This appendix provides a method for assessing the propensity of a dwelling to have high internal temperature in hot weather. It does not provide an estimate of cooling needs. The procedure is not integral to DEAP and does not affect the BER result. The cooling calculation is carried out as per the Cooling tab.
The calculation is related to the factors that contribute to internal temperature: solar gain (taking account of orientation, shading and glazing transmission); ventilation (taking account of window opening in hot weather), thermal capacity and mean summer temperature for the location of the dwelling.
1. Obtain a value for the effective air change rate during hot weather. Indicative values are given in Table P1.
Table P1: Effective air change rate for summer period
| Window opening | Effective air change rate for summer in ach | |||
|---|---|---|---|---|
| Trickle vents only | Windows slightly open (50 mm) | Windows open half the time | Windows fully open | |
| Single storey dwelling (bungalow, flat) Cross ventilation possible |
0.1 | 0.8 | 3 | 6 |
|
Single storey dwelling (bungalow, flat) Cross ventilation not possible |
0.1 | 0.5 | 2 | 4 |
|
Dwelling of two or more storeys windows open upstairs and downstairs. Cross ventilation possible |
0.2 | 1 | 4 | 8 |
|
Dwelling of two or more storeys windows open upstairs and downstairs. Cross ventilation not possible |
0.1 | 0.6 | 2.5 | 5 |
Cross ventilation can be assumed only if the at least half of the storeys in the dwelling have windows on opposite sides and there is a route for the ventilation air. Normally bungalows and two storey houses can be cross ventilated because internal doors can be left open. Three storey houses or other situations with two connected storeys where one is more than 4.5 m above ground level often have floors with fire doors leading onto stairs thus preventing cross ventilation. Slightly open refers to windows that can be securely locked with a gap of about 50 mm. Often this option will not give sufficient ventilation.
Windows on ground floors cannot be left open all night because of security issues. Windows on other floors can. Fully open refers to dwellings where security is not an issue (such as an upper floor flat) or secure night-time ventilation is available (including by means of grilles, shutters with vents or purpose-made ventilators). In most cases where there are ground and upper floor windows, windows open half the time is applicable, which refers principally to night-time ventilation (ground floor evening only, upper floors open all night).
If there is a mechanical ventilation system providing a specified air change rate, that rate can be used instead.
2. Calculate the ventilation heat loss, Hvsummer, using the formula,
H vsummer = 0.33 * n * V
Where,
3. Calculate the heat loss coefficient under summer conditions:
H = total fabric heat loss + Hvsummer
4. Calculate the total solar gains for the summer period, Gsummer using the solar flux for the summer period from Table 1c.
Gsolar summer = Σ (0.9 x Aw x S x g⊥ x FF x Zsummer)
where,
Solar gains should be calculated separately for each orientation and for rooflights and totalled according to Equation (P3)
For data to calculate Zsummer see Section P3.
Assume that the summer internal gains (Gi ) are equal to the winter net internal gains (these are calculated in the ‘Internal gains’ section of the DEAP software), so that the total gains are:
G = Gsolar summer + Gi (P5)
5. Calculate the temperature increment due to gains in summer, from the following, where H is the heat loss coefficient of the dwelling: Temperature increment = G / H
6. Obtain the summer mean external temperature, Tesummer , from Table P2. Alternatively, the mean for the months June – August for the dwelling location may be used if available.
Table P2: Summer mean external temperature
| Region | Mean External Temperature (used for estimating mean internal temperature in summer) |
|---|---|
| Ireland | 15 |
7. Obtain the threshold internal temperature to estimate likelihood of high internal temperature. This is the mean internal temperature during the summer period plus an increment related to the thermal mass.
Tthreshold = Tesummer + (G/H) + ΔTmass
where,
if HCP ≥ 0.28HCP (heat capacity parameter) is the internal heat capacity of the dwelling divided by the total floor area, as entered to the DEAP calculation software.
