Colonial Split level

"Best bet is to install rigid board on the underside as well and then back with a fibrous insulation that is pre-compressed so as not to drop down and cover with an air barrier like Tyvek to hold it in place." -----------------

I don't agree. With f.b. against the sheeting the cavity will be much colder to allow any air leaks around the Tyvek (a water resistive barrier, not an air barrier (they removed that claim after it first came out)) to deposit moisture condensing on the colder joists. With the f.b. on the joist bottoms they are decoupled from ground source temperature/moisture by the foil-facing and R-value composition. A warm cavity doesn't rot the framing when wet, joists would stay warmed from above in this method (thermal break). It wouldn't get wet from rising damp ground under the cantilever due to the foil-facing. Canned foam the edges and caulk the joist bottoms before installing the f.b. Caulk over the f.b. before installing the plywood air-tight, caulk edges after install to prevent air infiltration/exfiltration. Foamboard the rim joists, canned foam all edges to prevent same and because solid wood is only R-1.25 per inch; Fig.2: http://www.buildingscience.com/documents/information-sheets/crawlspace-insulation/

The solid wood rim joists expand/contract with the seasons, letting air in/out; Fig.3; http://www.buildingscience.com/documents/insights/bsi-023-wood-is-good-but-strange/

Use some store-bought foam outlet/switch covers to stop moving air at exterior walls, carefully pull the baseboard trim and caulk the drywall/floor joint at all walls; http://www.homeenergy.org/show/article/nav/troubleshooting/page/2/id/1360

http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/

Air seal the house leaks; http://www.homeenergy.org/show/article/year/1995/id/1173

Slow the stack effect with air-sealing; http://www.buildingscience.com/docu...ir-leaks-how-they-waste-energy-and-rot-houses

Savings: http://www.ornl.gov/sci/buildings/2012/B11 papers/80_Bibee.pdf

Gary

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;

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/docu...ld-weather-condensation-using-insulation/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/book/export/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/ibp/irc/bsi/90-controlling-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/documents/insights/bsi-009-new-light-in-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_Weathe...rior Insulation Building Science Bulletin.pdf


Gary

GBR in WA said:

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;

Click to expand...

All the links and data are great but real world deployment of that is a different story.

The original poster mentioned that:

I have a 1961 Colonial Split Level that has had numerous upgrades performed. Vinyl siding has been installed and the areas where the main floor extends beyond the basement has vented panels installed (covering below the floor joists).

Probably looks a bit like this home here:



While putting foam board over the joist is ideal in this case, it not not feasible in a majority of these cases with the siding already done. Where the starter strip, soffit J-channel, and other accessories (inside/outside corners, J-channel, etc) finish out cannot be effectively modified to work with rigid foam across the joists.

Doing so with any measured thickness will throw off the alignment of all those items and I don't think the original poster wants to re-side the home.

Putting rigid foam across the joists is also only really effective from the elimination of the thermal bridge if you can install it across the ribbon board as well and eliminate that thermal bridge. If not, you will still have the cold section at the front of the home and still have the potential for the condensation.

The rationale for the foam board inside the cavity is solely to air seal the underside of the floor and to have a layer of insulation that is stuck to the underside and will not drop down and therefore loose its R-value component.

While that cavity will certainly be colder, the thermal bridging of the joists (usually 2x8) will still keep them relatively warm (and keep the dew point higher) given that they are only going to be and R-7 at those locations.

In a perfect world, we are re-siding the home and wrap that whole section with rigid foam and bring it into the warm wall. That is just not the case in most cases and I suspect this one as well.

Pictures of 1961 house with new vinyl siding etc

Hope this works. As you can see this is a quality vinyl job (in appearance) that I don't want to damage or reduce its value. Drew