Thanks! If I understand correctly now; foamboard next to the floor sheeting, no f.b. on the bottom of the joists. If that is correct: foamboard on the outside of a warmed cavity warms the cavity above dew-point so no vapor retarder is necessary under the drywall (on a wall). It is also a thermal break for the studs. With f.b against
the flooring (stopping the room heat), the cavity and studs are much colder, close to the dew-point and temperature of the outside air and are radiation coupled to the ground; if the cantilever is close enough. With f.b. on the bottom, is is similar to an exterior wall (turned on its side) = works best if f.b. is outside of cavity
insulation (rather than inside the cavity insulation; next to the heat source) Figs. 1, 2, and 3
"Figure 4: Hybrid insulation approach—Although riskier, particularly in cold climates and higher interior humidities, hybrid walls offer slightly higher R-value and can be moisture-safe in many applications. Note that the ratio of exterior insulation value to the framing cavity R-value defines the cold-weather condensation risk.
If R-19 (RSI3.5) of air permeable insulation was added to the stud space, the R-value of the assembly would rise by about R-7 over the empty studspace scenario: that is, almost 2/3 of the insulation value of the R-19 batt would still be lost. However the sheathing temperature would drop below 30 °F (-1 °C) and the risk of condensation would be higher. The relatively small increase in heat flow control provided by the batt insulation is achieved at the cost of much increased condensation risk." From: http://www.buildingscience.com/docum...nsulation/view
Remember the other link with the empty space above the cavity insulation? Insulation values in the cavity/foam outside ratio establishes condensation potential. Tyvek could stop air, but water vapor (way smaller molecules than air) travels both ways through it, it is not selective. The foil-faced foam board stops all water vapor and insulates the cavity (insulation included) to warm it above the dew-point temperature for condensation.
From another link;
"Ironically, the use of insulation in wall cavities along with rigid foam on the building exterior can actually increase the risk of condensation if the wall system is improperly designed. The reason is that cavity insulation slows the flow of heat outward and has the effect of keeping the back side of exterior sheathing cooler, thereby making condensation more likely.
This has led some builders to skip cavity insulation altogether and put all of the insulation on the outside of the walls, what's been dubbed "outsulation." Holladay discusses a technique called PERSIST (short for Pressure-Equalized Rain-Screen Insulated Structure Technique) that was developed in the 1960s by the National Research Council of Canada as an example of just how far it's possible to take the exterior insulation route." From: http://www.greenbuildingadvisor.com/...ort/html/17157
With your f.b. tight to the floor sheeting and cavity insulation with a housewrap, water vapor would go right through the h.w. and cavity insulation to condense on the bottom side of the foamboard because it would be near the outside air temperature. Wet fiberglass would lose 60-70% of its R-value; http://archive.nrc-cnrc.gc.ca/eng/ib...ling-heat.html
The joists would act as thermal sinks, robbing you of more heating dollars, as they may be 16" on center= 25% of the floor cross area at only R-12. They are vapor-open (Fig.4)
unprotected by f.b. on the bottoms;http://www.buildingscience.com/docum...n-crawlspaces/
From Tyvek, on exterior foamboard; "Advantages of Exterior Insulation Wall Design
The main advantage of exterior insulation wall design is increased thermal performance and decreased sensitivity to condensation because the dew point is moved outside the wall cavity. Metal framed construction benefits most from the exterior insulation by reducing thermal bridging. Thermal bridging occurs when the highly conductive steel studs break the continuity of the cavity insulation. Heat will choose the most conductive path, the path of least resistance, to bypass the insulation."
"However, in the exterior insulation wall design the wall cavity is part of the interior conditioned space therefore the moisture loads in the cavity are controlled by HVAC and not by vapor diffusion.
" Bold is mine. From; http://www2.dupont.com/Tyvek_Weather...20Bulletin.pdf