8. Use Table P3 to estimate tendency to high internal temperature in hot weather.
Table P3: Levels of threshold temperature corresponding to likelihood of high internal temperature during hot weather
| Threshold temperature (Tthreshold) | Likelihood of high internal temperature during hot weather |
|---|---|
| < 20.5oC | Not significant |
| ≥ 20.5oC and < 22.0oC | Slight |
| ≥ 22.0oC and < 23.5oC | Medium |
| ≥ 23.5oC | High |
Results should include:
If the orientation of the dwelling is not known, the assessment should be for the orientation giving the greatest tendency to high internal temperature.
Zsummer is given by;
Zsummer = Zblinds (Z + Zoverhangs – 1) (P8)
subject to
Zsummer ≥ 0.1 Zblinds (P9)
Where;
Table P4 gives values for Zblinds, and Tables P5 and P6 give values for Zoverhangs.
If there are no overhangs, Zoverhangs = 1.
Unless specifically included in the design specification a default of dark coloured curtains should be assumed closed during daylight hours (f = 1). Shutters with window closed is compatible with windows open half the time in Table P1 as the latter refers to night-time and Table P4 refers to daytime.
Table P4 : Shading factors for blinds, curtains, or external shutters
| Blind or curtain type | Zblind |
|---|---|
| Net curtain (covering whole window) | 0.80 |
| Net curtain (covering half window) | 0.90 |
| Dark-coloured curtain or roller blind (note 1) | 0.85 |
| Light-coloured curtain or roller blind (note 1) | 0.60 |
| Dark-coloured venetian blind (note 2) | 0.88 |
| Light-coloured venetian blind (note 2) | 0.70 |
| Dark-coloured external shutter, window closed (note 3) | 0.24 |
| White external shutter, window closed (note 3) | 0.27 |
| Dark-coloured external shutter, window fully open (note 3) | 0.85 |
| White external shutter, window fully open (note 3) | 0.65 |
Notes to Table P4
f x Zblind + (1 – f)f x Zblind + (1 – f)Where the overhang is at least twice as wide as the window (such as balconies on blocks of flats) use Table P5. In other cases, use Table P6. Interpolation may be used between rows of these tables. Usually, the same value of Zoverhangs can be applied to all the windows on a given façade on the basis of an average depth-to-height ratio.
Table P5: Zoverhangs for wide overhangs
| Depth/H | Orientation of window | ||||
|---|---|---|---|---|---|
| N | NE/NW | E/W | SE/SW | S | |
| 0.0 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |
| 0.2 | 0.92 | 0.89 | 0.88 | 0.83 | 0.77 |
| 0.4 | 0.85 | 0.80 | 0.76 | 0.67 | 0.55 |
| 0.6 | 0.79 | 0.72 | 0.66 | 0.54 | 0.38 |
| 0.8 | 0.73 | 0.65 | 0.58 | 0.43 | 0.32 |
| 1 | 0.69 | 0.59 | 0.51 | 0.36 | 0.30 |
| 1.2 or more | 0.66 | 0.55 | 0.46 | 0.31 | 0.29 |
| This table is to be used where the overhang is at least twice as wide as the window. | |||||
Table P6: Zoverhangs for normal overhangs
| Depth/H | Orientation of window | ||||
|---|---|---|---|---|---|
| N | NE/NW | E/W | SE/SW | S | |
| 0.0 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |
| 0.2 | 0.94 | 0.91 | 0.89 | 0.84 | 0.79 |
| 0.4 | 0.90 | 0.85 | 0.79 | 0.72 | 0.64 |
| 0.6 | 0.88 | 0.81 | 0.72 | 0.62 | 0.53 |
| 0.8 | 0.86 | 0.79 | 0.66 | 0.55 | 0.50 |
| 1 | 0.85 | 0.77 | 0.61 | 0.52 | 0.49 |
| 1.2 or more | 0.84 | 0.76 | 0.57 | 0.50 | 0.48 |
| This table is to be used where the overhang is less than twice as wide as the window. | |||||
Table 1c: Solar flux for summer period [W/m2]
| Orientation | Solar Flux |
|---|---|
| North | 78 |
| NE/NW | 93 |
| E/W | 111 |
| SE/SW | 115 |
| South | 110 |
| Horizontal | 185 |
Notes