Optimum U-value - installation cost vs saving on heating

[Tony C]

Hi, Can I ask self-builders what U-value you went for for Floor/Roof/Wall ?

Is it worth spending more money on insulation to achieve better U- value than Building reg spec to save some heating cost (and of course good ecologically)?

I am going with UFH and it is a single story detached house with plenty of outside walls, it would be great if someone could quote the example result on heating bill, house size and U-value combo.

 

[ProDave]

All our walls roof and floor are in the order of 0.14   150 square metre 1 1/2 storey house (upstairs room in roof) total winter heating bill £234 using an ASHP and UFH and this is in the north Highlands so would be lower in less harsh climates.

 

Triple glazed and air tight with MVHR is the bit you need to consider as well to get the energy down.  If you don't detail the air tightness, your heating losses might be dominated by ventilation heat loss.

 

@JSHarris has produced a good spread sheet to model your heat losses and it certainly came up with figures incredably close to what it has actually turned out for my house.

 

Don't be afraid to over spec the floor insulation as you will lose some of your heat to the ground.

[Conor]

I'm building with ICF... To go from BC standard to "passive" will cost about £10k extra in insulation and upgraded windows. It means my heating system can be a lot smaller and simpler, saving me in the region of £5k. The remaining £5k extra will pay back in heating cost savings (£500pa) in 10 years. But tbh the comfort value of having a house at a constant 20c most of the year is worth way more than that.

 

(I could be really brave and forgoe central heating entirely and use storage heaters, saving even more)

[Ian]

Mine was Building Regs min compliance 2+ years ago in North Wales (building regs slightly different from England & Scotland)

It's a bungalow of 71 sq.m internal space. Area weighted average 'U' value for walls, floor and roof was 0.15 and 3G windows 0.8 'U' value average. No special measures for air tightness and no MVHR. Air tightness at test was 3.7 m3/h/m2 [Edit: 'U' values for each element were 0.1 for floor (UFH); 0.13 for roof and 0.21 for walls]

Bulk LPG with combi boiler for all hot water, cooking and UFH

Mains electric

Blower sewage treatment plant

Building is a holiday home and is used most weekends

1200 litres of LPG cost £360 on first fill of the tank and it lasted 24 months before it needed refilling so £180/year + standing charge. Its on a 'frost'setting of 12 degC and boosted at weekends in Winter to normal temperature. The building takes about 3 days to fall back to 12 degC after the weekend

Electric usage in 2018 was 1528 kWh

Water was about £70 total for 2018 inc standing charge (water in N Wales is £1.3 for 1,000 litres)

 

Edited May 14, 2019 by Ian

[Jeremy Harris]

41 minutes ago, Tony C said:

Hi, Can I ask self-builders what U-value you went for for Floor/Roof/Wall ?

Is it worth spending more money on insulation to achieve better U- value than Building reg spec to save some heating cost (and of course good ecologically)?

I am going with UFH and it is a single story detached house with plenty of outside walls, it would be great if someone could quote the example result on heating bill, house size and U-value combo.

 

 

The spreadsheet @ProDave mentions is here: Heat loss calculator - Master.xls  It has the U values, areas and airtightness figures for our build, but these can just be overwritten.

[Tony C]

Thank you for everyone for all the info, much appropriated! I am expecting the tender quotation sometime next month....so depending how it goes I will decide how much insulation I go for. (at the moment our architect added quite a bit! more than 220mm PIR).

[Simplysimon]

6 hours ago, Tony C said:

 (at the moment our architect added quite a bit! more than 220mm PIR).

what type of construction are you doing?

[Tony C]

12 hours ago, Simplysimon said:

what type of construction are you doing?

Typical section drawing here :

Timber frame construction, Plywood internal finish, Corrugated aluminum exterior. Kingspan K112 Framing Board 60mm x 3 =180mm total

[Simplysimon]

@Tony C there's a lot of care going to be required with all those 60mm sheets to avoid any gaps and potential cold areas. possibly worth considering i beam or twin stud construction with a blown insulation.

