Wood Stove efficiency rating
I was reading an article from the Jotul site that mentioned "old" stoves with an efficiency rating of 70%-75% with a reduced air supply (as believed the case in most homes) would drop down to 35%-40%. The article went on to say newer stoves achieve up 80%, but only drop to 70%-75% under the same reduced air supply conditions.
What causes this reduced air supply? 1. The air-tightness of new home construction or 2. The closing of the fresh air intake on the stove itself?