Domestic and non-domestic energy performance certificates review: supplementary notes

6. Ventilation Sensitivity: Draught lobbies, Flues, and Extract Fans

The calculation of the air infiltration rate in a full SAP assessment is a function of many individual building. construction, location and climatic variables:

• number of storeys
• floor construction
• wall construction
• draughtproofing of the doors and windows
• the presence of a draught lobby
• number of open fireplaces
• number of flues
• number of extract fans
• number of passive vents (e.g. air bricks)
• presence and type of mechanical ventilation system
• number of sides sheltered
• wind speed region

Some of these items are inferred from other data entry items in the full SAP assessment, for example, the description of the wall construction and the floor construction is used to assign the appropriate ventilation factor for different wall types and floor types respectively. Other ventilation factors are direct inputs, for example, the number of open fireplaces, extract fans, and sides of the dwelling sheltered. The wind speed region is derived from the post code of the property. Mechanical ventilation systems, if present, are split into 6 different system types (currently), and once the type of system is selected, there are subsequent details to be provided.

Alternatively, rather than deriving the air infiltration rate from all of these variables, the ventilation rate may be measured on-site, and the results of an Air Permeability Test may be entered directly into the full SAP program.

By way of contrast, in RdSAP, only three ventilation variables are entered directly into the program:

• the percentage of draughtproofing of the doors and windows
• number of open fireplaces
• presence and type of mechanical ventilation system

Mechanical ventilations systems are reduced from 6 different system types in full SAP down to one of 2 types in RdSAP, with no additional system descriptors needed.

It is not that RdSAP ignores all the other ventilation factors included within the full SAP calculation; rather, the values are either inferred from other data entry items such as the wall and floor construction description selected (as occurs in full SAP) or use default values as set out in Table S5 of Appendix S of the SAP 2012 methodology (see Figure V1 over the page). It can be seen from this table that RdSAP:

• defaults to no draught lobby for houses and bungalows;
• assumes no extract fans for any dwelling built before 1975 in Scotland;
• makes no allowance for flues for any dwelling; and,
• includes no allowance for air bricks.

A sensitivity analysis was carried using an approved full SAP 2012 program^[35] to assess the impact of these defaults on the SAP and Environmental Impact scores, as well as the SAP-calculated CO₂ emissions, primary energy and fuel costs.

Figure V1: RdSAP Ventilation Defaults (source Table S5 from Appendix S of the SAP 2012 methodology^[36])

This sensitivity analysis used 183 of the 355 archetypes that were produced for the research that underpinned the report for the Scottish Government on "Developing regulation of energy efficiency of private sector housing: modelling improvements to the target stock" that was published in November 2015^[37]. These archetypes were produced using the data collected on each dwelling by the Scottish House Condition Survey (SHCS), as a representative sample of the 400,000 private sector properties within the Scottish dwelling stock scoring below SAP Band D (that is, achieving a SAP score of less than 55) as surveyed by the SHCS between 2010 and 2013. The 183 archetypes used here represented over 50% of all of the archetypes, included all the archetypes that represented more than 1300 properties each, and in total represented over 75% of the 400,000 properties scoring below SAP Band D.

A full SAP2012 analysis was undertaken originally on each of the archetypes, and included within the energy performance assessment, the presence or not of a draught lobby, the count of the number of flues in the dwelling, and a count of the number of extract fans – as these are data items collected by the SHCS.

The analysis of these 183 archetypes was rerun to update the results for each archetype to current version of the SAP 2012 software, and the SAP and Environmental Impact scores, as well as the SAP-calculated CO₂ emissions, primary energy and fuel costs were recorded. The SAP analysis was then rerun for each of the 183 archetypes with the actual draught lobby, extract fans and flue variables being replaced with the RdSAP defaults as set out in Table S5 from the SAP methodology (see Figure V1 above).

Of the 183 archetypes assessed, change from the actual variables in the full SAP assessment to the RdSAP defaults, found that

• 35 of the 183 archetypes (i.e. 19.1%) resulted in no change as the actual variables used in the full SAP analysis were the same as the defaults used in the RdSAP analysis;
• 5 of the 183 archetypes (i.e. 2.7%) resulted in a negative impact; that is, they have a draught lobby that would be ignored in RdSAP, and as a result the air infiltration rate increased, along with the SAP-calculated primary energy consumption, CO₂ emissions, and fuel costs. The SAP and Environmental Impact scores both decreased;
• 114 of the 183 archetypes (i.e. 62.3%) resulted in a positive impact. These archetypes have either flues or extract fans or both, but no draught lobby, that would be ignored in RdSAP. As a result, the air infiltration rate decreased, along with the SAP-calculated primary energy consumption, CO₂ emissions, and fuel costs. The SAP and Environmental Impact scores increased.
• 29 of the 183 archetypes (15.8%) showed a mixed response. These archetypes have a draught lobby and either flues or extract fans or both. The RdSAP result could be either positive or negative compared to the full SAP analysis depending on the actual counts of the flues and / or extract fans.

