Objective: Flame spread is one of the key aspects of flammability, and the flammability of solid materials in low-velocity environments is crucial for fire prevention in manned spacecraft. Under microgravity environments, buoyant flow is greatly reduced or even disappears, and the material combustion characteristics are obviously different from those in normal gravity environments. In confined spaces under normal gravity, buoyant flow is inhibited. Until now, a control mechanism of flame propagation in horizontal channels has not been fully understood, mainly due to the lack of systematic understanding of the influences of residual buoyancy convection and heat loss on the flame-propagation characteristics when the channel height changes, which is directly related to the selection of channel height in experiments. Therefore, in this work, the applicable conditions for the narrow passage are first studied, and the upper and lower critical heights are proposed and verified from the perspectives of buoyancy convection and flame heat loss. Based on previous studies, the channel height intervals for different flame-propagation characteristics are identified, and several critical channel heights are proposed. Methods: The flame-spread behavior of thermally thin solid materials in horizontally confined space is studied by experiments and theoretical analysis based on heat-transfer mechanisms. The effects of buoyancy and heat loss on flame-spread behavior are discussed. The controlling mechanisms and the significance of multiple critical heights were analyzed together with the flame characteristics. Results: When the channel height is too great, the buoyant flow will not be completely suppressed; this is the maximum critical height of the narrow channel in a simulated microgravity environment. When the oxygen concentration is 21%, the upper critical height L_{cr, h} is 9 mm, and when the oxygen concentration is 18%, the corresponding L_{cr, h} is 7 mm. The experimental results confirmed the existence of the upper critical height L_{cr, h}, which is in good agreement with the theoretical prediction. When the channel height is low, excessive heat loss will extinguish the flame and make combustion unsustainable. This is the lower critical height, L_{cr, l}, for a simulated microgravity environment in a narrow channel. Conclusions: Two critical heights were identified for the narrow channel that can reproduce the flame characteristics in microgravity, L_{cr, h} and L_{cr, l}. When the channel height is greater than L_{cr, h}, buoyancy has a noticeable effect on flame spread, and heat loss becomes important when the channel height is less than L_{cr, l}. Between the two heights, heat conduction is greater than the buoyancy convective heat transfer, and it has a controlling effect on flame propagation. The two limiting heights are verified by theoretical analysis and experiments. Combined with the literature results, three regimes were identified, namely the quenching-height regime in the area where the channel height is between L_{cr, l} and L_{cr, h}, in which flame propagation is affected by excessive heat loss and oxygen supply conditions (forced flow), the weak buoyant flow regime when the channel height is between L_{cr, h} and L_o, and the regime in which the buoyant flow is fully developed when the channel height is higher than L_o. These results revealed the flame-propagation and extinguishing behaviors in confined spaces and will contribute to a deeper understanding of the influence of confined spaces on flame propagation in materials.