Year-round temperature control - 18" WSM

[Steve - Polecat]

Hello all,
I got my first WSM this past July and have been learning a TON. I've been cooking in all kinds of weather and temperatures, too.
Are there any 'rules of thumb' for adjusting to the outside temperature in order to maintain smoker temperature? For example, what percentage faster does fuel burn for every, say, 10 degrees lower the ambient temperature is? It is much easier to maintain 250 smoker temperature in August than it is in December. There's nearly a 60-70 degree difference in outside temps, so how do I plan on the amount of charcoal to use/when to add, etc.
I'm doing a couple of Boston Butts (about 8 lbs. each) and I'm having trouble getting much more than a 4-5 hour cook before needing to add fuel. I'm trying to maintain around 275 and I have bottom vents 3/4 open and the top vent fully open. It's currently 30 degrees F and the unit is tucked away from wind or direct weather.
Thanks in advance for a advice and help!
Steve

 

[JungWoo]

I had the same experience cooking in 30 degree temps on my 22" WSM. I just picked up a welder's blanket from harbor freight, I'm going to give that a shot later this week and report back.

I don't seem to lose efficiency as much on my 22" kettle...

 

[Jim C in Denver]

Are you using water in the pan for your winter cooks?

The water acts like a brake on the cooker temp. That can be helpful in the summer. But in the winter, it isn't much needed and is wasting a lot of BTUs and fuel.