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Vapour Barrier on first floor over underground garage

Vapour Barrier on first floor over underground garage

Vapour Barrier on first floor over underground garage

For the case of the first habitable floor above a below-grade unheated parking garage (say the first floor of a condo or apartment building with underground parking) in a cold climate like Canada and the northern U.S., I expect that it would be normal to have insulation on the underside of the floor slab to reduce heat loss from the condo suite to the unheated garage. I also expect that most modern below-grade garages like that would be unheated to save energy.


1. In a modern properly designed building, would there be a vapour barrier on the top side of the insulation? (i.e. on the underside of the slab, which would be the warmer side of the insulation).

2. Would there also be an air barrier at the slab underside or would an air barrier not be used?

3. To roughly what percentage of the normal outside atmospheric carbon dioxide concentration would the carbon dioxide content of the air at the garage ceiling level (i.e. underside of the first habitable floor) be expected to reach?

4. Does the garage mechanical ventilation system automatically activate when the CO2 reaches a certain concentration, and approximately what might that concentration level be?

Reason for above questions:

Forgive me if the above questions are perhaps ignorant but I am an old structural engineer who knows not much about mechanical engineering despite working with mechanical engineers on projects. The reason for the questions is to determine if there should be any concern about the carbonization of the bottom 30 mm of the slab due to higher CO2 concentrations that might be expected in a parking garage due to vehicle exhaust. If the bottom 30 mm of concrete cover to the rebar in the slab become "carbonated" then the bottom rebar would be subject to corrosion in the presence of oxygen and water. My opinion is that carbonation in concrete is such a slow process that this is not a real concern, but I know a structural engineer who perhaps thinks that it is, so answers to the questions that I posed above might help decide the question at least in a qualitative way.

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