###### Abstract
[PERSON] et al. [24] conducted a series of experiments with a compartment vented via the roof only to understand the effect of the horizontal opening on the pool fire behaviour, where six different horizontal openings and three pool fire sizes were used. This work also did not consider the fire behaviour with both horizontal and vertical openings present. As expected, the Heat Release Rate (HRR) and the oxygen concentration at extinction was highly affected by the opening size. This study was based on the studies carried out by [PERSON] [25] and [PERSON] et al. [26], and it proposed a unique ventilation factor (\(\partial\)) which couples the horizontal opening and the pool fire areas as the following:  
\[\partial=\alpha\;A_{0}^{5/4}/A_{f} \tag{1}\]  
where \(\alpha\) is a constant of 250 m\({}^{1/2}\), \(A_{0}\) is the total horizontal opening area, and \(A_{f}\) is the fuel bed area. It is important to note that in compartments with only vertical openings there is an empirical dependence of ventilation controlled burning rate on the commonly-known ventilation factor (\(v_{f}\)) which is equal to \(A_{v}\sqrt{H_{v}}\), where \(A_{v}\) and \(H_{v}\) are the areas and weighted height of the vertical ventilation openings, respectively [27]. Additionally, compartments with horizontal openings showed different fire dynamic behaviour compared to those with vertical openings [20]: pulsation, bidirectional flows, and ghosting flames (where flames are observed away from the burner) have been observed in different experimental and numerical work (e.g., [28; 29; 30]). These studies, however, did not cover cases with both horizontal and vertical openings present, the effect of the horizontal openings on the time to flashover and external plume size, they also did not validate the FDS code in these conditions. To the authors' knowledge, only one study has been carried out to estimate an empirical (fictitious) ventilation factor for the case with horizontal and vertical openings. [PERSON] et al. [31] assumed one vertical and one horizontal opening and presented an approximate nomogram. As shown in Figure 5, it was assumed that all the hot smoke will be leaving the compartment in straight lines through the horizontal opening, while the vertical opening will only pass cold air one way to the inside of the compartment. Based  
Figure 4: Illustration of pre-flashover (**a**) and post-flashover (**b**) conditions inside a dwelling with horizontal and vertical openings. on these assumptions, a fictitious ventilation factor that couples between the vertical and horizontal opening was proposed as the following:  
\[\left(v_{f}\right)_{fict}=v_{f}+2.3A_{h}\sqrt{h} \tag{2}\]  
where \(A_{h}\) is the area of the horizontal opening, and \(h\) is the vertical distance from mid-height of the window opening to the roof of the compartment. These assumptions do indeed not cover all the real case scenarios as presented in Figures 2-4, where the hot smoke exits the compartment via both the vertical and horizontal openings; the air flow is not in straight lines, and indeed there could be multiple vertical or horizontal openings in the same compartment. To further this base of knowledge on horizontal openings in compartments, in the present study a quarter scale ISO-9705 room (similar the compartment used by [PERSON] et al. [9]) is used to demonstrate the following:  
1. The effect of adding four corner horizontal openings or one slot central horizontal opening, in addition to a vertical opening on the ventilation, fire dynamics, and external plumes;
2. The effect of adding collapsible covers on these openings (e.g., covered with polycarbonate) on the fire spread between dwellings' parameters (e.g., the time to reach flashover and the heat fluxes to the surroundings);
3. The validation of the FDS software using the previously mentioned cases;
4. The use of the experimental and numerical results to create an empirical ventilation factor that couples the HRR with the vertical and horizontal openings' dimensions.