May 20, 2013

Unvented Flat Roofs: A Technical Discussion

Flat roofs have often been built with the insulation all on top of the vapor closed drainage plane – as this avoids dew point condensation issues inboard of the roofing.  However in many situations the insulation may need to be primarily or entirely below the flat roof membrane and between the roof joists – if so, what are the best options?

At pitched roofs with the insulation between joists we know a back vented roof is a great way to keep the roof functioning well, as stated in a prior 475 blogpost.  But what’s true for a pitched roof is not necessarily true for a flat roof.  And while we can back vent a flat roof, is it the most effective and safest approach?
The debate has been going on for at least 30 years. Do you need a ventilated cavity? How big should it be?  How much length can you vent under a flat roof? Does it actually have sufficient airflow to vent out the humidity? What drives this ventilation?  …etc etc…

In Germany, a leader in the research of wood framed building science, the thinking shifted around the year 2000 – and consequently their building code/construction guidelines were updated.  The current best practice is to fully insulate the rafter space and NOT vent the flat roof.

To document current best practices in this regard we have three reference points, with explanations and links below:

  1. A German building science trade article:  Ventilated or not? Protection against condensation in wooden flat roofs, May 2000, by Robert Borsch-Laaks, in the magazine The New Quadriga.   German original here.  English translation here.
  2. A power point presentation: Unvented Flat Roof in Wood Construction: Structure, Materials, Risks, by Martin Mohrmann MSc.  In German with English translation here.
  3. A new 475 blog post: The Ten Golden Rules for Foam-Free Flat Roofs, here.

1. Building Science Article: Ventilated or not? Protection against condensation in wooden flat roofs

The New Quadriga is a German wood-frame construction magazine.  The article gives an in-depth description of the development of the building science for wood flat roof construction.  It notes that the ventilation requirement was based on the research of Prof. Liersch (TU Berlin) at the end of the 70s.  His research showed that the ventilation could get rid of the humidity that was the result of diffusion into the insulation (when using a vapor barrier on the interior).    However Prof Liersch’s study did not include humidity brought into the insulated flat roof cavity by convection (air leaks).  The article notes that the ventilation cannot overcome the significant humidity drive into the flat roof structure caused by small air leaks.  Consequently the article concludes that the concept of venting a flat roof is no longer safe, and instead don’t vent the flat roof and install a smart vapor retarder inboard.

Read the translated article here.

2.  Power Point Presentation: Unvented Flat Roof in Wood Construction: Structure, Materials, Risks

The presentation by Martin Mohrmann MSc describes through extensive details and diagrams the basic points illustrating that vented flat roofs can be problematic and how to safely achieve an unvented flat roof.  Translation superimposed on the presentation by 475 can be found here.

3.  475 Blog Post: The Ten Golden Rules for Foam-Free Flat Roofs

In this blog post we summarize the knowledge gained from the above two documents – and other sources – while making it relevant to US cold climate construction.  The Golden Rules include:  3% minimum pitch, dark membrane, no shading, check wood moisture content, use intelligent vapor retarder, do not install humid insulation, no cavities in insulation, verified air tightness, do not vent, breaking rules is possible but must WUFI.  Read the blog post here.

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30 Responses to Unvented Flat Roofs: A Technical Discussion

  1. Rudy November 11, 2015 at 11:15 pm #

    I have a room addition that is a flat roof in Southern California. It was added on in the late 70’s. I bought the house in the mid 80’s. I noticed wet insulation when I did dry wall work on the ceiling and this must have happened when I had a roof leak in the mid 90’s. Now in 2015 I have been smelling mildew for years now and can’t seam to find it so I have decided to completely remove the dry wall and insulation from my kitchen flat roof and go with an exposed ceiling. What are the draw backs of going with an exposed ceiling with a flat roof room?

    • S. O'Farrell April 2, 2016 at 6:28 pm #

      there is no draw back to exposed rafter spaces, but if you want insulation it will have to be put on top of the roofdeck and a new built up roof installed. Insulation will need to be a rigid foam type.

    • John April 4, 2016 at 10:57 am #

      The retrofit can be foam-free provided you are able to follow our 10 Golden Rules for Foam Free Flat Roofs. As always, we are happy to take a look at details and give recommendations. It sounds like the situation would be improved by allowing the roof assembly to dry out, something that can be done with Intello covering the interior side of the flat roof assembly. For more on fiberglass in this assembly, see Yes, Unvented Roof Assemblies Can Be Insulated With Fiberglass, and drop us a line with additional questions.