[Tony C]

On 14/05/2019 at 09:30, JSHarris said:

 

The spreadsheet @ProDave mentions is here: Heat loss calculator - Master.xls  It has the U values, areas and airtightness figures for our build, but these can just be overwritten.

Thank you JSHarris, I am punching in the numbers now.

I am not installing MVHR, what number I should be entering to "Air changes per hour" and "MVHR efficiency" ?

[Jeremy Harris]

26 minutes ago, Tony C said:

Thank you JSHarris, I am punching in the numbers now.

I am not installing MVHR, what number I should be entering to "Air changes per hour" and "MVHR efficiency" ?

 

 

I'd put in the ventilation rate that you're aiming for using the window trickle vents and mandatory extraction fans, which is probably somewhere around 3 to 5 ACH worst case.  With no MVHR the MVHR efficiency will be 0%. 

 

Be prepared for this to cause a high heat loss, though.  If I change the ventilation rate to 4 ACH and set the MVHR efficiency to 0% for our house it increases the total heat loss of the whole house by about 900% (the heat loss at -10°C outside with MVHR is about 1.672 kW, with no MVHR it rises to about 15.095 kW).

[ProDave]

16 minutes ago, JSHarris said:

 

 

I'd put in the ventilation rate that you're aiming for using the window trickle vents and mandatory extraction fans, which is probably somewhere around 3 to 5 ACH worst case.  With no MVHR the MVHR efficiency will be 0%. 

 

Be prepared for this to cause a high heat loss, though.  If I change the ventilation rate to 4 ACH and set the MVHR efficiency to 0% for our house it increases the total heat loss of the whole house by about 900% (the heat loss at -10°C outside with MVHR is about 1.672 kW, with no MVHR it rises to about 15.095 kW).

Which is a good example of why in a well insulated low energy house you really do want to make it air tight and fit an MVHR.

[SteamyTea]

1 hour ago, Tony C said:

I am not installing MVHR

What is your reasoning for that decision?

[jack]

1 hour ago, SteamyTea said:

What is your reasoning for that decision?

 

I'm interested too. Of all the things we've learned from building a new house, the quality of life improvement from having MVHR installed would put it at the very top of the list of things I'd include in a house if I were doing it all again.

[ProDave]

I too used @JSHarris heat loss spreadsheet and it gave predictions that have proved very accurate over the first winter.

 

I tried "removing" the MVHR and my predicted worst case heat loss (-10 outside +20 inside) went from 2.2Kw up to 15.6Kw

 

Without MVHR I would have needed a substantially larger heat pump to heat the house, I would probably have to run the UFH at a higher temperature, and I would probably have to have heating upstairs (currently only heating downstairs)

 

The saving by omitting the MVHR I am certain would have instead been spent on a larger heating system, so no real capital saving, but a MUCH larger ongoing heating bill.

 

It really is a no brain decision to fit MVHR, that is even before you begin to consider the much improved air quality in the house.

[Tony C]

 

8 hours ago, SteamyTea said:

What is your reasoning for that decision?

 

Its partly to do with design of the building (it will have exposed joist) , also building in London meant space is premium, not enough space to install, perhaps not far enough from habitable spaces also (Noise concern).

[Jeremy Harris]

There's no noise concern; our MVHR is much quieter than the mandatory bathroom/shower/WC/kitchen extractor fans that are needed if you don't fit MVHR.

[ProDave]

+1 to the above. Our MVHR is in the plant room next to our bedroom, and even without a door fitted to the opening, you cannot hear it in the bedroom at normal speed,

 

I was surprised when I ran the numbers just how much heat you will lose through ventilation losses in a leaky house without mvhr even with decent insulation levels.  There must be some cupboard you can fit one in, they need not be that big.

[Ed Davies]

11 hours ago, JSHarris said:

I'd put in the ventilation rate that you're aiming for using the window trickle vents and mandatory extraction fans, which is probably somewhere around 3 to 5 ACH worst case.

