I've heard this phone call before - "Air blows in the hall, but the door is tight." Door weatherstripping is something that gets looser over time, and can admit air.
The weatherstripping is an important part of the energy efficiency of any door. And window!
The door itself is insulated metal, or insulated vinyl, or even wood, which is a good insulator. And any surrounding windows will be double pane and be pretty efficient.
But the weak spot is the weatherstripping.
This is a thermal image of the bottom of the front door.
It is very interesting because the warmest temperature is about 70F.
Yet the weather outside is sunny and 28F.
Imagine how hot that space under the door is in July!!
Why is it so warm under the door? Because it is 10am and the sun is shining directly onto the front door. The door weatherstripping is weak.
There is no storm door, so the area is heating up from the direct sunlight. But with a gap this large imagine how much heat is escaping when the sun is not on the door! Remember, heat seeks cold, AND MOVES TOWARD IT. With a gap this large, and when it's very cold outside, it would feel like cold air is literally blowing in from under the front door.
Yet, when facing the door, this gap is not visible. The thermal camera shows it quite well though.
Sweeping the rest of the house I was able to show them similar situations with other doors where the same area under the door was cold.
A storm door outside the front door would enhance the energy characteristics of the space dramatically. That space of dead air between the two doors is a great energy seal. And controls the sun's heat!
My recommendation: sometimes the simple answer is THE answer. If you feel air moving, there is a reason! It may not be obvious to the naked eye, but would be to a thermal camera. So call a thermographer. And remember, the side benefit is that all thermographers are really cute.
Jay Markanich Real Estate Inspections, LLC 
Based in Bristow, serving all of Northern Virginia.
Office (703) 330-6388 Cell (703) 585-7560
Good morning Jay. I find a lot of times that the door is improperly installed and there is a gap around the weather seal. I do have to agree with you, thermographers are good-looking.
This door is only 5 years old Michael and there is damage to the stripping. But also, the door comes factory perfect (ideally) so installation is a big deal. If imporoper, well, it's improper.
Jay, I agree that the problem is most often the installer. Enjoy your day!
Yes, that can make a big difference. bTW, I can't see the pictures.
May be Wayne. Nevertheless, the problem exists!
Debbie - I will post it again. Sorry. It was there when I first posted it! I am having so many problems with this "new" system. Try back later.
There, fixed. I hope.
This post could not have came at a better time. We are actually going Saturday and buying a door.... Now the decision is what color to buy
James - buy the one without the yellow stripe on the bottom!
I just had a door installed in my garage that was leaking not only air, but water like a sieve. Good timely information - thanks for sharing Jay
Could be any of a couple of things Jennifer, but I hope you called the installers back!
Jay, It's something that I know needs work in my own home. We had a new door installed in the basement and I think the strikeplate should have been installed a bit closer to the front of the door, the end result is the door isn't sitting tightly to the weather stripping. I need to get the hole and plate moved up a bit, because finding larger weather stripping doesn't seem to be an option, at least not from the big box stores. Bill
Bill - installation can be everything. The unit usually comes very square and with cross members to hold it tight. The installation can get it out of whack.
This was one of the first things I did when we purchased our house 3 years ago. I replaced all of the seals (side, top and bottom) on all exterior doors. What a difference!
Jay.... come to the light, come to the light, but leave that dang camera there! :-)
Stephen - and it would! Weatherstripping not only doesn't last forever, but gets weaker all along.
Fred - I feel it! Is it love? Is that a voice? Uh,oh - I think it's Wrench.
Some yeas back my in-laws could not understand the cold air they were feeling. I tried to tell them the issue but was scoffed at.
So, I went over and just fixed it and they were surprised how warm their feet stayed.
Amazing what a little strip of rubber can do, William!