  2. Daniel Lunstrum September 14, 2016 at 10:12 am #

    I need to re roof a 19,000 sf. flat roof building, that has no ventilation, no insulation,( in the truss system ,16″ on 2″ centers). It currently has 4 layers, including rock, buffalo board,a couple rubber membranes, about 5-6 inches of roofing to tear off to the deck. The roofers bid is to install a 2″ ISO board ( R-12 ), and also a layer of Dens Deck for a class 4 rating. I have found a closed cell bubble, foil faced on both sides, 3/16 thick, 4′ or 6′ widths and claims a R-16 value . My thoughts is to apply the bubble insulation directly on the deck, the ISO board, another layer of bubble, ( both layers will be taped to stop leakage or heat transfer/loss), the Dens deck, the roofing material, which is IB PVC membrane system. What would your thoughts on the insulation be ? Could I be making any future problems for this building ? The bubble insulation is a product from, Insulation4Less, called Prodex Total 72 or 48. Could you please advise, as for I have 4 other building projects with the same issue. The buildings are all in Wyoming. Thank you,

    • John September 14, 2016 at 10:31 am #

      Hi Daniel,

      Thank you for taking the time to explain the situation. As a rule we advise again spray foam and building assemblies that are vapor-closed. Our focus is on encouraging insulation options that are not foam-based, and that allow condensation to dry out through vapor diffusion. We lay out our rules for flat roof assemblies in this post:
      Our thoughts, though it might not be what you were looking for, are to scrap most of the elements you mention, and go for a foam-free, vapor-variable approach. Let us know if you would like to take that leap – if so, we’re here to help.

  3. Jeff Williams November 29, 2016 at 1:46 pm #

    I have garage with a flat roof. Ceiling joists are 2 x 8 at 16″ oc. There is little or no insulation in rolled asphalt roofing system and a plywood deck.

    I’d like to insulate either between the joists or across them, prefer between as then I do not need to deal with the lighting and conduit.

    Home is located in Orange County California.

    What is the best method?

    • Ken November 29, 2016 at 4:21 pm #

      Hi Jeff,
      If you follow our 10 golden rules: ….then using fiberglass, mineral wool or cellulose between the joists is fine, with INTELLO Plus membrane stapled and taped airtight below enclosing the insulation. We then recommend using furring strips to form a service cavity for wiring, then apply the finish.
      Let us know if you need any further assistance. We’re happy to help.
      – Ken

  4. Jessica December 21, 2016 at 10:52 pm #

    I have a flat roof on the back of my house over my kitchen. There is about 1 ft of space max from the drop ceiling to the roof plywood. Every winter we have leakage all over which we believe is caused by lack of ventilation. There is no venting and this is a new modified bitumen roofing put on a year ago. We think it’s frost built up on the plywood and then when the weather gets warmer it melts and goes through the insulation to the ceiling tiles. Is there anything we can do so this doesn’t happen every winter? We thought a new roof was the cure but it actually made it worse because it’s better insulated. Thanks.

    • Ken December 22, 2016 at 10:12 am #

      Hi Jessica,
      You can tackle the ventilation problem with Lunos through-wall system.

      We would also recommend keeping the humid conditioned interior air from getting to the cold condensing roof structure surfaces by installing an airtight smart vapor retarder INTELLO Plus inboard of the insulation.

      Where are you located? Happy to provide more assistance as needed.

      – Ken

      • Jessica December 22, 2016 at 9:39 pm #

        I live in Minnesota. Thank you for your quick response. Can you tell me exactly where the vapor retarder would go? Would it go above the insulation next to the plywood or under the insulation next to the drop ceiling?

        • Ken December 23, 2016 at 6:09 am #

          Yes, It would go below the insulation – typically stapled to the underside of the roof joists. THen you tape seams and any penetrations to ensure good airtightness.

  5. Nick December 27, 2016 at 10:24 pm #

    I live in Pittsburgh in a house built in 1912. Previous owners constructed an addition and utilized a flat roof. This summer I replaced the entire roof and instead replacing the flat roof it was pitched and shingled. This flat section was vented, and uninsulated. The old rubber roof was left in place when it was framed out. In order to properly insulate this section of the house will the old roofing material need to be removed in order to insulate between the joists or can insulating merely be laid on top of the old roofing. My concern would be laying down insulating on the old flat roof and causing moisture buildup in the ceiling.