 

3 to 5 ACH is indeed likely some of the time but surely a house with minimal leakage for building regulations not to require mechanical ventilation (5 m³/m²/h, I think) would have a much lower air change rate most of the time in practice.

 

50 pascals corresponds to a wind speed of around 8.8 m/s (just under 20 mph) which is relatively uncommon even in exposed locations. Also, a blower door test blows (or sucks) on all four walls and the floor and ceiling whereas wind tends to, in effect, blow on just one wall and suck on the opposite one. IIRC, a rule of thumb is that the actual ACH, on average, is likely to be about 1/20th of the test result.

 

Of course, what's of interest is the typical ACH during the worst bit of the heating season. Still…

[ADLIan]

22 hours ago, ProDave said:

I tried "removing" the MVHR and my predicted worst case heat loss (-10 outside +20 inside) went from 2.2Kw up to 15.6Kw

 

Are those figures correct, MVHR reduces heat loss by a factor of 7? I can see 10-20% improvement when using MVHR provided it is correctly designed. A poorly specified/designed MVHR system can potentially increase energy use!

[Jeremy Harris]

14 hours ago, Ed Davies said:

 

3 to 5 ACH is indeed likely some of the time but surely a house with minimal leakage for building regulations not to require mechanical ventilation (5 m³/m²/h, I think) would have a much lower air change rate most of the time in practice.

 

50 pascals corresponds to a wind speed of around 8.8 m/s (just under 20 mph) which is relatively uncommon even in exposed locations. Also, a blower door test blows (or sucks) on all four walls and the floor and ceiling whereas wind tends to, in effect, blow on just one wall and suck on the opposite one. IIRC, a rule of thumb is that the actual ACH, on average, is likely to be about 1/20th of the test result.

 

Of course, what's of interest is the typical ACH during the worst bit of the heating season. Still…

 

 

It's definitely the worst case, which is why I mentioned using the aimed for ventilation rate using trickle vents and extraction fans. 

 

If we hadn't fitted MVHR we'd have had to install five extraction fans in order to comply with the regs, so I took the view that I'd rather just have two holes through the walls rather than five holes through the walls (plus trickle vents). 

 

Using the same ventilation rate with no MVHR as we have with MVHR almost doubles the cold weather heat loss, though, from 1,672 W at -10°C to just under 3,000 W, and that's not allowing for the additional heat loss that would occur whenever one of the mandatory extract fans is running.

[Ed Davies]

For a two-storey box (house) with an 8x8 metre ground floor and 5 metre height the surface area (walls, ground floor, upstairs ceiling) would be 336 m². Assuming leakage of 5 m³/m²/h at 50 pascals and practical leakage being about 1/20^th of the air test measurement that'd be 336 × 5 / 20 = 68 m³/h.

 

Air has a density of around 1.28 kg/m³ and a specific heat capacity of 1006 J/kg·K so for a temperature difference of 30 K between inside and out that'd result in a heat loss of:

 

30 K × 68m³/h × 1.28 kg/m³ × 1006 J/kg·K / 3600 s/h ~= 730 W.

 

I can see slightly different assumptions resulting in @JSHarris's “1,672 W at -10°C to just under 3,000 W” but not @ProDave's “worst case heat loss (-10 outside +20 inside) went from 2.2Kw up to 15.6Kw”. I suspect @ProDave is taking the 50 pascal leakage rate too literally but maybe there's something else going on.

 

Apart from anything else, windy conditions don't tend to be extremely cold.

[SteamyTea]

1 hour ago, Ed Davies said:

Apart from anything else, windy conditions don't tend to be extremely cold.

Too true.  The Easterly at the beginning of this week, that made it unseasonally cold down here, but warm in the North, is a very rare event.  Can't think I have ever known one that has lasted more than a day or two.

Some years ago, I plotted temperature against sunlight, wind direction and speed.  The biggest predictor of warm temperature was increased speed.