Need Info on moisture trapped in roof systems due to insulation.

[Ron247]

I am looking for information or articles about moisture that gets trapped in roof systems due to insulation causing problems. Examples would be a shallow parallel chord wood truss roof with un-vented insulation under the trusses. I have seen pictures of wood rot and rust on nails due to excessive moisture trapped in system. This would not be from roof leaks. This would be moisture forming in the area. I am told this is more of an issue on the west coast than the east coast for some reason I do not know.

Also, is there any moisture issues from the standard PEMB vinyl backed insulation that is placed over the purlins before the roof panel is installed. I have never seen a moisture problem from that but I have been asked by someone about it.

 

[dhengr]

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.

 

[dik]

The building envelope has to be weatherproof and condensation has to be eliminated. Water is a major cause of deterioration to both wood and masonry structures. Once brown rot (aka dryrot) has been observed, a substantial part of the strength may have been lost.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Ron247]

Thanks all for the info. It helps.

 

[Ron]

Having the insulation under the trusses creates the potential for condensation in the void space. Likely the moisture source is from the interior, not the exterior, unless you have roof leaks. Since you are in Alabama, you can have a range of thermal conditions that would contribute to this, depending on whether you are in north Alabama or south.

The simple solution would be forcing ventilation of the cavity.

 

[dik]

In these environs, construction has the insulation on the bottom chord of the wood truss (actually between the chord members) with the space between the insulation and the roof sheathing being vented... generally be soffit and gable wall vents... sometimes roof mounted and occasionally 'whirly birds' (for real ventilation).

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BenMacier]

You can predict this risk of condensation considering indoor/outdoir conditions with a software like BuildDesk U.

* Finding a solution is great * Knowing how to implement it is fantastic * Believing it is the only one and best is naive ?

 

[Ron247]

dik,
The roof mounted ventilation sounds like a quicker solution that venting the insulation.

I have to check into BuildDesk.

 

[dik]

All three are done... generally starting with a soffit inlet.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik