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Headspace and fermenter size are also concerns when it comes to aging beer with living ''Brettanomyces''. This includes sour beers, [[Brettanomyces_and_Saccharomyces_Co-fermentation|non-sour beers with ''Saccharomyces'' and ''Brettanomyces'']], and [[100%25_Brettanomyces_Fermentation|100% ''Brettanomyces'' beers]] that are aged. The larger the headspace, the more air will be sucked in when a vacuum occurs. The smaller the fermenter, the more headspace becomes a problem. Smaller vessels, in general, have a larger surface area to volume ratio. Therefore, they have more potential for exposure to oxygen. A large headspace in a smaller vessel exacerbates this problem, therefore it is advised to top up small fermenters and flush them with CO<sub>2<sub?> after primary fermentation or if significant evaporation occurs during aging. For example, a 1-gallon jug should be filled all the way to the neck if possible. A 5-gallon carboy could also be filled to the neck, but a little more headspace is permissible since it is a larger volume. Barrels are porous and the liquid inside them slowly evaporates. Some brewers combat this by topping up their barrels on a regular basis; this also helps keep the top staves from drying out (higher humidity can help limit evaporation; see the [[Barrel#Using_Barrels_for_fermentation_and.2For_aging|Barrel]] page).
One misconception about aging beers is the claim that CO<sub>2</sub> is heavier than air and forms a blanket that protects the beer from oxygen. This is not true unless CO<sub>2</sub> is constantly being produced from the beer. The [https://en.wikipedia.org/wiki/Ideal_gas_law Ideal Gas Law] states that unlike solids or liquids of different densities, the gasses will of different densities eventually mix. See [http://beerandwinejournal.com/can-co2-form-a-blanket/ Dr. Chris Colby's explanation of this on Beer and Wine Journal.], and this [https://www.youtube.com/watch?v=_oLPBnhOCjM science video documentary demonstration of how gasses eventually mix] (note that the molecular weight of bromine used in the video is 160 g/mol and the weight of CO<sub>2</sub> is 44.01 g/mol, so CO<sub>2</sub> would diffuse into air faster than bromine <ref>[https://pubchem.ncbi.nlm.nih.gov/compound/Dibromine Bromine. PubChem. Retrieved 1/1/2016.]</ref><ref>[https://pubchem.ncbi.nlm.nih.gov/compound/280 Carbon Dioxide. PubChem. Retrieved 1/1/2017.]</ref>).
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