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 generally avoid using foam above or below the roof deck (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 475 blog post Unvented Flat Roofs: A Technical Discussion, we have established The Ten Golden Rules. By following the Ten Golden Rules you are ensured a robust roof assembly.
The Ten Golden Rules are:
1. Minimum flat roof pitch 2% (1/4":12) and 3% (3/8":12) when considering potential deflection
Shedding water is always the essential first step in preventing leaks and keeping your roof safe. The New York architect Chris Benedict leads in the right direction by going further, using 1/2":12 flat roofs.
Ponding water from inadequate pitch.
2. Roof Membrane Should be Dark (in Climate Zone 5 and Higher)
The colder the climate and the higher the percentage of cloud cover, the higher the absorption value of the roof should be. This sounds counter intuitive, as we've all been directed towards white roofs. However, the additional heat provided by the sun will allow any humidity within the wood structure/insulation to be driven inwards (the only way the assembly can dry). In Germany >80% absorption is recommended. (In the US heating climates (>5), or as you head to southern climates (zone 4,3,2 - east of the Rockies), you could have a lighter roof, because of higher solar radiation levels, but we do recommend a WUFI calculation to determine how light.)
3. No shading of roof membrane
Any shading will prevent the sun from driving the humidity out of the assembly. To meet this golden rule, it means no pavers, no terraces, no gravel, no green roofs. Even if you have surrounding structures that might shade the roof or pitched PV panels, you will need to perform a WUFI analysis to see what's possible (see #10 for more on that).
Wood moisture meter (credit: thehumansolution)
4. Check wood moisture content before insulation and installation of vapor 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 between 9-15 M%.
5. Use an intelligent vapor retarder inboard
Class III vapor retarders are semi permeable (1-10 perms) and will allow too much humidity into the assembly in winter - potentially leading to condensation on the sheathing.
INTELLO Plus smart vapor retarder for flat roof insulation
Class I and Class II fixed vapor retarders will prevent humidity ingress from the interior, in winter, but will also reduce the inward drying capacity in summer to close to zero. An assembly with a Class I or Class II vapor retarder will lack sufficient drying capacity, as humidity from vapor ingress through small air/water leaks will get stuck between two vapor retarders, and can cause humidity build-up and structural damage.
However, INTELLO Plus and INTELLO X airtight smart vapor retarders located inboard prevent wetting from the interior in relatively dry wintertime conditions, acting as a vapor barrier at 0.13 perms - while allowing drying to the interior over the relatively more humid summer season, 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. Do not install humid insulation
Do not install humid insulation or damp-spray cellulose into the cavity, as they introduce an overwhelming amount of moisture into the structure. Additionally, in case you are using batt insulation, you should install the smart vapor retarder immediately after the insulation is installed in winter to avoid rising moisture levels in the structure during construction.
7. No cavities/air spaces in the insulation.
Damage at uncontrolled air cavities under flat roof (credit: Mohrmann)
Uninsulated cavities are colder than their surroundings. These empty cavities will create convective loops, be colder and consequently moisture will gather and condensate in the high points in the cavities.
8. Verified airtight
It is most critical to blower door test the roof assembly (and the building in general). 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. Pressurize and depressurize and locate all leaks and seal them. The functional airtightness should be below 0.05cfm/sq.ft. at 50 pascals.
9. Don't vent the roof (generally speaking, it doesn't work)
Successfully vented flat roofs require an enormous free venting cross-section. Most vented flat roofs don't work properly, and can actually cause problems, as a humid vent space can cause condensation and rot the cold wood roof deck above.
10. Want to break a rule? (Or not.) Have a professional WUFI it
You can follow all the rules above and even break a few - but you must do a WUFI of your roof, to confirm the risk profile. In most cases we've found - a foam free solution is possible.
At unvented roofs, the building code, as we’ve noted before, is written for the foam industry, and does ask for air-impermeable insulation, or foam. However, the intent of the code can be readily met with an assembly that includes fibrous insulation and an inboard air barrier and vapor control layer - and is regularly 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 WUFI the assembly or get an architect/engineer stamp of approval on the drawings. We are happy to help you.
Again, have a certified professional WUFI the assembly or get an architect/engineer stamp of approval on the drawings. Building code typically dictates how such assemblies are allowed legally, and design professionals may be required to take responsibility.