Is internal wall insulation worth it on a cavity wall?

[Colin Green]

Yesterday we had our offer accepted on a house and we're now planning what to do with it. One thing we want to do is improve the efficiency. We're going to add an air source heat pump and a 7kWp solar array, and we're considering internal wall insulation. The house was built in the 70s and has cavity wall insulation. Even so, it has an EPC rating of D (1 point below a C) with the loft fully insulated.

I've just done some simple calcs and the heat loss through the walls, windows, roof and floor with a 30°C temperature difference comes to 3.75kW. Adding 62mm Kingspan plasterboard to half of the internal walls (some rooms we don't want to lose any floor space) and the heat loss comes down to 3.2kW. For the sake of 540W on a really cold night, is it worth the effort?

 

[motorbiking]

I'd rethink the air source heat pump and maybe go for split system aircon that can heat and cool in the summer working with the gas central heating. You may also want to think about a power wall or similar with a 7kw install.

Does it have any aluminium original double glazing - these are like cold magnets and can have U values above 5 in places.
50mm PIR is a lot on an already insulated wall. I'd maybe think of only doing the North facing external walls. You may find its hard to take the walls back to brick as the "plaster" they use in the 60s and 70s was more like internal render.
what about the floor - is it uninsulated concrete?

Other options are to extend the house and wrap it in new walls.

 

[JohnD]

I've just done some simple calcs and the heat loss through the walls, windows, roof and floor with a 30°C temperature difference comes to 3.75kW.

If correct, that is remarkably low.

Spending money to reduce it a fraction will have a poor payback.

What will you do with the solar power you get in summer?

 

[Colin Green]

I'd rethink the air source heat pump and maybe go for split system aircon that can heat and cool in the summer working with the gas central heating. You may also want to think about a power wall or similar with a 7kw install.

Does it have any aluminium original double glazing - these are like cold magnets and can have U values above 5 in places.
50mm PIR is a lot on an already insulated wall. I'd maybe think of only doing the North facing external walls. You may find its hard to take the walls back to brick as the "plaster" they use in the 60s and 70s was more like internal render.
what about the floor - is it uninsulated concrete?

Other options are to extend the house and wrap it in new walls.

3 of the windows are the original metal framed with the thinnest of thin sealed units retrofitted. They'll be replaced with UPVC to match the rest of the house. Thanks for the tip about the plaster. That sounds like a lot of work for a few millimetres of room.

The floor is solid concrete. I don't fancy either digging it all out and relaying it over insulation, raising the floor and loosing door height, or having to take out all the door lintels and raising them.

Wrapping the walls is not a likely option. We will be right on the edge of a conservation area in the "modern zone". Alterations to the rear, plus solar panels and heat pumps are fine. Changes that can be seen from the road would take a lot of persuasion.

 

[Colin Green]

If correct, that is remarkably low.

Spending money to reduce it a fraction will have a poor payback.

What will you do with the solar power you get in summer?

The first 50mm of insulation does the most work. It is diminishing returns from there on. We're going to see the house again next week to measure up. I'll see if I can accurately measure the thickness of the wall to gauge the thickness of the cavity. I can redo the calculations then.

I drive an EV and will be having a Zappi charger. It has a current transformer on the mains feed to the house to measure how much solar power is being exported. It adjusts the charging rate of the car so that the car uses all the excess solar power but doesn't import any from the grid.

 

[foxhole]

Heat pumps require excellent insulation to be viable given electricity price rises .

 

[omph]

3kW is on the lower end assuming your calculations are right.
Also, internally insulating is a bad idea because you want to use the thermal mass of the wall to feed heat back into the house. There is also more chance of cold bridging/mould issues.
Why don't you get the cavities filled if you haven't already ?
Heat pumps are most efficient when run 24/7, and you will need to upsize your radiators to type 33/44 for a design temp of DT10/15 (assuming a room temp of 20c) if you want maximum efficiency at lower temps.

 

[Colin Green]

Heat pumps require excellent insulation to be viable given electricity price rises .

We're going for low carbon rather than lowest cost. The solar panels will cover the first part of the heating cost. Electricity would have to be 4 times the cost of gas to make it cost more overall.

 

[Colin Green]

3kW is on the lower end assuming your calculations are right.
Also, internally insulating is a bad idea because you want to use the thermal mass of the wall to feed heat back into the house. There is also more chance of cold bridging/mould issues.
Why don't you get the cavities filled if you haven't already ?
Heat pumps are most efficient when run 24/7, and you will need to upsize your radiators to type 33/44 for a design temp of DT10/15 (assuming a room temp of 20c) if you want maximum efficiency at lower temps.

3.75kW is just through the fabric of the building, not account for air changes or leakage.
The cavities are filled already. IWE is to improve on that. There is close to zero chance of us getting permission for external insulation.

 

[Colin Green]

The comment on radiator sizes is a good one though. I'll have to check that one out.

 

[foxhole]

We're going for low carbon rather than lowest cost. The solar panels will cover the first part of the heating cost. Electricity would have to be 4 times the cost of gas to make it cost more overall.

It may well soon be .

 

[omph]

3.75kW is just through the fabric of the building, not account for air changes or leakage.
The cavities are filled already. IWE is to improve on that. There is close to zero chance of us getting permission for external insulation.

Assuming -3c outside, 3.75kW is still low and I wouldn't bother with internal insulation unless you have a room that is still a bit nippy. I would focus on upsizing rads so you can run them at 30-35c when -3c outside. Once done, lower your boiler temps (if possible) and see how far you can go until you start to feel a little nippy. You can use that to gauge what weather compensation heating profile you may need and whether you are contented with the potential performance or want to go lower. If you can maintain temps below 30-35c for your worst case scenario then you can easily achieve a COP of 5, 6 or higher.

You can also insulate the floor if you haven't already.

 

[JohnD]

The solar panels will cover the first part of the heating cost.

No, because solar power is high on long sunny summer days, when you don't need it
And negligible on short cloudy winter days, when you do.

Electricity would have to be 4 times the cost of gas to make it cost more overall.

It is.

 

[flameport]

Heat pumps are most efficient when run 24/7,

Only true when they supply underfloor heating, just as with any other heat source for UFH.
Radiators and a heat pump can be used in the same way as radiators heated by gas boiler, oil boiler or any other heat source.

Heat pumps require excellent insulation to be viable given electricity price rises .

Also not true, the heat loss of a building does not have any impact on the differences between fuels.
A poorly insulated house will certainly require a much more powerful heat pump compared to a well insulated one, but that's the same for a gas boiler or any other type of heating.

 

[robinbanks]

The first 50mm of insulation does the most work. It is diminishing returns from there on.

Mmm, you could take that argument for any thickness though; it's more like the first X millilmeters stop Y% of the heat loss, the next X millimetres stop Y% of whatever the first X mm didn't stop (so if 100mm stops 90%, 200mm stops 90%, plus 90% of the 10%..)

..I don't think it's ideal to assert that some particular thickness is a magic perfect amount beyond which it's diminishing. The ideal amount is contextual, rather than a fixed value