made that clear.

I've looked into this a fair bit recently for work and some other stuff I'm involved in. I'm coming round to the notion that the 'sum of the parts' building physics approach is not necessarily that useful - especially for assessment of energy performance (and hence retrofit opportunities) without building inspection.

For example, the best scenario 9" solid wall is a better insulator than a worst scenario cavity filled wall of similar materials. You simply don't know how well a building performs until you actually see it, and understand how it's being used/abused.

Anyway, I digress. More here is anyone's interested:

In message <10fs7ui$cn1j$ snipped-for-privacy@dont-email.me, at 11:48:34 on Sat, 22 Nov

Why would I insulate my side of the party wall, when that's what's heating my house for free?

Because its your side that is heating an empty property for free

Are you sure that his house *isn't* the empty one?

In message <10ft0tm$nmtb$ snipped-for-privacy@dont-email.me, at 18:54:46 on Sat, 22 Nov

Wrong! Mine is the empty(ish) property being heated by next door.

How else would I have obtained all the temperature readings *inside* the empty house...

In message snipped-for-privacy@mid.individual.net>, at 21:42:24 on Sat, 22 Nov

It's empty of people (for all but a couple of days a month), but has rather a lot of other things lurking inside.

It does not really matter; the calcs show the rate of heat transfer through a wall - typically from the warmer side to the cooler. U value, area and temperature difference are the main factors (assuming there is a decent air seal between the properties)

More detail here:

It is not heating your house, it is just lowering the rate of cooling. Adding insulation would lower that rate of cooling more.

Yes, OK, I take your point. But don't agree. Exposure (or not) to sun, wind and rain, as well as susceptability to the standard/type of construction and maintenance, can be highly significant variables.

This webinar:

Even allowing for some hyperbole, I can't see how anyone could conclude that it doesn't matter.

What I can see is why such variables are not generally considered when thinking about, for example, retrofit.

I'd suggest that's a universal rule.

In message <10g011i$1r320$ snipped-for-privacy@dont-email.me, at 22:15:16 on Sun, 23 Nov

Without the party wall (ie if that was an outside wall) the unheated house would be somewhere like the average external temperature, which at the time I did my measurements was somewhat lower than the inside of the house.

So I conclude that heat leaking through from next door's heated half of the semi, is warming my half above what it would otherwise have been.

In message snipped-for-privacy@mid.individual.net>, at 07:56:06 on Mon, 24 Nov 2025, alan_m snipped-for-privacy@admac.myzen.co.uk> remarked:

Most of that heating would be on the roof [adjacent housing and vegetation mean very little direct sunlight on the south facing walls], which is of course insulated from the house; and last time I looked warm air rose (ie didn't fall).

I don't think the thermal storage of the house would last days.

But I didn't say "instantaneous", that would be absurd.

Maybe I should have added "daily" in between

Without the party wall (ie if that was an outside wall) the unheated house would be somewhere like the

average external temperature, which at the time I did my measurements was somewhat lower than the inside of the house.