Just Say No to [Swiss] Cheesey Attic Floors!

As the biting cold of winter hits us, many building owners this week will start to see signs of moisture damage on parts of their walls or ceilings. As we keep temperatures in our homes consistent, the difference between indoor and outdoor temperatures (the ole’ Delta T as they call it) becomes greater, and this difference drives air up and out through holes in our ceiling (see Photo 1 below) and into the cold attic space. We call it the ole’ Stack Effect – just slightly less powerful than the force Darth Vader used, but equally as nefarious.


Photo 1: Does your attic floor look like this? Then don’t design it like this otherwise you have lots of holes, gaps and cracks to air seal. Remember, air seal first, then insulate!


A particularly good example of an ice dam on an roof’s eve. The cause is typically 80% air leakage and 20% conduction (lack of insulation).

The summer’s high humidity kept stored in our buildings stays high until the cold winter drives it out. Usually by late January there’s not a lot of humidity stored in the house because the stack effect drove it all out. But in early winter, humidity levels indoors are rich and so it’s common at the first bite of winter to see signs of attic condensation… indoors. The combination is destructive if not remediated promptly.


This is your sheathing…on moisture… in winter. The cold night sky absorbs heat off roofs and any moisture in the attic usually condenses on the back side as in this newly sheathed attic.

Below are examples of stack effect and careless workmanship.



The nails are made of metal which is very conductive. The cold night sky, or brisk winter winds keeps these nails colder than the wood sheathing and they are the canary in the coal mine for attic moisture.



You wouldn’t fill your car’s gas tank by aiming the nozzle 3″ from the hole, so why do it with your bath fan in the attic? The bath exhaust fan pipe aimed at the hole in the roof sheathing had equally disastrous consequences. The above plywood sheathing was merely 3 years old.

Because we do lots of discomfort, mould and condensation diagnostics on existing buildings, we tend to see the worst of building envelope design. Though it is possible to design really funky Passive Houses like Libskind’s signature pre-fab Villa, the sad truth is that the run of the mill builder isn’t there yet. Not even the plumbers can get it right 100% of the time:

The plumbing vent likely contributed lots of moisture to this icy attic sheathing.

The unglued joint in the plumbing vent likely contributed lots of moisture to this icy attic sheathing.

The interface between the living space and the attic needs to be simple for high performance buildings. If you really need ornamentation then build a high flat ceiling that’s air tight and drywall, then just build soffits under it; at least then you’ll have a continuous attic floor. It takes a special builder to detail complexity successfully, so if you’re not ready to hire the best of the best builder, don’t design ceilings like this: