The sill & band joist tend to be huge air leaks as well- sealing & insulating them with foam is always a good idea.

And the above-grade portion of uninsulated concrete is good for less than R-1. Insulating down to at least the mid-winter frost depth with foam board is also always a good ideal.

Insulating & sealing the basement of my ~80 year old 1.5 story bungalow to ~R20 took ~20% off the heating bill. Interior basement wall & air temps coast along at ~65F, when the first floor is maintained at 68F. (The uninsulated floor slab is now the larger heat loss for the basement.)

Good points. Very obvious the delta T is greater at the above ground porion of the basement wall. I had suggested they seal and insulate the sill and band joinst, but it didn't happen in the first section.

I imagine the first floor heat will move down and along the floor joists to and through the exposed concrete wall and although the BTU exit area is much reduced (from the entire basement wall to the roughly 8" above the sill) I wonder if there could be condensation and humidity problems above the basement drywall ceiling?

Air movement from the conditioned space onto the now colder sill & band joist is potentially a problem. But as long as it's air-sealed the condensation & rot risk is low.

I still commonly see unfaced fiberglass batting installed against band joists & sills, which is the worst of all worlds: The effectiveness of the insulation is compromised by the ease of convection through the FG (that R19 batt becomes effectivelyR7) and the wood is now colder, with higher condensation potential. Spray foam (any type) will block that that conditioned-air/wood contact, eliminating the condensation issue.

Just to be clear Dana, if faced fiberglass is used, the face goes to the warm interior, right?

Posted By Farmboy on 17 Feb 2010 10:37 PM
Just to be clear Dana, if faced fiberglass is used, the face goes to the warm interior, right?

If you're talking about the just sill & band joist, yes but...




...it's next to impossible to get a perfect air seal using faced fiberglass on sills & band joists, making it a thermosiping vapor-still with the wood as the condensing surface. MUCH better to use foam here.

And never used faced fiberglass for the below grade portion or it WILL trap moisture in the studwall, leading to a mold problem.  (Hope I wasn't too late on this one...)

Thqnks, Dana. I believe the basement walls and ceiling are closed in and that no concerted effort was made to air seal the sill/band joist. Before the drywall was done, I didn't feel a noticeable draft in that area, so hopefully it won't be a problem.

Posted By Farmboy on 03 Mar 2010 01:29 AM
Thqnks, Dana. I believe the basement walls and ceiling are closed in and that no concerted effort was made to air seal the sill/band joist. Before the drywall was done, I didn't feel a noticeable draft in that area, so hopefully it won't be a problem.

The draft you're concerned about isn't outdoor air coming in, but indoor air getting through the fiberglass to condense on the sill & band joist.  The drywall doesn't extend up between the joists and form an air-barrier there, so some air is getting to it.

In central KS the condensation risk is low, but greater than zero. While average outdoor temps Dec-Feb are below the dew point of 68F 30%relative-humidity interior air, the average daily highs are somewhat above the dew point, so at least SOME drying occurs on a regular basis even through the winter (not true in colder climates.)  As-built expect some hygric accumulation in the wood during the winter months, but whether it's enough to cause a problem depends on the volume of interior air reaching the cold wood.  Fiberglass doesn't slow it down much, but if the house is otherwise pretty tight there may not be much pressure driving it.

Now that's a clear, concise explanation that helps me understand the relationship between dew point and the source of air infiltration.

Regarding "..the average daily highs are somewhat above the dew point, so at least SOME drying occurs on a regular basis even through the winter...". We just went through an all time record of NO daily highs equal or greater than 60 degrees since sometime in November, so there's a higher chance of some condensation. But I understand what you're saying. One reason we didn't move to the eastern part of Kansas was the higher humidity. Thanks!

Posted By Farmboy on 04 Mar 2010 11:35 PM
Now that's a clear, concise explanation that helps me understand the relationship between dew point and the source of air infiltration.

Regarding "..the average daily highs are somewhat above the dew point, so at least SOME drying occurs on a regular basis even throught the wintoer...". We just went through an all time record of NO daily highs equal or greater than 60 degrees since sometime in November, so there's a higher chance of some condensation. But I understand what you're saying. One reason we didn't move to the eastern part of Kansas was the higher humidity. Thanks!

The dew point of 68F indoor air with ~30-35% relative humidity is ~35-38F.  If it's 40F outside, unless you're adding humidity to the indoor air to some ridiculous level or the place is so tight & unventilated that it's relative humidity is higher than 30%, some drying is occurring (even if it's raining outside.   But when the temperature of the band joist & sill is below 35F, indoor air that reaches it can and will condense as liquid (or frozen) water on that surface.  If that colder-than 35F condition persists (as it does for weeks or months in colder climates) water accumulates in the wood, mold & rot will occur, which is why interior side air barriers and vapor retarders are necessary.  But if it's temp rises above the dew point on a regular basis (as is does in central KS) the water that didn't get absorbed by the wood can evaporate, and the risk of problems is lower. But it's still a question of how much water condenses (a funtcion of air-volume reaching the condensing surface), and how much drying time vs. condensing time does it get every week.

