Full Fill Cavity insulation. 0.036 vs 0.032

[Triggaaar]

We've just started our kitchen extension (foundations completed, brick and block about to start).

 

The finished room will 7.5m wide x 9.6m long, 2.7m high (internal). The east elevation will be mostly glass sliding doors. to the garden.

The south elevation will be brick & block with 150mm full fill cavity. Inside that we'll have 25mm PIR in between battens, and then plasterboard and wet plaster. The blocks are Celcon, with a thermal conductivity of 0.15W/mK.

Half of the north elevation will be the existing walls, which are brick and brick, with 100mm cavity, which was filled some years ago with beads. The other half will be new brick and block with 100mm full fill cavity. Again with 25mm inside.

The west then joins the rest of the old Edwardian house.

 

The plan is to have a concrete oversite (on top of hardcore) and then 100mm of PIR rated at 0.022W/mK, and a warm roof with 140mm of the same PIR. Do those thicknesses sound alright? I know we'll meet building regs, but I don't know if it's worth doing any more.

 

Re the cavities, do you think it'll make much difference whether the 100mm cavity wall insulation is 0.036W/mK or 0.032W/mK?

Since the insulation is thinner than the opposite wall, I wondered whether it would be worth putting a slightly better insulation in. It's not much more expensive, but then the difference might be so negligible it's still not worth it.

 

Thanks

[MikeSharp01]

The lower one is 10%+ better than the higher one.. So provided it is not 10% more expensive it would, all other things being equal - which they often aren't, be a better bet, if more than 10% more expensive the payback steps out a bit.

 

[A_L]

1 hour ago, Triggaaar said:

aRe the cavities, do you think it'll make much difference whether the 100mm cavity wall insulation is 0.036W/mK or 0.032W/mK?

Both the 100mm and 150mm cavity walls have a U-value about 0.014/0.015 less with the 0.032W/mK option. This translates to an energy saving of less than 1kWh/m²/year

 

 

1 hour ago, Triggaaar said:

The plan is to have a concrete oversite (on top of hardcore) and then 100mm of PIR rated at 0.022W/mK, and a warm roof with 140mm of the same PIR. Do those thicknesses sound alright? I know we'll meet building regs, but I don't know if it's worth doing any more.

A U-value reduction of 0.05 saves about 3kWh/m²/year

[Triggaaar]

Thank you both for the replies, much appreciated.

 

1 hour ago, MikeSharp01 said:

The lower one is 10%+ better than the higher one.. So provided it is not 10% more expensive it would, all other things being equal - which they often aren't, be a better bet, if more than 10% more expensive the payback steps out a bit.

 

I assume the 0.036 is 12.5% better than 0.032. It costs 81% more. But that percentage wouldn't mean a lot if it was cheap to start with. For that one wall, we're talking £70 more.

 

 

23 minutes ago, A_L said:

Both the 100mm and 150mm cavity walls have a U-value about 0.014/0.015 less with the 0.032W/mK option. This translates to an energy saving of less than 1kWh/m²/year

 

A U-value reduction of 0.05 saves about 3kWh/m²/year

Thanks. You clearly have the answer I'm looking for - now I just need to try and understand it

 

The wall areas I'm taking about total 55m2. So you're saying it would be 55kWh / year. Do I just times that by the cost of our gas (say 3p/kwh) to get a yearly saving of £1.65?

[Ed Davies]

8 hours ago, Triggaaar said:

I assume the 0.036 is 12.5% better than 0.032.

 

I'm not sure if that's a typo or a misunderstanding: 0.032 W/m·K is better than 0.036 W/m·K as less conductivity (greater resistivity) is what you want.

[A_L]

14 hours ago, Triggaaar said:

The wall areas I'm taking about total 55m2. So you're saying it would be 55kWh / year. Do I just times that by the cost of our gas (say 3p/kwh) to get a yearly saving of £1.65?

 

Nearly ?, about 1kWh/m²/year less will leak out of the heated space but allowing for the inefficiency of the boiler you will probably use (about) 1.25kWh less of  gas per meter squared per year of wall area.

[Triggaaar]

12 hours ago, Ed Davies said:

 

I'm not sure if that's a typo or a misunderstanding: 0.032 W/m·K is better than 0.036 W/m·K as less conductivity (greater resistivity) is what you want.

Oops, yes, that was a typo.

 

 

7 hours ago, A_L said:

 

Nearly ?, about 1kWh/m²/year less will leak out of the heated space but allowing for the inefficiency of the boiler you will probably use (about) 1.25kWh less of  gas per meter squared per year of wall area.

Thanks. Yes, I had actually thought about boiler inefficiency, but thought I'd let it go

 

So if the heating was required to keep the room warm all year, then I could be saving about £2. But then the heating won't even used for half the year, so I'd be lucky to save £1. In other words, don't bother going for the 0.032 over the cheaper 0.036.

 

How about the floor and roof insulation? I'm currently planning on 100mm floor, and 140mm roof (PIR 0.022). Is there much point in increasing either?

 

Many thanks

Edited November 27, 2019 by Triggaaar

[A_L]

19 minutes ago, Triggaaar said:

So if the heating was required to keep the room warm all year, then I could be saving about £2. But then the heating won't even used for half the year, so I'd be lucky to save £1.

 

No £2, the model only counts losses when heat is required to maintain demand temperature (Full SAP) but yes premium versions are rarely worth it from a saving point of view.

 

41 minutes ago, Triggaaar said:

How about the floor and roof insulation? I'm currently planning on 100mm floor, and 140mm roof (PIR 0.022). Is there much point in increasing either?

 

Adding 50mm of PIR to either will reduce the U  value by about 0.04 and save approximately 3.5kWh of gas per m² per year