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The Ten Golden Rules for Foam-Free Flat Roofs

May 20, 2013

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 not only ensured a robust roof assembly but you can also avoid the need for WUFI calculations.

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 dampspray 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. Breaking one or more rules can be done, but requires thought/WUFI.

If you do break the above rules – you don’t have to resort to foam immediately.  We can WUFI your roof and come up with foam free solutions in most cases.

Contact us at info@foursevenfive.ca and indicate your roof assembly from outside to inside.  Include preferred materials used, color of the roof, include sections in both directions and location at a minimum.

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13 Responses to The Ten Golden Rules for Foam-Free Flat Roofs

  1. peter marciano May 22, 2013 at 2:29 pm #

    Who actually does this in a NYC flat roof? Are you talking about passive house buildings that have flat roofs or all flat roofs in general?

    I understand about having the sun drive the moisture inward but when? winter, summer? or do you need the space below to be cooler and dryer to force inward drying and when will that happen?

    I strongly agree with you that venting roofs is not the way to go, what do you propose for insulating these structures from the outside if not polyisocyanurate or XPS ?

    BUT To simply ignore the massive benefits from foam, when installed properly, even considering its disintegrating R value, is misguided – especially at this point when practically no one in the NYC market even considers topside insulation when undertaking a reno or new building but sabotaging the very recent acceptance of this practice is not good for our building stock or our energy consumption.

    We are not going to get to passive house overnight , getting any amount of ISO on top of a flat roof should not be discouraged, complicating the average owner’s, builder’s or architect’s understanding of roofing systems will only slow down the transition to PH buildings.

    P. Marciano NYC

    • Ken Levenson May 23, 2013 at 12:34 pm #

      Hi Peter,

      We’re talking about flat roofs in general and not just in NYC but across the US (focusing on cold climates).

      The vapor drive is inward in the summer – in combination with sun, any roof moisture can be baked back to interior.

      Replacement for foam at built-up roofs can be Roxul, Foamglas or wood fiberboard.

      We aren’t ignoring the benefits of foam – we’re just trying to show options. It’s quite certain foam will continue to dominate – but for those who are looking for non-foam options we are trying to provide some help.

      It’s really not about Passive House – just having choices in approach for high performance.


  2. TamLy October 30, 2013 at 6:38 pm #

    Hi Ken,

    Great article! I have a question on rule #6: Do not install humid insulation or damspray cellulose. What are some other insulation materials do you recommend?


  3. foursevenfive November 5, 2013 at 12:06 pm #

    Tam, we recommend cellulose, mineral wool, fiberglass, cotton, sheepswool, etc. However, you do not want to install these when they are wet, because this would introduce a lot of moisture in the construction. So nothing wrong with those materials, just keep them dry.

  4. Greg Whitchurch March 5, 2015 at 5:32 pm #

    I don’t disagree w/ Peter, but I think it’s important to note that pushing the envelope, so to speak, in any engineering field requires that the new thinking be energetically advocated in the face of current practice. 475 has been paying careful attention to building science for years. It has cast its lot w/ scientifically-understood, field-verified, innovative (& green where possible) bldg. practices. They’ve also demonstrated through their own design & construction just how the changes they propose work, what they cost, & how to address real world engineering & political constraints.

    I don’t believe that there’s any danger that their advocacy will have the effect of sabotaging recent improvements in roof design & construction – even though one might consider those improvements half-way, environmentally questionable, or even misguided overall. I wonder if the foam-based improvements you note as being recently accepted became so w/o some enthusiastic advocacy on someone’s part.

  5. Jeremy Werlin March 18, 2015 at 11:22 am #

    I’m designing and building a 1200 sq. ft. modern style home in western Colorado. Eves on south side only, 1:12 pitch roof. I am trying to save my client some money by blowing 16″ of cellulose (dry) in my 16″ TGI rafters. The cellulose will settle as much as 20% thus leaving an air gap against the sheathing and a potential condensation point. Do you still suggest an unvented rafter space in this situation? I don’t know how to achieve a warm roof without going back to the more expensive foam.

  6. Floris March 18, 2015 at 3:43 pm #

    Flatroofs, as noted above are not reliably vented by providing space above the insulation.

    It is better to fully pack the roof with cellulose AND use INTELLO Plus on the interior as densepack netting and a vapor variable vapor retarder. This will protect the insulation against condensation in winter, while allowing inwards drying in summer.

    That solution will avoid you from having to install foam.
    Please send us a detail of your roof and we would be happy to help, WUFI the assembly if needed for the inspector, etc.

  7. Bill April 8, 2015 at 6:49 pm #

    So, am I reading this correctly?
    Large apartment building: Largest single contiguous roof plane is 8000 sf. Plywood deck (slope – 2% min.). Open web wood joists, vary in depth from 18″ to 30″. Fiberglas insulation. Best practice?:

    Fill the space between the roof deck and the “intelligent” vapor barrier with the insulation. Hold the vapor barrier in place with netting, or string. No ventilation.

    • Floris April 9, 2015 at 9:59 am #

      Bill, Our recommendations are to follow the ten golden rules and depending on climate do a WUFI Pro analysis to double check if all is ok.
      You can use open web trusses. Blowing in fiberglass (or cellulose, wool etc) would be the preferred installation above INTELLO Plus. With counter battens under the INTELLO Plus, this membrane is also your insulation netting, so no need for netting/string under it. It will fill you 18-30″ space well and will manage the moisture if all other conditions are met in most climates/locations. Please send us drawings so we can help you with this project

  8. tim June 14, 2015 at 1:57 am #

    i’m planning to build a house in Philly suburbs with a flat roof and solar panels on top of the structure. In that regard Rule 3 from the above concerns me. what can you suggest in that case?

    • John June 15, 2015 at 3:40 pm #

      Hello Tim,

      Rule #3 can be addressed by Rule #10: “Breaking one or more rules can be done, but requires thought/WUFI.”
      It can absolutely be done, it just needs further analysis to consider the safety and performance of your particular assembly. Let’s get in touch to discuss further. Kent Lessly is in your area and available to work through details.

      • Michael July 11, 2015 at 4:31 pm #

        Currently building a 2000 square foot commercial buiding, which will include a flat roof (with about 3/8th-12 pitch), constructed of 9.5″ TJI joists.
        Prefer not to use spray foam due to cost, and am wondering what other methods you recommend for an unvented flat roof, also in the Philly suburbs.

        • Floris July 13, 2015 at 9:38 am #

          We recommend dense packing this roof with fibrous insulation as described in this blogpost and this Foam free flatroof in VT case study. Let us know if you have any other questions.

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