Overall, switching from the actual ventilation variables in the full SAP assessment to the RdSAP defaults, results in a reduction in the mean air infiltration rate of 0.08 air changes per hour (that is, a lower ventilation loss) with a consequent improvement in the SAP score of 0.62 points and the Environmental Impact score of 0.55 points. Fuel costs, CO₂ emissions and energy consumption are all reduced respectively by £17.81 per year, 107 kg of CO₂ per year, and 535 kWh per year (see Table V1 below). Within this sample of dwellings, RdSAP underestimates the ventilation losses overall because it defaults on the draught lobby, flues and extract fans, and therefore overestimates the SAP and Environmental Impact score, and underestimates the SAP calculated fuel bill, CO₂ emissions and energy consumption.

Table V1: Impact on energy performance indicators by change from actual ventilation factors to RdSAP defaults

	Change in Air Infiltration Rate (ach)	Change in SAP score	Change in Environmental Impact score	Change in SAP fuel costs (£/year)	Change in CO2 emissions (kg of CO2 /year)	Change in Primary Energy (kWh/year)
All 183 ATs	-0.08	0.62	0.55	-17.81	-106.58	-535.03
Draught lobby only (n=5 ATs)	0.042	-0.342	-0.3	13.716	90.586	411.25
Flues and/or extract fans - no draught lobbies (n=114 ATs)	-0.12	0.90	0.79	-25.29	-155.17	-764.47
Mixed (draught lobby and flues and or extract fans) (n=29 ATs)	-0.05	0.45	0.38	-15.35	-78.20	-441.97

This sample of 183 archetypes is not a representative sample of the total Scottish dwelling stock as it was only drawn from a sample of those dwellings surveyed by the SHCS scoring below SAP Band D. It was not representative of all dwellings scoring below SAP Band D as it was only concerned with the private sector (both owner occupier and private rented). The 183 archetypes were also heavily biased towards older properties, with 133 of the 183 falling into the pre-1919 age category. That said, these dwellings should feature in future programmes to improve the worst performing dwellings in the country.

No single ventilation factor on its own is going to make a significant impact on the ventilation rate because of the many variables that contribute to the derivation on the air infiltration rate.

However, it can be seen in Table V2 and Table V3 that the number of flues and extract fans can add up. There were 2 properties with 5 flues recorded and 1 property with 5 extract fans. As the number of flues and fans in a property increase the change in the air infiltration rate becomes more significant, with a consequential impact on all of the energy performance indicators assessed here. With 4 or 5 flues, present RdSAP is underestimating the SAP score by more than 2.5 points, and with 2 and 3 flues, the underestimate is between 1 and 2 points.

Table V2: Change from actual full SAP ventilation factors to RdSAP defaults on number of flues

	Change in Air Infiltration Rate (ach)	Change in SAP score	Change in Environmental Impact score	Change in SAP fuel costs (£/year)	Change in CO2 emissions (kg of CO2 /year)	Change in Primary Energy (kWh/year)
5 flues (n=2)	-0.38	2.63	2.41	-68.52	-425.27	-2401.99
4 flues (n=4)	-0.30	2.55	2.20	-76.68	-351.70	-1856.48
3 flues (n=10)	-0.24	1.75	1.58	-51.17	-275.89	-1505.63
2 flues (n=15)	-0.17	1.49	1.33	-48.59	-331.01	-1479.65
1 flue (n=53)	-0.10	0.72	0.59	-19.84	-117.15	-603.89
all flues (n=84)	-0.14	1.11	0.96	-32.56	-192.74	-970.08

Flues have a bigger impact than extract fans. In SAP ventilation rates, 1 flue is the equivalent of 2 extract fans.

As both extract fans and flues are a straight count within full SAP, RdSAP could be revised to allow the count of fans and flues to be included rather than use defaults than may underestimate considerably the number of fans and flues present in a dwelling. It would add a minimal amount of time to the survey.

Table V3: Change from actual full SAP ventilation factors to RdSAP defaults on number of extract fans

	Change in Air Infiltration Rate (ach)	Change in SAP score	Change in Environmental Impact score	Change in SAP fuel costs (£/year)	Change in CO2 emissions (kg of CO2 /year)	Change in Primary Energy (kWh/year)
5 extract fans (n=1)	-0.12	1.17	0.94	-48.36	-301.24	-1026.44
3 extract fans (n=15)	-0.16	1.42	1.27	-48.46	-230.22	-1282.96
2 extract fans (n=34)	-0.10	0.76	0.69	-23.34	-135.46	-722.59
1 extract fans (n=57)	-0.08	0.61	0.51	-14.72	-99.32	-458.73
all extract fans (n=107)	-0.10	0.78	0.68	-22.50	-131.04	-663.43