can any SAP bods out there explain why our code assessor has told us that in order to achieve the requisite heat loss parameter for a code 6 dwelling we need wall/floor/roof u-values of 0.07??

We do both CfSH and SAP (as consultants) as we can not see how it can be done otherwise (at least not with joint up thinking).

Firstly I would advise against going down the Level 5/6 route other than as a very expensive publicity stunt; The reliance on electricity producing renewables is turningh the home into a power station.

Level 4 is a realistic target for most new homes and can be designed in such a way that there is an upgrade path to level 5/6 if needed. The cost step up from level 4 can be huge.

Regarding your query:

The heat loss parameter in CFSH is for level 6 (only) is 0.8 w/m2/k

Individual U values are not actual that important; If your assessor is leading with these then they probably do not know how SAP works...

Starting with a typical home over 50% of the heat loss is due to thermal bridging and ventilation heat loss. Logicaly, when these are improved then you start reducing U values...

If a passive house (low or no heating) can get by on U values under U = 0.15 how can u values of half that level be required?

This is very subjective - I have achieved less than this, in flats, with U values over 0.2.

For a detached home it is hard to achieve, mainly becasue of errors in SAP!

I would say that if you are trying for Level 4, and above, by just using assessors (SAP and CfSH) then it is going to be a very hard process - what you actualy need are experienced professional energy consultants who can answer all of your questions - would you use an Architect with just one weeks training as an Architectural assessor?

I have no problem with "publicity stunts" and research projects - as long as you know why you are doing it; just because it is pointless to build such homes (in larger numbers) before the technology matures does not mean that we should not do so.

The specific problem with SAP that makes life very difficult is that it does not measure ventilation heat loss correctly (nor the parasitic fan consumption).

Ventilation (and associated systems) are included in the Heat Loss Parameter and so their treatment is vitaly important.

Some of the problems are:

SAP is using the permeability when it should be using the air change rate (it used to use air changes but got changed, I assume by mistake). We see errors arising from this in the range of +-60%.

SAP uses a 1/20 calculation to give the heating season air change rate. The 1/20 rule is an annual figure so not correct as wind speeds will be higher in winter.

The 1/20 rule is for moderately sheltered two/three storey homes.

1/30 should be used for single storey and/or highly sheltered homes

1/10 should be used for highly exposed homes. (ie double infiltration heat loss)

As vnetilation is typically around 40% this is significant (to say the least)

The SAP method is to reduce infiltration rates by up to 30% for highly sheltered (all 4 sides or a flat up to 3rd floor)

HRV systems - these are supposed to top up to 0.5 ach, not provide 0.5 ach as SAP suggests.This increases the heat loss form the HRV and also increases the Fan power consumption (the latter not in the HLP).

You have to use SAP for the regulations, so easy way out of the problems.

For such a high level of spending, though, you should also appoint an expert; Even Passiv Haus procedures may not tell you everything you need to know.

The same applies to simulation - I have seen overheating buildings pass a simulation becasue of buried detail in models.

To progress the issues with SAP -

The Solar (and other) gains are only included as part of the base temperature value. Yet, correct Soalr orrientation can reduce the heating load by, say, 20%.

In SAP and CfSH you get no benefit from this!
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I am working on a mill conversion at the moment:
For a mid floor appartment with nothing special (electric heating, close to standard U values and WHV) -
SAP fails
The HLP is 0.8 - the requirement for Code level 6.
Space heating load is 6.89 Kwh/m2a - Passiv Haus requires 15 Kwh/m2a
Heating costs are around £40 per year on electric!

The failure is primarialy because of heating hot water with electricity, but hoping to fix this with Solar

(This could get to code level 3).

This shows up the complexity of the assessments.

We have met the primary Passiv Haus critera of low heating demand and low heat loss parameter with standard (argon ) glazing and wall U values double the required standard. This also before improving the air tightness and fitting HRV (something I hope to persuade the Client to do as we would get the HLP down to around 0.5 as ventilation heat loss is at 60%).

The use of gas in the Target (TER) has always been the case - it is probably just a case of somebody else in BRE noticing.

Even if there is no gas at all you still have to use it in the TER!

It is actualy good that they are catching heat pumps - the last one we looked at was just a 20% CO2 improvement over gas - not exactly a low carbon solution is it?

The TER guidance is in the SAP2005 document - appendix R - reference values.

(Not wanting to offend but your SAP training (All of which is "sub the standard it should be") should have made studying this document mandatory, as well as PartL1A, the domestic heating compliance guide and the full set of accredited construction details...)

The Mill conversion that is aiming for CfSH level 3. We can get the higher level units to pass with HRV and Solar but have problems with North facing lower floor units; In this case an air source heat pump would make a difference and I would consider such a system even with a low COP (say 1.5 for hot water).

bampton - more problems with SAP -

Only 30% of low energy lighting is allowed even if 100% is fitted. So, for CfSH level 6 you have to have renewable electrical generation to provide power for the 70% of standard lighting that you will not have fitted - a few pound on light bulbs becomes thousands on PV

From SAP2005 "In UK houses, the average energy consumption for lighting is taken as 9.3 kWh/m2 annually if no low-energy lighting is used"

"each fixed low energylighting point achieving a saving of 75% over a point with a non low energy lamp"

For an average home (70m2):

Standard lighting 651 kwh
with 30% low E 504.5 kwh
with 100% low E 162.75 kwh


So SAP does not allow for 341 kwh of savings and forces you to generate this electricity!

Hi bampton - will do.

To continue the thread...

SAP does not adjust the heat load according to thermal mass.

- BRE did a study in 2001 for the gov. whicjh showed that heavy thermal mass buildings (esp expossed thermal mass) required significanlty higher heating loads in the morning. Some of the heat did flux its way back into the home but this is not efficient heat storage. (SAP is based on occupancy based on everybody being out of the home durring the working day).

SAP imposed a secondary heating load of 10% direct electrical heating (typically, if no other provision). This means that even a super insulated home has to get hit by a large amount of CO2. If you had a wood pellet boiler and no secondary heating then this could double the CO2 from space heating...

There is no freedon to adjust this value according to factors such as the passive performance, etc...

SAP is an annual system - accumulated degree days. It should use monthly values (they are available as BRE have a monthly model already). This could provide usefull information - such as designing out heating for given months (The summer overheating calcs should also be monthly or even a worst case heat wave).

.

Possibly the most damming aspect of the system is the lack of adequate software produced analysis and check lists - we have to do this manualy - eg:
+Pie charts showing CO2 sources and a breakdown of heat loss (has to be easier than reading detailed worksheets).
+A check list of design features for building control is required- a good example being the huge fraud that is Accredited Construction details - I know that many thousands of home designs are gaining a reduction of around 10% heat loss by 'ticking the box' but the builders do not even know about the design requirements and compliance requirements... If there was 20% less insulation (the equivalent) then it would be fraudulent practice.