Reflective roofs, known as cool roofs, have many benefits for low-slope buildings, such as energy savings from reduced air-conditioning loads due to lower roof heat build-up. In the following picture, it is clear the heat load on the right-hand building is greater than that on the left:
In Part 1, I showed that even in Northern climates, as long as building cooling costs exceed heating costs, cool roofs provide energy savings. This is true as long as two conditions are met: the building is air conditioned and is not heated with electric.
There are concerns in the roofing industry that cool roofs could promote moisture build-up in the roof system. However, condensation under roofing membranes in the North has been observed since well before reflective roofs became popular. In Part 2, I showed that:
Reflective roofs do not, in of themselves, lead to moisture build-up.
For any roofing system, care must be taken in the North to prevent moist air from the building interior reaching the underside of the membrane.
Moisture Control The Basics
Preventing moisture migration in a low-slope roof system is important in Northern climates. The worst case might be mechanically attached single ply over a single layer of insulation, shown here:
There are a couple of obvious problems with this type of installation:
Single layers of insulation have significant air leakage up between the insulation boards.
Wind will lift up the membrane between fasteners and pump building air up into the system.
If we consider a good, better, best approach, then good is to always use two layers of insulation with vertical and horizontally staggered joints:
Double layers of insulation with staggered joints reduce the amount of air drawn up under the membrane. As noted in Part 2, this type of installation rarely leads to moisture concerns. However, air can still get to the underside of the membrane via penetrations and from the edges.
A better approach is to fully adhere the membrane as well, shown below:
Looking at the schematic above, its apparent that interior moisture can still get into the roof system, even if not to the underside of the membrane. Also, fully adhered systems dont fully address edge and penetration air leaks up into the roof system. If elimination of moisture is desired, then we have to discuss air barriers.
Role of Air Barriers
What Is an Air Barrier?
An air barrier is any material that has an air permeance 0.004 cfm/ft2.
Air barriers are assemblies of materials and components (sealants, tapes, etc.) that have an assembly air permeance 0.04 cfm/ft2.
Air barriers can be moisture permeable or not. The terms vapor barrier and moisture barrier are outdated and should not be used; the correct term is air barrier that is vapor impermeable.
When to Use As discussed in an earlier blog post about moisture:
Most buildings have small amounts of occupancy-generated moisture. These include office, retail, and warehouse spaces.
There are some buildings with large amounts of occupancy-generated moisture. These include paper mills, laundries, buildings with indoor swimming pools, and the like. In these cases, it can be important to design the roof system so moisture-laden air cannot get up into the roof system at all.
So, use an air barrier in a roofing system when there will be large amounts of occupancy-related moisture. In addition, there are some building owners who simply want the extra assurance of an air barrier.
How to Specify: There are many details involved in air barriers, a number of which depend on factors beyond the scope of this article. One of the most important factors is the vapor permeability. We have to recognize that moisture might get into a roof system eventually. This might be due to leaks, small gaps around penetrations, and sometimes overlooked gaps in the air barrier itself. Single-ply membranes are excellent moisture barriers, so try to make sure that if an air barrier is installed, it is vapor permeable. Otherwise, moisture that gets into the system will not be able to leave.
A vapor-impermeable air barrier in combination with a roof membrane can result in trapped moisture and is not a good idea. If an air barrier is specified, it should always have some vapor permeability.
The amount of vapor permeability required depends on several factors, including location and expected indoor humidity levels. Always consult a building engineer or appropriate consultant for an exact air-barrier specification.
Installing Air Barriers
Think of air barriers in the same way you consider roof membranes. Preventing water from getting into a building requires a lot of attention to terminations, penetrations, and flashings. Air barriers must be approached in the same way; the goal being to prevent movement of air up into the roof system. Air barriers often consist of a roll good, seaming tapes, caulks, and coatings. Its not uncommon to see spray-applied foam being used to close up large holes and gaps, followed by a coating and tapes to transition to a membrane.
This article has covered the basics of air barriers for use in single-ply roof assemblies. Its important to note the IECC requires air barriers for all new construction in zones 4 to 8 unless the membrane is fully adhered. Clearly, air barriers are becoming more important and those of us in the roofing industry will need to become more familiar with their use, installation, and specification.