When insulating a flat roof wood structure, 475's goal, as always, is to do it in the most healthy, structurally sound, durable, and ecologically friendly way—achieving truly high performance. This means we try to avoid using foam above or below the roof deck whenever possible (for more on that, see our Foam Fails series.)

With foam out of the equation, we examine how to avoid condensation, rot, and mold in our structures. Based on the research noted in our post, Unvented Flat Roofs: A Technical Discussion, we have established The Ten Golden Rules. Follow these rules to ensure a robust roof assembly.

The Ten Golden Rules are:

1. Use a minimum roof pitch of 2% (1/4:12) or 3% (3/8:12)

Shedding water is always the essential first step in preventing leaks and keeping your roof safe. This is why, in reality, flat roofs are not perfectly flat. Roof pitch, also known as the grade or slope, is a measure of vertical rise to horizontal run, measured in inches per 12 inches (or one foot.)

We recommend that the pitch be a minimum of 2-3% to prevent water from ponding. This provides a buffer for potential deflection, or sagging, of the roof under a load over time. New York architect, Chris Benedict, leads by going even further, using a pitch of 1/2:12 for her flat roofs.

2. Use dark roof membranes in colder climates (Climate Zones 5 and higher)

This rule may sound counter-intuitive, as white roofs are often lauded in sustainable construction for their ability to drive down air conditioning consumption. But the colder the climate, the higher the absorption value required (and therefore the darker the color the roof should be.) The additional heat absorbed by darker colors will allow any humidity within the wood structure/insulation to be driven inwards, which is the only way the assembly can dry.

In Germany, >80% absorption is recommended. In the U.S., hotter climates (Zones 1-3), or as you head to southern climates (Zones 2-4 east of the Rockies), high solar irradiance allows for lighter-colored roofs. To be sure, a WUFI analysis can determine how much and which type of fibrous insulation can be safely used.

3. Don’t shade the roof membrane

Any shading will prevent the sun from driving the humidity out of the assembly. Meeting this Golden Rule means NO: pavers, terraces, gravel, green roofs, or solar panels. If you have surrounding structures that might shade the roof or pitched solar photovoltaic panels, again a WUFI analysis can determine what's possible.

4. Check wood moisture content before installing insulation and vapour control layer/air sealing

Verify and document the moisture content before installing the interior air barrier and insulation. Solid wood should have a moisture content percentage (M%) of 12-18M% and OSB/Plywood should be between 9-15M%. In either case, the International Building Code (IBC) and International Residential Code (IRC) require you to install wood with a moisture content below 19%.

5. Use a smart vapour retarder inboard

Class III vapour retarders (e.g. latex paint) are semi-permeable (1-10 perms) and will allow too much humidity into the assembly in winter via diffusion. This can potentially lead to condensation on the sheathing.

While Class I and II fixed vapour retarders will prevent humidity ingress from the interior in winter, they will also reduce the inward drying capacity in summer to close to zero. An assembly with a Class I or Class II vapour retarder will lack sufficient drying capacity, as humidity from vapour ingress through small air and water leaks will get stuck between two vapour retarders, and can cause humidity build-up and structural damage.

By comparison, INTELLO PLUS and INTELLO X airtight smart vapour retarders, located inboard, prevent wetting from the interior in relatively dry wintertime conditions, acting as a vapour barrier at 0.13 perms. Over the more humid summer season, they allow drying to the interior, with the ability to open up over 13 perms. INTELLO is HYDROSAFE, and by preventing wetting and promoting drying out, it provides maximum protection.

6. Never install humid insulation

Humid or wet insulation, including damp-spray cellulose, introduces an overwhelming amount of moisture into the structure. If installing batt insulation in winter, install INTELLO PLUS or INTELLO X immediately after to avoid rising moisture levels in the structure during construction.

7. Fully insulate cavities and other air spaces

Uninsulated cavities are colder than their surroundings, creating convective loops within and causing moisture to gather and condensate in the high points of the cavities. To prevent this, fully insulate cavities and other air spaces.

8. Verify the airtightness

Airtightness is the cornerstone of any high performance assembly. The roof should be airtight, both outboard at the roof sheathing or membrane, and inboard with airtight INTELLO PLUS or INTELLO X membrane. A blower door test is critical for testing the airtightness of a roof assembly (and the building in general.) Pressurize and depressurize the assembly to locate all leaks and seal them. The functional airtightness should be below 0.05 cfm/sq ft at 50 pascals.

If there are electrical connections in the assembly, a dropped ceiling or service cavity is required (see Golden Rule No. 10.)

9. Don't vent the roof

Generally speaking, it doesn’t work. Successfully vented flat roofs require enormous cavities to allow a reliably vented cross-section. Most vented flat roofs don't work properly, and can actually cause problems—a humid, under vented space can accumulate moisture in high spots that further blocks humidity from escaping. This can cause rot in the cold, wood roof deck above.

Successful venting is generally only achievable on higher pitched roofs. Roof pitches above 3:12 can easily be ventilated using timber strapping and SOLITEX MENTO 3000.

10. Utilize service cavities

Use of a service cavity—to route HRV ducts, electrical and plumbing vents through—minimizes penetrations in the smart vapour retarder/interior air barrier. This ensures it functions as intended, keeping humid air out of the insulation and as far away from the condensing surfaces as possible.

BONUS RULE: Want to break a rule? (Or not.) Have a professional WUFI it.

You can follow all the rules above, and even break a few, with a professional WUFI analysis.

Current building code for unvented roofs is written for the mainstream, spray foam industry and therefore specifies air-impermeable insulation, or foam. This is due to a lag in educating code officials on alternative safe and robust systems, as we noted in our blog post, Yes, Unvented Roof Assemblies Can Be Insulated With Fiberglass. However, the intent of the code can be readily met with an assembly that includes fibrous insulation and an inboard air barrier and vapour control layer—and is approved for use as such.

If you are in a cold climate and are considering one of our suggested assemblies, review it with your local building department official. Have a certified professional perform a WUFI analysis of the assembly or alternatively, have an architect or engineer approve the drawings. In most cases, we've found that a foam-free solution is possible.