Indoor Humidity is a Fact of Life, But Rot is NOT
Photo Courtesy of Alex France
In super tight homes, indoor humidity, left unchecked, can build up to ultra high levels. Particularly during the heating season, when indoor humidity is higher than outdoor humidity, a house develops a positive vapor pressure. The pressure seeks the path of least resistance in order to equalize. Vapor finds its way out through panel joints or minor spaces around door and window openings or other penetrations. In doing so, it will condense into water upon reaching the dew point; either near the cold side of the wall, but still within the wall, or just behind your building paper. For obvious reasons, water in either of those places is not a good thing. Wind driven rain and capillary action can cause water to run uphill, easily penetrating your siding or roofing. Furthermore, when exterior finishes are installed directly to the wall, with no air space, a difference in air pressure on either side of the siding will cause water to be drawn to the backside.
Moisture Migration is referred to often throughout our building science section. There are four very important things that you can do to help prevent this issue from ever becoming a problem in your home.
1) Any envelope penetrations (windows, doors, and vents) should be sealed with an appropriate sealant or gasket to prevent Moisture Migration at those locations.
3) Keep indoor humidity to less than 40% during the heating season.
4) Incorporate an appropriate air exchange system into your design.
5) Vent your roof, and vent walls when appropriate.
Build it Right
Our SIPs use OSB as exterior sheathing; this is ideal for attaching almost any kind of siding. Siding is intended to be the primary defense against moisture.
We recommend using felt paper rather than a more impermeable building paper or house wrap like Tyvek or Grace Ice and Water Shield. The reason for this is that no weather barrier is perfect, there will always be nail and staple holes at the very least, therefore, small amounts occasionally leak through. This very minor condensation and leakage is not a problem, unless it is prevented from drying out. Felt paper doesn’t impede the drying process, it actually speeds it by absorbing water from the OSB and bringing it to the surface to dry.
In order to deal with the inevitable small amounts of leakage a vented rain screen is essential for the long term success of your building project. A vented rain screen allows air flow to dry out any moisture and equalizes the pressure on both sides of the siding discouraging capillary action.
The rain screen is often simply vertical strapping over a water proof building paper. This works well for siding like clapboards but requires an additional layer of horizontal strapping for nailing shingles. We recommend a simpler solution of Home Slicker or Cedar Breather two products that wrap the building on top of the building paper. Home Slicker
and Cedar Breather
are created by Benjamin Obdyke.
Panel with Building Paper, Rainscreen, and Taped Joints
Both products are inexpensive and are easy to install. We recommend Cedar Breather for Shingles and Shakes, and Home Slicker for clapboard sidings. A vent at the top and bottom of the wall allows air to flow behind the siding keeping things dry. Of course, there are many details that are not fully described here; padding out window and door openings, corner trim, and overhangs, as well as extending flashing back to the sheathing and behind the felt paper. Building a vented rain screen is clearly the ultimate in performance when it comes to protecting your home from moisture damage.
Any roofing material can be used over panels. Some of the more popular materials include asphalt shingles, standing seam metal, corrugated metal, slate and even cedar shakes or shingles. Design, climate, aesthetics, and budget are the primary factors in determining which material is ultimately selected. Once the material is chosen, proper installation details must be researched and executed.
Steel Frame with Panels and Strapping
Venting a roof means that a space is created between the top of the panel and the roofing material. This space is for air to flow through. Convection pulls air from vents at the eaves to a vent at the ridge. A vented roof is referred to as a “cold roof”; a non–vented roof is referred to as a “hot roof”. A “cold roof” helps prevent ice dams by minimizing the amount of snow melt and in the summer it prolongs the life span of your roof by keeping the temperature of the roofing material lower.
However, the single greatest reason to want airflow between the panels and roofing is to help keep that area dry. Our well–sealed panel installation coupled with a properly designed mechanical air exchange system will minimize the amount moisture migration occurring, but cannot eliminate the risk entirely. No matter how water gains access, the best way to get it back out is to allow airflow into this space. Otherwise, water trapped directly between the top of the panel and back of the roofing may promote molding or even rot, not to mention limit the lifespan of the roofing material.
Venting a roof can be accomplished in many ways. Some methods work better for specific roofing materials. Below is a list of roofing materials and suggested venting methods.
Built–up roof with vertical strapping followed by an additional layer of sheathing to attach shingles to.
Use proper bottom edge and ridge venting, and the roofing will usually provide enough venting, though Cedar Breather or Home Slicker can also be used.
Also creates its own venting, if provided proper edging details.
Built up roof same as asphalt shingles or vertical and horizontal strapping.
May be done with strapping or with Cedar Breather applied.
strap and re-sheath with vented edges to allow air flow.
Proper use and installation of roofing felt, strategic – and limited – placement of an impermeable membrane such as Ice and Water Shield or Bituthene, and proper flashing are also very important details.
Good builders recognize that proper flashing details are critical to keeping water from leaking in around window and door openings and potentially causing expensive and frustrating damage. At Foard Panel, we are concerned with the quality of your home even after your panels are purchased and installed. We understand that all siding, no matter how well installed, will allow some water to get behind it. Therefore, we offer flashing instructions to stop water at that point. When applying felt paper and flashing, try to think like water; overlap paper and flashing so that water will always have a way to run down without getting behind any of your materials. See our notes in “Building Science Information”, in our guidebook.