AboutBruce E. Johnson Expertise I can answer any construction related question in regards to carpentry, concrete, drywall, masonry, structural elements of any type of building, residential or commercial. Interior or exterior.
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Riversong HouseWright wrote at 2006-06-25 14:49:18
Definitely insulate the floor over the crawl space, but use unfaced fiberglass batts (the Kraft facing on the underside will be a cold side vapor barrier and trap moisture in the insulation). This will reduce the heated volume of the house (avoid heating an unused crawl space).
As long as you do the suggested moisture sealing on the crawl space floor (and you have good gutters), you won't have a moisture problem there. But do NOT ventilate the crawl space. Joe Lstiburek, perhaps the premier building scientist in North America, recommends against crawl space ventilation in heating climates, as tests demonstrate it introduces more moisture and condensation. And keeping it sealed will make it a partially-conditioned space which will reduce the heat loss from the floor. During the heating season, moisture migrates outward, not inward.
You're right that bringing combustion air directly to the boiler will reduce the negative pressure. However, if the kitchen and bathroom fans are used when moisture is being generated, then negative pressure will be created when it is most needed - to evacuate the moisture sources.
Tightening up the house envelope will possibly eliminate the need for a wintertime humidifier since dryness is due to infiltration. If the humidity level rises too much (beyond 40%) then the house will need some controlled ventilation. But I suspect this is unlikely in your case.
As long as the drywall is sealed at all seams (including wall/ceiling junctures) and door and window frames and electrical outlets are sealed, then I wouldn't worry about moisture in the walls. Most moisture exfiltration is due to air movement, not diffusion.
