# How does a building lose heat through a gap in the external wall?

Consider a building in a cold climate with a temperature difference of 20 degrees C, i.e. external temp. is 0 degrees C and internal temp is 20 degrees C.

How does a building lose heat through a gap in the external wall?

Does the cold low pressure air draw the warm air out or does the warm moist air rising in the room leave a 'gap' below which draws in the cold air?

For the sake of argument, assume that the remainder of the building is hermetically sealed i.e. airtight, and the wall is a high performance wall e.g. U-value of 0.3W/m2K.

• Welcome to Physics Stack Exchange! What is a U-value? – DanielSank Nov 3 '15 at 9:48
• A U value is a measure of heat loss. It is expressed in W/m2k, and shows the amount of heat lost in watts (W) per square metre of material (for example wall, roof, floor etc.) when the temperature (k) outside is at least one degree lower. The lower the u value, the better the insulation provided by the material. – Richard14 Nov 3 '15 at 9:57
• If you put a heater in the corner how does it heat the room? You will have convection through the gap. – paparazzo Nov 3 '15 at 10:35

assume that the remainder of the building is hermetically sealed

There are very few such buildings, since occupants dislike suffocation and carbon-dioxide poisoning.

But in that case, warmer air will exit through the upper portion of the gap and colder air will enter through the lower portion of the gap.

The flows are caused by density differences, the heavier (higher density) cold air from outside falls through the lighter (lower density) warm air on the inside, displacing it from the lower regions of the building. The lighter warm air also rises through the heavier cold air.

The heating system in the building converts the cold air to warm air and provides the energy needed to drive the convection current.

If the initial condition is that the building is constructed (with the gap) and contains air at the same temperature and pressure as the outside, there will be no significant movement of air until the heating system is turned on. Turning the heating on will start convection currents inside the building that will soon lead to an exchange of air through the gap.

• Thanks RedGrittyBrick, sowhat causes the transfer to take place is it is simply Newton's Law of Cooling or does the temperature differential create a pressure differential pushing the warm air out and/or the cold air in? – Richard14 Nov 3 '15 at 10:06
• I think we are getting to the heart of it and thanks for your patience with me here. I understand from your answer that convection currents will form presumably because the air becomes less dense when heated but also more voluminous. Does this extra volume cause the warm air to then leak through the gap leaving a space for cold air to fill, which then accelerates the convection currents? – Richard14 Nov 3 '15 at 10:29
• And this effect is started through the cold bridging within the gap? – Richard14 Nov 3 '15 at 10:43