Worth worrying about heat loss through aluminum door threshold?
I’ve just replaced a door between the house and unconditioned garage, where the temperature regularly gets up to 110°F in summer (i.e. the garage is basically a greenhouse). I was really focusing on air-sealing so I completely replaced the jambs with exterior door jambs with Q-lon etc.
The bottom of the door I wasn’t sure about, and I had an aluminum threshold with a rubber gasket handy so I put that in at the bottom of the door (removing ~3/4 of an inch of door). I think it’ll provide a good air-seal (which is the most important thing, I’ve learned :), well it will after I caulk a few gaps around it.
However, I notice that it is pretty cold, very noticeable on the garage side. Clearly I’ve just installed a very efficient 32″ heat bridge. Is that worth worrying about?
The other alternative is using something like a door bottom seal, like these from Conservation Technology:
http://conservationtechnology.com/building_weatherseals_adjustable.html but that’s quite a bit more work. Clearly silicon conducts less heat!
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Replies
James,
You're right that an aluminum threshold is an undesirable thermal bridge. Whether it's worth worrying about is another question. I don't think this one thermal bridge will cost you much on your energy bills.
That said, there was no reason to install an aluminum threshold in this location. Aluminum is chosen because it resists rot; it is used for exterior doors exposed to rain. This door will never see rain. What you need here is weatherstripping -- and there are many options, as your research has shown -- not an aluminum threshold. An ordinary wood threshold would work fine here.
I'll very probably replace it with an wood threshold (looks nicer anyway) and silicon weatherstripping.
One puzzle for me, though, is why it feels cold on the warm side rather than hot on the cold side. If it was just straight conduction I thought heat flowed from hot to cold. That suggests that the lack of air tightness currently is the dominant factor, cold air flowing through and around it, due to air pressure is cooling the metal. Make sense?
James,
Heat does flow from hot to cold. In this case, your hand is around 90 deg F and the aluminum is probably cooler than that. Metals feel cooler (and hotter) than non-conductive materials because metals transfer energy better. Heat is conducted out of your hand, making it feel cooler than a wood threshold at the same temp.
Bill
Thanks Bill :) That's makes total sense to me and is very satisfying. I guess that's why serious people use infra-red cameras to figure this stuff out!