There are a lot of theories and anecdotal evidence about single-walled WP/B performance in below freezing conditions, but I'd like to point out another contributing factor to breathability (or the lack thereof). The flame retardant treatment used to pass CPAI-84 tests affects both the weight and the breathability of the tent. For example, the Epic tents made by Black Diamond do not pass this test, and thus cannot be sold in California, Canada, Michigan, etc. (Check the listing at REI.com for the LightHouse).

At one time, both Integral Designs and Bibler offered their tents in both CPAI-84 spec and also without the flame retardant treatment. I remember one poster saying that she used an ID MKIII on the Appalachian Trail and had a miserable time with condensation during the summer. I asked if the tent had a tag with the CPAI-84 statement, and she said that hers did. My ID MKIII doesn't meet the spec, and it's fine for breathability.

All of the Biblers that I have are non flame-retardant with the exception of a Pinion, and they are generally fine in the cold and the rain. But there are differences: the Eldorado and I-Tent have superb ventilation with the roof vents and and the doors that have three zippers so you can vent both from below (stake out the little flap at the bottom of the exterior door) and from above (protected by the wired brim). But the Torre tent has no roof vents (and also built in vestibules), so it is more prone to frosting up on the ceiling (and of course the poles and the seam tape).

But I think I read once in some Bibler literature that the bigger the tent, the less condensation. I have an unusual Bomb Shelter that was probably made for the Special Forces. Instead of ToddTex, it has GoreTex (says so on the inside) with the same type of scrim as found on jackets. Not only is it supremely dry inside under all conditions (even without roof vents), it is also big. I don't know what to attribute this to: perhaps GoreTex is more breathable than ToddTex, perhaps the lighter scrim (also found on eVent tents) is better at passing moisture than the fuzzy nexxus stuff, or perhaps bigger is better.

Here, I've found the statement from Bibler about condensation vs. size:

Do I have to "heat up" the tent's interior to make it breathe?

The myth about single wall tents is that they require a pressure differential, interior to exterior, in order to breathe and prevent condensation. If this were true, you would find less condensation in a smaller tent or bivy sack than in a larger tent. In fact, the larger the surface area of fabric, ie: the bigger the tent, the less condensation occurs.