u-value: the be all and end all? And is there a SAP/EPC ready reckoner?

[eandg]

I'm comparing different quotes/services from SIPs companies and one's best energy performing kit is 0.15 compared to 0.11 at another. What does that equate to in terms of SAP/EPC ratings and energy bills? And can the difference be overcome through additional measures elsewhere?

 

 

[jake]

generally u-vales tend to be lower ( toward the 0.1 figure, or less) the less compact the building. basically the more exposed wall and roof areas you have the the lower u-value you'll be looking for.

can you maybe outline roughly what the difference in the two sip construction types are? and whether either or both are supplied with reflective membranes inside and out?

[Ferdinand]

For estimating heating and energy bills, you would probably be better off with the @JSHarris heat estimation spreadsheet (which does not currently seem to be where it is normally linked from here http://www.mayfly.eu/).

 

EPCs are not a reliable way of estimating energy costs, especially for non-typical houses. However you can get a trial set of one version of the software that will let you build a complete model. Same page for link.

Ferdinand

 

[Jeremy Harris]

Often airtightness and MVHR will make a more significant change to heat loss than chasing the last decimal place of fabric U values, plus decrement delay needs to be considered, as that plays a very large part in perceived comfort.  If push came to shove I'd go for a construction with a slightly poorer U value if that gave a longer decrement delay, just for the comfort factor.  There's a good explanation of decrement delay in this article: http://www.greenspec.co.uk/building-design/decrement-delay/

 

37 minutes ago, Ferdinand said:

For estimating heating and energy bills, you would probably be better off with the @JSHarris heat estimation spreadsheet (which does not currently seem to be where it is normally linked from here http://www.mayfly.eu/).

 

EPCs are not a reliable way of estimating energy costs, especially for non-typical houses. However you can get a trial set of one version of the software that will let you build a complete model. Same page for link.

Ferdinand

 

 

The spreadsheet is here: Heat loss calculator - Master.xls

[SteamyTea]

I remember that Paul in Montreal (from the other place), did not go for a particular good U-Value when he did his refurbishment.  He spend his time and money on air tightness.

He got good saving on his running costs.

I once started a spreadsheet that balanced the losses from air leaks so that they matched U-Value losses.  I never got around to putting in cash values, might have to revisit it sometime.

[eandg]

Thanks all. 

 

Is there a measure of air tightness? Is ICF better for this than timber kit or SIPs? 

 

Ecology require a minimum SAP rating of 85 for any home they will lend on - would a walls u-value as low as 0.15 sit within that? 

[Jeremy Harris]

1 minute ago, eandg said:

Thanks all. 

 

Is there a measure of air tightness? Is ICF better for this than timber kit or SIPs? 

 

Ecology require a minimum SAP rating of 85 for any home they will lend on - would a walls u-value as low as 0.15 sit within that? 

 

Yes, it's measured during the air test that's done as a part of building regs compliance usually.  A reasonable target is to aim for an airtightness that's a fair bit better than building regs require, especially if you want to get an EPC rating that's up around 85.  Having good airtightness is only a benefit if you also fit MVHR, though, as much of the benefit comes from the heat recovery provided by that system.

 

A wall U value of 0.15 W/m².K might be OK, but it very much depends on the totality of all the parts of the building, so the insulation values of the walls, floor and roof, the elimination of thermal bridges in the structure (as these can easily undermine otherwise good insulation levels) and the airtightness level and MVHR efficiency.

[SteamyTea]

https://www.bre.co.uk/filelibrary/SAP/2012/SAP-2012_9-92.pdf

Only 234 pages

[ADLIan]

There is no simple SAP estimator as it is complex with many independant and interdependant inputs. It is not particularly sensitive to elemental U-values especially at the levels you mention. Taking care with air tightness and linear thermal bridges will give better returns.

For a new build built to basic Building Reg standards and gas heating you should be acheiveing a SAP of approx 80-85. As SAP is based on fuel COST your choice of heating fuel and heater efficiency will have a big impact - electric resitance heating may reduce the above figure by 10-15 points!

[eandg]

Thanks again. Plan is for ASHP and MVHRs to be installed and as airtight and energy efficient as we can get. Just now it's looking like we'll be picking from one of the very few companies who've bothered to come back to us so we may have our hand forced on that! 

[epsilonGreedy]

The House Builders Bible has tables showing the thermal performance and likely fuel costs for an average sized house built to 2013 and Passivhaus standards. The annual bills for hotwater and space heating are estimated at £800 and £224. The u-values for the 2013 house are assumed to be 0.18, 0.22 and 0.18 for roof, walls and floor. The 2013 model house is does not feature MVHR which accounts for 25% of the difference in thermal performance.

 

Given that either of your u-value options is substantially better than the model 2013 house and you will have MVHR, then I suggest your option of 0.11 and 0.15 for different kits equates to less than £50 p/a in fuel bills for an average sized house.

[Jeremy Harris]

19 minutes ago, epsilonGreedy said:

The House Builders Bible has tables showing the thermal performance and likely fuel costs for an average sized house built to 2013 and Passivhaus standards. The annual bills for hotwater and space heating are estimated at £800 and £224. The u-values for the 2013 house are assumed to be 0.18, 0.22 and 0.18 for roof, walls and floor. The 2013 model house is does not feature MVHR which accounts for 25% of the difference in thermal performance.