    • John January 4, 2017 at 11:18 am #

      Hi Nick,

      Good questions – and ones that probably warrant a greater discussion. We have another blog post that delves into the complexities of our best-case-scenario for this:

      If the current roofing membrane is good and watertight – you can probably leave it. You could insulate inboard, between the studs, and airseal inboard of the insulation with Intello Plus. As long as you’re following the other rules in our post, you should be ok. But please do contact us if you have photos or additional details / considerations that can inform your decision. We’re here to help.

  6. BOB Haynes February 12, 2017 at 4:27 pm #

    BOB, Hamilton ontario Canada. I am about to construct a flat roof 25ft by 40ft, 2″ by 10″, filled with fiberglass batts ,r 20 + r 12 total r 32, non ventilated ,would i be wise to try to squeeze two 5 1/2′ r 20 batts in the 9 1/2′ space to get two the r 40 that i really want ?

    • Floris February 13, 2017 at 9:34 am #

      Bob, IN a 2×10 – which is actually 9.25″ high – you can theoretically get to an R-value of max 4/in *9.25 = R-37 with fibrous insulation. You might get slightly higher R per inch, if you pack more of such insulation in, including fiberglass – however if you go over 2.5lbs/cf you will start to lose a little bit of r-value (it is the still air that insulates, the insulation is there to help keep the air still/stop it from convectively looping).

      So forcing 2x 5.5″ batts in 9.25″ might “overfill” it, and not get you R20+R20=R40. But you might get slightly over 2 lbs/cf, which would be great. Make sure the batts completely fill the cavity and with INTELLO Plus on warm side – So your assembly is protected by airtightness and with smart vapor control.
      Being slightly denser than ideal is preferred than a not completely fill an unvented cavity, as that results in lower R-value – also if density is low (like with some USA batts is the case – batts in Canada are denser and have in general a slightly higher R/in).

      To calculate if over/under filling would be an issue (not getting around 2-2.5lbs/cf and you get the best r-value. You can check what the weight of the fiberglass batts and see what the resulting density is if you double of the 5.5′ batts in 9.25′ space.

  7. davelukasek February 25, 2017 at 9:32 am #

    I have a 40′ x 14′ flat porch roof with a pitch of <2 degrees. Construction is 1 x 6 tongue and groove cypress over 3 x 8 cypress beams. Felt was nailed directly to the t&g, followed by modified bitumen. The roof is seven years old and, while it still has life in it, the heat which radiates down into the porch is intense, especially in summer here in Florida. I was planning to apply a 100% liquid silicone product over the existing roof in order to A. not have to redo the roof in a couple of years and, more importantly, B. reflect the sunlight and reduce the heat radiating into the porch area. I happened to have the roofer who installed that roof here recently to install seamless gutters for me and mentioned to him what I planned to do. He told me that the porch roof "needs to breathe" and that putting a non-breathable coating over it like the silicone product would cause the wood to rot. A general contractor friend of mine agreed with him, but an architect friend of mine said it shouldn't be a problem since the t&g decking is exposed on the bottom side and thus can "breathe" from that side. I'm hoping to get an answer from a disinterested party as to whether applying the silicone coating is a bad idea or not.

  8. Mark March 23, 2017 at 1:42 pm #

    I have a stone house built in 1958. It currently has a flat room with a newer EPDM roofing system installed within the last 3-4 years. The interior under side of the rafters is Western Red Cedar ceilings and the exterior soffit has the same with approx. a 24″ over hang. Unfortunately the installation of the EPDM looks rather unsightly as they continued the termination strip over the fascia as there is no parapet. In order to clean this up I would like to reinstall the soffit and reface the fascia in some manner. There currently is a separation within the cedar soffit and screen for venting. When reinstalling is it important to keep that venting in place? I was considering screening the open rafter spaces or installing blocking, however did not want to further promote an environment for moisture. I do not believe there is insulation between the rafters and I would imagine that the cedar ceilings in the interior allow some air flow. I have not noticed any significant moisture issues nor mildew smell. Any recommendations for this application?

    • John March 24, 2017 at 11:19 am #

      Hi Mark,
      Thanks for taking the time to explain – there’s a lot going on here. I’m afraid more than we can diagnose properly in the comments section. We’d recommend you have a professional take a look in-person. We’re happy to try our hand at taking a stab if you can send us some photos and a quick sketch of your wall / ceiling assembly. We would need to take a deeper dive to understand it fully. I wish we could give you more now, but it’s not a completely straight forward situation. Best of luck.

