Question

I'm a custom home builder in the Pacific Northwest that had read your 1998 paper titled, "Drainage, Ventilation Drying, and Enclosure Performance" and wondered if you still felt that rainscreen wall ventilation was a requirement for wet coastal climates (like Seattle and Vancouver, B.C.). You indicated that you are more convinced than ever that ventilation should be used in conjunction with rainscreen systems. However, you made the comment that ventilation is not necessary all of the time. Since I'm trying to develop an overall building approach regarding rainscreen systems for the homes I build, I thought that a quick review of my building situation might be helpful in answering the question:

"Do I need to ventilate the TOP of my rainscreen walls as well as the bottom of the walls?"

I've purchased and read the Canada Mortgage and Home Corporation (CMHC) book titled, "Woodframe Envelopes In The Coastal Climate Of British Columbia." Their position on venting the TOP of rainscreen walls is that venting the top of the wall may cause additional water to enter the wall as a result of negative pressure created in the rainscreen cavity during wind-driven rain storms. They believe this is especially significant at the top of buildings. Their position is that not venting the tops of rainscreen walls reduces the amount of pressure-driven rain entering the wall to begin with, a strategy that they prefer to use because of the limited drying potential of rainscreen walls during the cold and humid winter months (from November through April) here in the Pacific Northwest.

I've decided to incorporate rainscreen design into all of my projects. I expect that the exterior walls will be comprised of the following materials (as viewed from the inside of the building to the exterior cladding):

- (2) - coats of latex topcoat paint
- (1) - coat of latex drywall primer
- 1/2" or 5/8" gypsum drywall (sealed using the Airtight Drywall Approach)
- 2" x 6" hemlock-fir studs (milled from spruce, pine, and fir trees)
- 5 1/2" R-21 fiberglass insulation, kraft paper faced
- 1/2" plywood sheathing
- 1 layer of asphalt-impregnated building paper; taped and sealed; functions as both a moisture & air barrier
- 3/4" pressure treated furring at 16" o.c.; creates 16"-wide rainscreen cavities
- fiber-cement shingles; primed and painted on all 6 sides
- Concrete masonry or stone veneer wainscoting; 42" tall - installed below the Hardishingles

The only question I now have to answer is: "Do I ventilate the top of the wall? (I will be ventilating 100% of the bottom portion of the wall)"
Ventilating the top of the wall is a bit of a trim nightmare in terms of making all of the various material dimensions work together properly. Not ventilating the top of the wall eliminates these headaches. However, I'm most interested in the performance of the wall, not the inconvenience that venting the top of the wall may cause me and my subcontractors. I don't want my customers to call me and say, "I've found mold."

One additional point - I'll be using a Heat Recovery Ventilator (HRV) to manage the ventilation and humidity of my projects. Dr. Joe Lstiburek recommends NOT using a polyethylene vapor barrier UNLESS you are building in a climate with 8,000 or more heating degree days AND where you can control interior vapor to 50% or less Relative Humidity. It is his opinion that the building industry's indiscriminate use of polyethylene vapor barriers has largely been responsible for the majority of building failures in the last 15 - 20 years. He recommends NOT using an interior wall vapor barrier in order to allow interior walls to dry to the interior. Of course my local building codes require an interior vapor barrier (although I have requested an alternate to use the rainscreen cavity in lieu of an interior vapor barrier). I've decided that if I have to use an interior vapor barrier I'll use kraft paper since its permeability increases as it becomes wet. However, I think I'd prefer not to use any type of interior vapor barrier. What is your opinion on the use of interior vapor barriers?

Answer

In the wall system you describe, I would ABSOLUTELY ventilate for better performance. I am sure that once you have worked out your details for the first time, you will no longer find ventilating to be a trim problem. The risk of water entry at the top of the wall can be completely negated by proper use of overhangs (i.e., mpre than 6"!!!) or parapet design to shelter the top vent opening. The CMHC opinion is based on a few peoples ideas who have NO scientific evidence to support them.

Pressure moderation (not equalization) is not a big deal either way with your wall system, as it drains so freely there is no real concern, Painting all six sides and ventilating is important to reduce rainwater absorption and to dry out the shingles respectively.

Low permeance vapor barriers like poly are definitely not needed in your climate, and in fact may, as Lstiburek says and I have seen, cause moisture problems. The approach you take is generally very good.

Be sure to develop details for windows, low roof to wall intersections, doorways and decks. This is where the water gets in.

I am not keen on sealing asphalt paper as an airbarrier -- stick with sealing the drywall and just lap the building paper well. If you want an exterior air barrier or wind washing control layer, use a housewrap and tape it properly.

The only improvement I would make, and this might be food for thought on your next project, is to apply a thin layer of extruded or expanded foam over the framing. This warms up the stud space in cool weather and thereby keeps the studs warmer and drier. It also essentially stops inward vapor diffusion from wet cladding.