I tend to use 68F/30-35% is used as a "typical" indoor humidity, but is by no means absolute. To figure it more precisely, use a psychrometric chart, or use a psychrometric calculator (if you want to adjust for altitude).  Measure the indoor temp & RH (there are cheap RH monitors available online or even some box-stores, including Wal-Mart), then find the intersection of the RH curve and your indoor temp on the chart (or enter it into a psychrometric calculation tool.)  Draw a line laterally to the left to where it intersects the 100% line, then draw a line down from there to find the dew point temperature.

An indoor relative humidity is at the low end of the "comfort & health" recommendations HVAC and health organizations.  You can be comfortable down to 25% at which point evaporation off your skin makes it feel noticeably cooler than the sensible temp reading on the thermometer.  Winter indoor air tends to be drier than summer air, because the outdoor dew points are so much lower, so ventilation air when warmed to room temp has a much lower RH.  If actively humidifying the air, keep it at 30% in winter, no higher and the structure will have less water to remove.  If it's naturally higher than that in winter, heat recovery ventilation or a room dehumidifier can usually get it down to 30-35% without a huge duty cycle in winter.

In summer the indoor humidity levels rise because the outdoor air has a higher dew point, higher humidity content, but theres no threat of condensation within the wall structure unless it's very hot & humid and you're air-conditioning the interior, so that the dew point of the OUTSIDE air occurs somewhere within the wall.   (In FL and along the gulf coast vapor retarders are sometimes necessary on the exterior side of air conditioned buildings.)  In general, summertime indoor relative humidities are still comfortable up to about 60-65%, but dust-mite allergic people are advised to keep it drier than 50% (at which point the mite populations don't survive.)  Above 70% relative humidity the mold potential on building materials explodes- mechanically dehumidifying to ~50-60% is "right" for most people from both comfort & health.

Thanks for the info on how to use the psychmetric charts. I think you should write a book called "Principles and Applications of Insulation, Energy Efficiency and Mold Control for Dummies". I'm saving this thread for reference. Cheers Dave

Posted By Farmboy on 05 Mar 2010 11:06 AM
Thanks for the info on how to use the psychmetric charts. I think you should write a book called "Principles and Applications of Insulation, Energy Efficiency and Mold Control for Dummies". I'm saving this thread for reference. Cheers Dave

It's pretty much already been written.  (Note the recommendations in different climate zones for maximum wintertime interior relative humidity in that document.)

Mold problems in climates like yours are more commonly caused by bulk-moisture intrusion such as foundation, siding, or roof leaks, followed by air-transported moisture deposition on condensing surfaces, which is what we're talking about here.  Air-sealing both the exterior and interior faces of insulated assemblies is key, but often ignored in practice, even in colder climates where the risk is higher. 

KS is on the warmer edge of a "cold" climate as defined in that article, which means the risks of problems from vapor-diffused moisture transport to condensing surfaces is fairly low. Vapor retardent interior paints on the first floor walls/ceiling or the basement ceiling aren't a terrible idea, but nowhere near as important as the air-leakage issues. Don't put vapor retardent paints on basement walls, or you run the risk of trapping ground moisture in the wallboard & stud wall.

When it comes time to re-side the place, air sealing the exterior sheathing before the siding goes up is always a good idea, as is building in a ventilation gap (aka "rainscreen") between the siding and sheathing to allow bulk wetting of the siding to drain & dry, which helps in two ways: It decreases the uptake of bulk water of rain penetration into the sheathing layer, and increases the drying capacity of the sheathing layer since it's now exposed to an easy flow of exterior air.  Both aspects reduce the risk of problems related to any lapses of best-practices regarding air sealing & vapor control on the interior side.  The perfect house has yet to be built, yet most seem to survive somehow, eh? ;-)  But anything you can do to lower the moisture content of the structural wood going forward increases the likelihood that it'll be around and problem-free for decades to come.

Actually, if it's in the budget when it comes time to re-side, air-sealing with a layer of rigid foam insulation all way down to the sill will raise the average winter temp of the sill & band joist further lowering the risk, while providing a thermal break over the framing timbers dramatically increasing the R-value of the wall assembly.  If you tack some copper flashing (as termite control) to the sill, digging down 2' and putting a similar thickness of foam to the exterior of the foundation at the same time will raise the wintertime sill temp even more.

Houses are never really "done", eh? (You just went ahead and re-did the foundation after "only" 100 years. :-) )

Quite an informative article. The most important thing I learned is that one needs to use the wall assembly most capable of functioning properly in your specific climate. The psychometric chart is very helpful as I spend more time simulating various scenarios Since we'll be building our dream home with ICF, our main concerns willo be the wood truss roof and windows/doors moisture control. I think this discussion and associated references have highlighted what it takes to attain a comfortable and healthy indoor environment. Thanks again.