The Ten Golden Rules for Foam-Free Flat Roofs

The Ten Golden Rules for Foam-Free Flat Roofs

When insulating a wood structure flat roof, 475's goal as always, is to build this in the most healthy, structurally sound and ecologically friendly way, achieving truly high performance.  This means we try to generally avoid using foam above or below the roof deck (see our 475 series Foam Fails).

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 3% (3/8":12) and 2% when considering deflection (1/4":12)

Because shedding water is 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.  Dark membrane will help inward drying.

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 a 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

This means no pavers, no terraces, no gravel, no PV, no greenroofs and no surrounding structures that might shade the roof.  Any shading will prevent the sun from driving the humidity out of the assembly.

Wood moisture meter (credit: thehumansolution)

4. Check wood moisture content before insulation and installation of vapor control layer/airsealing


Verify and document the moisture content before installation of interior air barrier and insulation. Solid wood should be 12-18 percent moisture content (M%) 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

Fixed vapor retarders (class I and II) will prevent winter humidity ingress, but do also reduce the inward drying capabilities in summer to close to zero. Such assemblies will lack sufficient drying capacity and this can cause humidity build up and structural damage -  as humidity from vapor ingress through small air/water leaks get stuck between two vapor retarders.

However the INTELLO airtight smart vapor retarder located inboard prevents wetting from the interior in relatively dry wintertime conditions, acting as a vapor barrier at 0.17 perms - while allowing drying to the interior over the relatively more humid summer season, with the ability to open up over 13 perms.  By preventing wetting and promoting drying out, INTELLO provides maximum protection.

6. Do not install humid insulation

Do not install humid insulation or damp-spray cellulose into the cavity, as this will 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. Not doing so would lead to rising moisture levels in the structure as well.

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 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. 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.

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.

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