 

Given that either of your u-value options is substantially better than the model 2013 house and you will have MVHR, then I suggest your option of 0.11 and 0.15 for different kits equates to less than £50 p/a in fuel bills for an average sized house.

 

 

So much depends on individual details, including the size, shape, surface area and volume of the house, that it's near-impossible to make a generalisation that a particular set of U values and air tightness will save a specific sum of money.  Even the choice of fuel could give about a 3:1 range of cost variation (maybe more).  If a house is larger than average, but is a simple square shape, then wall, roof and floor U values become less significant and airtightness becomes more significant.  Conversely, if the house is smaller than average, and has a more complex shape, then wall, floor and roof U values will become more significant and airtightness less so.  It's a variation of the elephant and mouse metabolic rate analogy.

 

 

[epsilonGreedy]

4 minutes ago, JSHarris said:

So much depends...

 

 

All true but the absolute numbers in my post will help the OP comprehend that chasing ever better u-value figures is a game of diminishing returns which for a 2-story rectangular house 1800 sq ft in size, translates in trivial annual cashing savings.

[Jeremy Harris]

2 minutes ago, epsilonGreedy said:

 

All true but the absolute numbers in my post will help the OP comprehend that chasing ever better u-value figures is a game of diminishing returns which for a 2-story rectangular house 1800 sq ft in size, translates in trivial annual cashing savings.

 

 

Here's some absolute numbers for our build.  If I changed the U values to 0.13 W/m².K for the floor and roof, 0.18 W/m².K for the walls and 1.4 W/m².K for the doors and windows, and change the airtightness to 5m³(h.m²) at 50 PA, with no MVHR, then the heating fuel consumption increases by 239%.

[epsilonGreedy]

2 minutes ago, JSHarris said:

Here's some absolute numbers for our build.  If I changed the U values to 0.13 W/m².K for the floor and roof, 0.18 W/m².K for the walls and 1.4 W/m².K for the doors and windows, and change the airtightness to 5m³(h.m²) at 50 PA, with no MVHR, then the heating fuel consumption increases by 239%.

 

 

The mathematical cynic would observe that since your house is CO2 neutral and generates a profit as an energy producer then a percentage increase in your fuel consumption takes us into a new undiscovered financial dimension.

[Jeremy Harris]

4 minutes ago, epsilonGreedy said:

 

The mathematical cynic would observe that since your house is CO2 neutral and generates a profit as an energy producer then a percentage increase in your fuel consumption takes us into a new undiscovered financial dimension.

 

 

I was ignoring the PV system, as that doesn't heat the house, but just offsets the overall energy use over a year.  The bottom line is that the house heating demand for only a modest worsening of the insulation (the values I quoted are still better than building regs limiting fabric values), and reverting to airtightness which is twice as good as building regs require, results in a pretty massive increase in heating cost.

[SteamyTea]

Really a case of making a simple model of the house in Excel and changing the numbers.

[MikeSharp01]

20 minutes ago, SteamyTea said:

Really a case of making a simple model of the house in Excel and changing the numbers.

I guess the accuracy ( & resolution) of the output of any model is some function of the model complexity but also the combinatoral complexity of the model and reality which you cannot see until the house is built. The PHPP is relatively complex when compared to 2013 so is more able to show the effects of critical variables. In the end they both suffer when put up against build quality and material selection which feels like it has a big effect while not being a model feature any where. So only once built can you assess the real energy in / out evelope accurately but once built  you cannot be sure which bits of the fabric are doing what specifically only generally. Air tightness can be measured and modelled but it's much harder to model the actual effects of the build in its environment. As I believe @JSHarris found the micro climate has a big effect on the actual outcome and thus in turn effects the energy in / out equation. I think that this just means that we probably need to give the building a final rating after several years of use so as to have an accurate picture of the usage. However in the era of global warming we are probably getting incrimental wins from our homes which the models - based as they are on historic patterns don't account for. 

[Ian]

5 hours ago, eandg said:

 

Ecology require a minimum SAP rating of 85 for any home they will lend on - would a walls u-value as low as 0.15 sit within that? 

It’s heavily dependent on shape of your building and other factors such as choice of energy source, renewables etc.

 

2 years ago I built a holiday home to the minimum current (Welsh) Building Regs. Area weighted U values for walls, floor and roof were 0.15. Windows were 0.8. No MVHR or PV. Air tightness was 3.7. Fuel source is bulk LPG.

SAP score was only 71

The building is a small 2 bed bungalow 

[ADLIan]

@IanLow SAP probably due to use of bulk LPG. Would be very diferent with mains gas.

[eandg]

2 hours ago, Ian said:

It’s heavily dependent on shape of your building and other factors such as choice of energy source, renewables etc.

 

2 years ago I built a holiday home to the minimum current (Welsh) Building Regs. Area weighted U values for walls, floor and roof were 0.15. Windows were 0.8. No MVHR or PV. Air tightness was 3.7. Fuel source is bulk LPG.

SAP score was only 71

The building is a small 2 bed bungalow 

Thanks. Not sure how helpful that is apart from to suggest that it's a minefield! 

 

I've noticed the higher (wall) u-value of the two SIPs companies do their own insulated plasterboard (up to 75mm insulation) which presumably reduces the value considerably.