On Sunday, February 19, 2017 at 8:52:35 AM UTC-5, wrote:
DerbyDad03 wrote: - show quoted text -
"What's the R factor of that flap vs. the R factor of the rest of the door?
Assuming a quality door, of course. "

If rest of the dwelling is well insulated and
weather stripped, especially roof/attic,
infiltrations issues through the mail slot
will be minimal.

As a test, in my two-story house with attic,
on a cold day I cracked open a down stairs
window. Istuck my hand in the opening and felt cold,
but not coming in. I then went upstairs and
opened the attic stair-hatch about halfway down,
then returned to cracked open window downstairs.
Cold air was blasting in as heated air escaped
into that attic! The moral? Insulate top-down.
Dodges drafts!

Would you leave that window open all the time? I think not. While
the mail flap is less "leaky" than an open window, it's still an
opening that reduces the R value of the door.

BTW if simply opening your attic stairs causes heat to "blast" into
the attic, you may need to improve the insulation for the opening.

If all you have is the basic 1/4" plywood cover, you are losing a lot
of heat into the attic even when it's closed. When I open my attic stairs,
this is what I see:

http://i.imgur.com/o0K2Bpc.jpg

I built it to fit snug enough in the framing that the handles are needed
to pull it down.

Here's an attic view of the same type of device. Mine actually sits inside
the opening, on top of the framing for the stairs, as opposed to just resting
on the floor like the one in the image below. Mine has a much tighter fit.

https://www.conservationmart.com/ima...dium/10134.jpg