How to Control High-Risk Moisture Environments
The best way to protect high-risk moisture environments – for walls and roofs
It does not get any riskier than constructing an enclosure over an indoor swimming pool. Except maybe an enclosure over a pressurized and humidified hospital. Or, how about an enclosure over a conditioned museum designed by a famous architect who is incapable of drawing straight lines? Even better, how about an enclosure for an art gallery where the art is the gallery itself? Oh, I forgot one more thing. Let’s put the enclosure on the top of a mountain in a cold climate.
Start with Perfect Walls
Of course, the enclosure is going to be some variation of the “perfect wall” where the water control layer, the air control layer, the vapor control layer, and the thermal control layer all are exterior to the structure and the cladding system – whatever it is – is back ventilated and drained.
So how do you back ventilate and drain cladding? And how much of a gap do you need? With a non-reservoir cladding, not much of one – I tend to like ¼ inch to 3/8 inch. But this gap needs to be continuous and, in many cases, the cladding needs to be supported to transfer the loads to the structure. Drainage mats tend to be pretty ideal for such applications.
With a reservoir cladding, things can get tricky. Stucco is particularly tricky. Inward vapor drive from a wet stucco rendering needs to be intercepted. This can be done by installing a vapor throttle inward of the air gap or immediately behind the stucco rendering. Alternatively, it can be done by increasing the air gap to enhance back ventilation.
What we have learned over time is that it is nice for the wall to dry outwards into the air gap and then subsequently to the exterior.
What About Roofs?
Walls are relatively easy. So how do you do this for a roof? How about a metal roof? Or a tile roof? Or a slate roof? Well, it is pretty easy to point out that the metal roof and the tile roof have to be back ventilated and supported. We pretty much know how to do this for metal roofs and tile roofs. The slate roof? Kinda. Supporting a slate roof is tricky and essential. Slate is simultaneously exceptionally durable and fragile. It does not like it when someone walks on it. Hence, the need for full support. The best we can do is install it over a highly vapor permeable and water-resistive underlayment, and fully support it. The good news is there is some vapor movement and redistribution around the slate members. Slate roofing is not a complete air barrier and vapor barrier.
With tile roofs the best approach is a batten and counter-batten approach where the battens are installed over a highly vapor permeable and water-resistive underlayment. There is no real controversy here.
With metal roofs, back ventilation is not typically done, for reasons that make no sense, when a roof goes over a high-risk environment. A drainage mat is ideal for this application. It provides complete support and allows vapor flow in the air space provided. The drainage component is far less important than the vapor redistribution provided by the air space. The drainage mat needs to go over a highly vapor permeable and water-resistive underlayment.
More Moisture, More Problems
The back ventilation controversy with metal roofs comes from experience with metal roofs over low-moisture-load enclosures. Lots of warehouses with metal roofs installed directly on sheathing have worked just fine. Lots of commercial buildings with metal roofs installed directly on sheathings have worked just fine. Lots of factories with metal roofs installed directly on sheathings have worked just fine. Over swimming pools, hospitals, museums, art galleries or data-processing centers, the failures have been legion. The difference is pretty obvious. No interior moisture… no problem. Lots of interior moisture… lots of problems.
The problems go away with two approaches that need to be combined. First, keep the moisture from the inside from getting to the underside of the metal roof. Second, recognize that you are never going to be perfect in keeping the moisture from getting to the underside of the metal roof, therefore you have to provide a means for getting rid of it once it gets there.
You do the best you can with air control layers (a.k.a. “air barriers”) and vapor control layers (a.k.a. “vapor barriers”) in keeping as much interior moisture as possible from getting up into the roof assembly. “Best” means pretty darn good. But note that perfect never happens. That’s where back ventilation comes in.
The combination of air control and vapor control layers, coupled with back ventilation, is hard to beat. Works for walls. Works for roofs. Especially metal roofs and tile roofs.
To that, DELTA®-TRELA is one of the best vapor permeable air barriers on the market. It’s continuous, so it keeps external air and water out of the building, but it allows moisture and vapor to escape, for more effective drying.
About Joe Lstiburek, Ph.D., P.Eng., ASHRAE Fellow, Principal, Building Science Corporation:
Joe Lstiburek is the founding principal of Building Science Corporation, one of the most influential, innovative, and respected building science firms in North America. Dr. Lstiburek’s work ranges widely, from providing expert witness testimony to overseeing research and development projects, to writing for the ASHRAE Journal. His commitment to advancing the building industry has had a lasting impact on building codes and practices throughout the world, particularly in the areas of air barriers, vapor barriers, and vented and unvented roof assemblies.
Dr. Lstiburek is also an acclaimed educator who has taught thousands of professionals over the past three decades and written countless papers as well as the best-selling Builder Guides. His commitment to education earned him the hailing, “the dean of North American building science” by the Wall Street Journal. You can find additional details on Dr. Lstiburek on our About the Blog page.
For more tips from Dr. Lstiburek, watch his latest webinar: How to protect below-grade living spaces.