  9. Geoffrey April 28, 2017 at 1:14 pm #


    I have a 1924 wood-frame home in Memphis, TN. The roof is approx. 12/12 pitch on either side with a 14′ flat span in between. Where the rafters meet the ceiling joists is a rim joist.

    In the late 50’s, the attic was converted into an apartment. In doing so, they blocked the gable vent with a crawl space. Furthermore, they packed insulation in the rafters and ceiling joists covered with drywall. From then on, there was NO venting whatsoever in the roof.

    I had to remove a large portion of the ceiling for repairs when I discovered this issue. I’ve since removed the entire ceiling and set off to fix this the best I could.

    Keep in mind: this is a retrofit I am hoping to improve the best I can.

    For all intents and purposes, we can ignore the knee walls for simplicity, and treat this question/issue like a 12/12 cathedral ceiling that meets a flat for 14′ span, then 12/12 down the other side. (there are crawl spaces behind the knee walls). I have also furred down all the rafters and joists to allow for r21 and r30c, in addition to a 1.5″ air channel above the insulation and below the decking. In the future a re-roof of the flat section may be an option, or adding a crawl space for venting, but right now it is not. The flat section is asphalt roll.

    I installed soffit vents around the house, then ran rafter vents up to the flat section. With advice from an engineer, I drilled 6x 1″ holes through the ceiling rim joist, allowing air to escape from the flat section into the pitch rafter channels.

    My intent at this point is to install off-ridge vents as close to the top of the pitch as possible on either side of the flat section. The vent hole int he roof will likely be 12-20″ down from the “ridge” depending on the product. This is less than ideal, since the holes at the end of each joist channel won’t be able to “see” each other, but it’s the best I can do.

    The chimney effect *should* bring air up through the soffit vents and out the off-ridge vents. However, the off-ridge vents will function as intake vents when wind blows against them, and could ultimately force air down the rafters. I realize this is a complicated problem, and difficult to determine a guaranteed solution.

    All that being said, is there any reason I shouldn’t proceed with what I have to work with at this time?

    What are the dangers of the venting process I have described. Keep in mind, as a retrofit for a 1924 home with a 1950s attic conversion, air-sealing the ceiling and roof as-is would be nearly impossible given my resources.

    Thank you!

  10. Ken Stradling June 5, 2017 at 8:42 am #

    Three years ago, I had my large leaky flat roof repaired with Dura-Last Membrane. In the process, the vents incorporated in the scuppers were sealed by the new membrane roof, thus no more vents except internally through my ceiling pot lighting. Since then we have developed a mildew smell. Any suggestions?

  11. Philip November 17, 2017 at 12:23 pm #

    This is a very interesting article but I wonder if the concept is accepted or understood by most roofers. I live in Montreal in a small flat-roofed detached house and my current 30+-yr roof is built-up (asphalt and gravel) — time to replace it! Currently the roof is vented (as was the practice when my house was built in the 50s) and I am wondering if in my situation, I should continue with a vented roof or fill in the airspaces and remove the venting?

    I am leaning toward covering the roof with modified bitumen membrane (worried about EPDM punctures from squirrels). Another issue is, you recommend dark (vs white reflective) membranes, but our municipal bylaws forbid that. The options are green roof or a reflective roof with minimum SRI of 56. (I am shying away from green roof due to maintenance and local animals!).

    Do you have recommendations in terms of venting or not in my situation? If I go non-venting, will that imply a major, expensive retrofit? Also, do you have experts in the Montreal area that I might consult?


    • John November 17, 2017 at 5:39 pm #

      Great questions — the possible solutions get a bit bigger than the comments section can support. We do, in fact, have a 475 representative based in Montreal. His name is Huston Eubank. We’ll have him get in touch with you to talk details. Thanks for getting in touch.

  12. Jim Sytnick November 21, 2017 at 5:54 pm #

    I have a roof with a low slope. The roofer put 2 vents on it, in Winnipeg Manitoba we have snow that stays for 5 months. The vents are covered in snow. Should the roof have the vents? The roof was redone this year & there was water under parts of it. Will the snow melt by the roof vents & will water get in the roof? It is a newer roof on top of an older kitchen (addition) roof.

