## SAP 2012

Thought it might be interesting to compare EN 12831 to other calculation methods. A while back now I helped CarbonCoop build a SAP 2012 assessment tool and that has a section for estimating air change rates that I should probably have paid more attention to at the time!

Here’s a screenshot of the ventilation section in my more recent minimal SAPjs implementation of what used to be called MyHomeEnergyPlanner:

**Try the tool here:** https://openenergymonitor.org/sapjs/

**SAPjs source code:** https://github.com/TrystanLea/SAPjs/

**OpenBEM underlying model code:** https://github.com/TrystanLea/openBem

The SAP 2012 calculations are split into infiltration (air movement through the building structure) and ventilation.

### Basic structural infiltration rate:

1. Number of floors

```
infiltration = (number_of_floors - 1) * 0.1
```

2. Dwelling construction

```
if timber frame:
infiltration += 0.25
if masonry:
infiltration += 0.35
```

3. Suspended wooden floor

```
if suspended wooden floor:
if unsealed:
infiltration += 0.2
if sealed:
infiltration += 0.1
```

4. No draught lobby += 0.05

5. Draught proofing

```
+= 0.25 - (0.2 x % draught proofed / 100)
```

6. Shelter factor

```
shelter_factor = 1 - (0.075 * data.ventilation.number_of_sides_sheltered);
infiltration = infiltration x shelter_factor
```

7. Wind factor

```
windfactor = windspeed / 4
infiltration = infiltration x windfactor
```

**Example (My house):**

A 2 story house, masonry, solid floor, with draught lobby, 75% draught proofed, sheltered from 3 sides and at a mean January wind speed of 6.5 m/s

```
infiltration before shelter factor = 0.1 (floors) + 0.35 (construction) + 0.25 - (0.2 x 0.75) = 0.55 ACH
shelter_factor = 1 - (0.075 x 3 sides) = 0.775
infiltration after shelter factor = 0.55 ACH x 0.775 = 0.43 ACH
windfactor = 6.5 m/s / 4 = 1.625
infiltration after wind factor = 0.43 ACH x 1.625 = 0.69 ACH
```

### From air permeability test

The SAP 2012 calculation also has the option to enter an air permeability blower door test value. Ours was 8.9 m3/hr/m2 **without** taping up the extract vents and stove.

```
air permeability = 8.9 m3/hr/m2
```

This gets converted to air changes per hour at 50pa by multiplying by the external envelope area and dividing by the dwelling volume which in our case is:

```
8.9 x 240.2 m2 / 205.8 m3 = 10.4 ACH @ 50pa
```

This divide by 20 approximation is then applied:

```
10.4 / 20 = 0.52 ACH
```

We then return to step 6 the shelter factor above and apply the wind speed correction:

```
0.52 ACH x 0.775 (3 sides sheltered) = 0.403 ACH
0.403 ACH x 6.5 m/s / 4 = 0.65 ACH
```

Which is pretty similar to the first approximation I guess! Both of these figures are in the ball park of what I measured (or estimated given that some estimation is still required with the measurement) using the CO2 sensor approach of around 0.6 ± 0.2 ACH for January. The average wind speed in January according to the MetOffice for our area was 6.8 m/s.

At least SAP 2012 confirms that CIBSE pre-2000 figures are way out!

### Wind speed correction

The wind speed correction in SAP 2012 is interesting but I also wonder if it can overestimate heat loss. Here’s the wind speed and temperature for our area in January, there is a rough correlation between wind speed and outside temperature, milder weather generally brings higher winds, convenient for heat loss (but less convenient for wind generation when you need it…).

Correlation (each datapoint is an hour):

Does the SAP wind speed factor work for higher wind speeds? E.g during the 20th to 24th of January storm, winds were around 12.5 m/s this would suggest a multiplication factor of 3.1x which if applied to 0.403 ACH from the base blower door rate = 1.26 ACH… and I measured an average of ~1.2 ACH using the CO2 sensing! not bad at all!