First the warning:
If you seal too well, the exhaust from the furnace or hot water heater will not naturally go up the chimney, but instead stay around.
The by product of burning Natural Gas and Other fuels is Carbon Monoxide … a silent killer.
Care should Always be taken as well as continuous verification that sufficient air is reaching the exhaust ducting. I also use a secondard carbon monoxide detector near the furnace/Hot Water Heater to alert me for when I have gone too far.
So, I pulled out the ramshackle fiberglass insulation from the cubbies. My plan was to calk the top bottom and sides of the board facing out as well as any other holes I found.
What I found were some pretty significant leaks. The worst places were where two vertical boards met. I forget the proper construction name, but basically there are a few stacked 2-by 4’s or 6’s and on top is a 2x 10. These 2×10’s where they met had the worst leaking. Sometimes where the 2×4’s met there was leaking too, but the gap height was only 2 inches, so the volume was less… but still significant.
There was usually a 1/4 in gap between the two boards and significant air flow as entering through the gaps. The second most leaking place was the gas pipe coming through the cinder block foundation. There were many other lesser leaking places. sometimes the gaps between sill plate 2×4’s had a leak. What I did not find, was any leaking between or underneath the 2×4 sill plates. I sealed them just to be sure, but never felt cold air coming through.