Ron247:
You are most likely talking about a fairly common condition, depending upon the exact climate conditions. Consider the following gen. details, an exterior wall or roof system with a makeup something like this: interior sheathing/surface covering (sht. rk., plywd., paneling, etc., wall or ceiling), framing (wd. struds, your shallow parallel chord wood trusses, etc.), insulation material in the void spaces, ext. sheathing or not, ext. siding or roofing mat’rls. of many variations; our fairly std. details for these cross sections through these systems. The missing details, and their exact placement in the above cross sections are a vapor barrier mat’rl./layer and a wind/rain screen mat’rl./layer.
Now consider the weather conditions, in the south you guys deal with a relatively humid, hot exterior and a cool (A/C conditioned) interior for most of the year, whereas, in the north, we deal with a cold/dry exterior for a good share of the year, and a warmer, moisture laden interior spaces. The temp. drive/movement is from warm to cooler, and the moisture drive/movement is from moist to dry. And, in the north, with this heat and moisture vapor movement through the insulated space and to the outside, at some thickness point in the insulated space, you will/might reach the dew point temp., at which point the moisture vapor will condense. This might wet the insulation or it might condense on a solid surface which is near this point in the space thickness. The moisture vapor will certainly condense on the underside of roof sheathing plywd. or on sheet metal roofing mat’rls. It will literally run down, drip off of these surfaces or form hoar frost and then melt when warmed a bit, by the sun. You must vent these spaces, outside the insul. layer to try to remove any moisture vapor to minimize these problems. Our solution, in the north, is a good, well-sealed, vapor barrier immediately outside the interior sht. rk. (on the inside edges of the studs) to try to keep moisture out of the insul. space; and then a good moisture plane system/mat’rl. (vapor permeable but water resistant) btwn. the ext. sheathing and the siding (on the outside of the sheathing, to allow a small amount of moisture vapor to escape, but to keep rain moisture out. You can sometimes vent wall cavities too. All wall penetrations must be well flashed and sealed and properly incorporated into this rain screen system. Roof spaces are vented above the insul. layer to get rid of any moisture vapor in this space.
The moisture vapor problem may be slightly less severe if there is no long term cold/freezing temps. since a well-defined dew point temp. isocline does not form for a long periods of time. But, with sufficiently high humidity you can still get condensation at temps. well above freezing, and we see this as dew on the grass or a windshield. Animals in a factory style barn produce considerable heat and moisture (humidity) within the barn, and I’ve seen relatively flat barn roofs acting like it is raining under the right conditions. Alternatively, the moisture will condense on the underside of the metal roofing, and well wet the thin draped insul. to the point that it falls down, into the bldg.
These are absolutely significant issues as we make bldgs. tighter in an effort to improve energy efficiency. Bldgs. used to breath to mostly alleviate these problems, but as we caulk every joint and eliminate any air movement through the bldg. envelope/system, the moisture can collect at various locations in the framing. Google Joseph Lstiburek, Building Science Corp., he’s a real authority on these subjects, and building to try to counteract and manage them.
