New to the process of sealing an older home, and have a question.  On the one hand, I'm finding in article/video research that sealing every single opening in the attic floor and walls before laying any insulation is essential and initially made obvious sense to me, but then, leaving eave vents completely uncovered and flowing is a must.  Why can't I help seeing this as a contradiction.  What was the point in sealing/caulking every possible source of air flow in the house's upper level ceiling/attic floor, wouldn't these just all serve as mini eave vents?

You are correct - insulating a home is complicated! Whether or not to ventilate an attic is a current hot topic among energy geeks, and depends on a number of factors.
1. Where is the house? Climate makes a huge difference in insulation techniques
2. Where is the insulation? Is it on the attic floor (cold attic) or on the roof (conditioned attic)?
3. How do you access the attic? (many people try to have it both ways - with a stairway to a what they imagine is a "cold attic", for instance, where the attic door is not an exterior grade insulated weatherstripped door and the stair walls and the stairs themselves are not air sealed and insulated the same as exterior walls. This is an exercise in futility.)
4. Do you use the attic? (If so, and if there is access from the house, it should be a conditioned space.)
5. How old is the house and what is the roof construction?

there are more issues, but we can start with these.

Is your "attic" conditioned or unconditioned space?

Posted By Gmq on 31 Jan 2013 11:51 PM
New to the process of sealing an older home, and have a question.  On the one hand, I'm finding in article/video research that sealing every single opening in the attic floor and walls before laying any insulation is essential and initially made obvious sense to me, but then, leaving eave vents completely uncovered and flowing is a must.  Why can't I help seeing this as a contradiction.  What was the point in sealing/caulking every possible source of air flow in the house's upper level ceiling/attic floor, wouldn't these just all serve as mini eave vents?

Holes in the ceiling creates a stack effect on the conditioned space, depressurizing it which sucks outdoor air in elsewhere, which is the opposite of what you want.  In a heating dominated climate in winter that allows conditioned space air with a much higher moisture content into the attic where it will adsorb into the wood of the rafters & roof deck when they are at a temperature below the dew point of your humid conditioned space air.

Sealing up the ceiling cuts that moisture & heat flow, and attic ventilation dilutes it with much drier outdoor air, lowering the dew point of the attic air. That limits the amount of moisture getting into the wood, lowering the risk of mold/rot getting started. Eave/soffit venting (but not ridge venting) also limits the stack effect flow by keeping the attic pressure about the same as that of the upper floor ceiling.  (If you have both soffit and ridge venting, it's important to have considerably more soffit vent area than ridge area so as not to depressurize the attic, driving stack effect flow from conditioned space.)

In an older house if you have no signs of mold/rot/moisture issues on the underside of the roof deck there is no need to further open up eave ventilation or add ridge vents- it's doing just fine with the ventilation it has.  Sealing it up between the attic and conditioned space is still important though, since after insulating the attic will run somewhat cooler (sometimes a LOT cooler) in winter.

If you're in US climate zone 1 or 2 (hot, cooling-dominated areas) attic ventilation usually adds more moisture to an attic than it removes (except in the driest parts of the southwest), and it may even be beneficial to even seal up any pre-existing attic ventilation. There are exceptions to this though, so if you're on the gulf coast or southnern AZ you MAY want to seal it up, but I'd need to know more about what's going on in the attic before fully recommending it.