I don't know how common this is, but I found an area under my front door where the exterior wall hadn't been built down all the way to cover the ends of the joists. To 'close' the space between the joists, the builders had taken chunks of 2x6's and placed them in the spaces. This, of course, leaves huge gaps because floor joists are skinny in their center portion. To make it worse, the joists had been sistered for some reason. So a 2x6 sistered to a joist leaves a nice channel in the middle.

I managed to seal all these gaps with spray foam and then replaced the little squares of fiberglass that had been concealing it for the past 25 years. There's only one spot where the spray foam nozzle simply couldn't reach due to distance and a narrow gap for accessibility.

The pay off was nearly immediate! This morning I checked my entryway floor and it was clearly much warmer than it had ever been. I've always thought I'd paid enough attention to things like this, but I guess it pays to dig deeper. I'll have to climb under my front porch in the spring to seal up any more concealed holes, unless anyone out there wasn't to volunteer to dive under the snow and do it now? :-)

I extend mine by using straws. If they are not a tight fit into each other just overlap and tape them. Doesn't have to be pretty, you'll throw them away when the can is empty.

davidandkasie: There's plenty of other proper vents provided for that purpose. This is obviously some kind of construction flub or shortcut. Our house was built back when the inspectors were known for doing curbside inspections. Fortunately this doesn't seem to have caused any structural issues that I can detect.

I'm actually making slow progress towards getting the crawlspace to be sealed. We monitor the humidity down there with a digital remote sensor and the winter is the best time to start some of this work due to the dryer weather.