    • John November 22, 2017 at 2:39 pm #

      Hi Jim,
      Firstly, just wanted to make you aware: we have a whole Canadian site over at, where you’ll find everything stocked in Canada all listed in Canadian dollars.

      Now to the question at had: that is a tough one. If your vents are properly made and waterproofed, there should be no leaks from melting snow. We can’t tell if there is a concern regarding your roof vapor management. In order to find out, we’d need further analysis, such as the build up of assembly, the airtightness of the assembly.. we’d want some idea of moisture % in the assembly. In other words, we would recommend you hire a professional to look at the property in person to determine the needs. We can certainly provide guidance in the case of a gut retrofit, but precise diagnosis takes more fine detail than we can provide here.

      Best of luck – let us know how we can be of assistance.

  13. Denis January 8, 2018 at 8:40 pm #

    Hi all, I have a 15×15 flat roof in one part of my house with only one maximum vent in the center. Should I add wall vents intake on the wall. Or the maximum will be enough

    Thank you for your response


    • John January 19, 2018 at 12:38 pm #

      Hi Denis,
      I’m afraid we will need to get more information to help with your issue. If you can contact us by phone when you have an opportunity we will need to walk through the assembly to see if we can provide assistance.

  14. Dave February 17, 2018 at 10:45 pm #

    Hi I Have a 1930s House in western NY our weather gets cold in winters and often up and down the rest of the year. I have a add on room in the back of my house that’s 10’x10′ The room is always cold like a fridge in the winter. I Insulated the crawl space with R30 along the cinder block walls, in the between the 2″x10″ studs and laid it on the floor as well. The room was still cold. I ripped the drywall ceiling down and found a plastic vapor barrier over faced R11 and under that was a drop ceiling with more R11 at the bottom of the 2″x8″ rafters with almost a 6″ gap between the plywood and the R11. R11 is also running down the walls. I’m wondering what the best way to reinsulate is. I was going to seal all the air gaps with a can of foam insulation fur out the studs with 2″x2″s and install 9″ R30 with new R13 down the walls. I’ve read other places that if flat roofs are sealed they don’t need ventilation. Is that true? or Do I need baffles and proper ventilation?

    • John February 20, 2018 at 10:48 am #

      Hello Dave,

      Thanks for getting in touch. If you have photos of your process those might be good to inform as well. We have details we can share for woodframe retrofits. We’ll put you in touch with our New York State representative: Nick Shaw. We should be able to find an answer that works for you.

  15. Andrew February 22, 2018 at 3:41 am #

    Hello, I’ve been reading all the questions and answers with great interest as I have a problem with a flat roof that I can’t seem to explain. The house is located in Seattle and was built in 2013. It’s a 3 story, freestanding townhouse with a rooftop deck over a flat roof. The issue is that now for the second time the paint in the room below the rooftop entrance bubbled away from the wall and ceiling and it looked liked water was flowing in between the paint and sheet rock. So I opened up the ceiling and wall but surprise, everything on the inside was dry and no signs of leaks from the roof. Looking closer I found what looks like a support 4×4 right above where the wall damage is that is wrapped vertically and around the bottom by a thick 1” wide metal band. The bottom of the metal band showed slight corrosion but nowhere else. As everything was dry inside the cavity (including the insulation) is it possible that condensation started building up on that metal bracket and then just seeped down through the corner of the wall and ceiling? Additionally on the other side is a small laundry closet with a stacked washer and dryer. I’m at a loss so far (much like the contractors I had on site and the roofer) because there is no sign of any leakage from the roof. Is it possible that this single metal band ( which also nobody could explain its purpose) can have enough condensation buildup to cause the described damage. The metal band is about 1” wide by 1/4” thick, comes down 12” on one side of the 4×4, wraps around the base and then runs another 12” up the other side and is screwed to the 4×4. The 4×4 sits about half on top of the framed wall and extends to below the plywood sheathing of the roof. Sorry about the ramblings, but I’m trying to relay as much info as possible. Thank you in advance for any advice you have. Andrew

    • John February 22, 2018 at 11:37 am #

      Hello Andrew,

      When we hit this level of complexity we like to take it out of the comments bin and address the situation directly. I’m going to connect you with our representative for Washington State, Lucas Johnson. Additionally, any photos you have of your discoveries would be much appreciated. Look out for an email from us shortly and we’ll do what we can to help.

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