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Community and Q&A

Insulating a Low-Sloped Roof

Andrew808 | Posted in General Questions on

I have a question about insulating a roof in Hawaii — we don’t get temperatures below 75, humidity is moderate; so, wondering what applies in our environment.


CURRENT ROOF: 1961 single story home, 2000 ft2. — 2:12 pitch + a portion that is nearly flat

50% of home is open beam ceiling

50% has a very shallow attic with R-19 fiberglass insulation laying on top of the ceiling

Roof is ¾” plywood + 2-ply bitumen peel and stick

Our ceiling (over half the home) is leaky, with can lighting

R19 fiberglass insulation on top of the ceiling – nothing against the underside of sheathing

Very shallow attic space (~ 24” at peak – very challenging for humans to access)

Soffit vents (plan to increase these) and gable vents

We do have a powered attic fan installed on the leeward side of the home in a gable vent — it seems to reduce humidity when on (I have bluetooth device in the attic)


All canec was removed in 2016 with this roof – Canec was made from sugarcane and probably had an R-value of ~ 10


PROBLEM: Our home is terribly hot — 97+ degrees in the open beam portion, ~ 89 in the drop ceiling portion. The open beam area heats and cools fast, but the bedrooms with the drop ceiling actually retain heat longer and can be ~ 3 degrees warmer than the open beam area at night (I suspect that better attic ventilation may help — more soffit vents?)



We have two proposals from our roofer:

Replace the open beam area plywood with 2×6” Cedar T&G (aesthetics and natural termite resistant)

Between 1 and 3” of Polyiso Foam on top of current plywood / cedar planks


Option 1 — vented roof:

Furring strips on top of foam with cuts laterally to allow heat to flow between bays

Openings under flashing to let air in

Roof vents every other furring strip bay near the ridge to allow air to escape

Roof vent would only penetrate to the foam layer, not through it

Idea is to allow heat absorbed by the foam to escape?

Radiant barrier – TECHSHIELD on top of furring strips

Polyglass Polyfresca reflective capsheet on top


Option 2 – non vented:

Perhaps increase the foam to 3”

Some form of ¼” decking

Polyglass Polyfresca reflective capsheet




We are very concerned about anything that could lead to water intrusion, damage, and mold. What is the risk of water intrusion with a vented system using roof vents?


I noticed in comments that roofs don’t vent in less than 3:12 pitch (stack effect?)  If so… sounds like we will not achieve the venting desired?


Benefit vs Risk of a vented system like this


Can we visually inspect the vents to ensure they were installed correctly?


       We hear of skylights leaking a lot — how are these different?


Do we get enough heat in Hawaii that the foam insulation would need to be vented?

Do we get a significant gain in cooling from the air gap and radiant barrier?

We had solar attic fans that were installed incorrectly, leaked, and caused water damage to our home. I understand from multiple roofers that when properly installed with a curb on low-slope roofs this should not be a problem.


Should we consider a solar attic fan for our drop ceiling section (attic section)?


I note from reading that in the non-vented assembly we should air seal our attic to prevent moisture on the underside of the sheathing. Is this true in our climate too?


I like the doghouse idea — is that just to vent the attic?

Our proposed “vented roof” is to vent the insulation (air wash?)

Can you achieve similar effects with properly installed solar attic fans?


Thank you in advance for all your help and inputs!


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  1. user-2310254 | | #1


    I'll give your post a bump and ask a question. Do you have (or plan to add) air conditioning?

    1. Andrew808 | | #2

      Hello Steve,
      Thank you for the question. The home used to have split A/C, and we may add it down the road. Our primary intention is to achieve maximum passive cooling and rely primarily on trade winds and fans.

      Thanks for the bump -- I have been anxiously awaiting some expertise.


  2. user-2310254 | | #3


    No worries. Just didn't want to see your Q&A split into two threads.

    Martin has a great article on low-slope roofs here:

    Based on the article, I would avoid trying to vent the roof if it were my home. It would be much better to install all the insulation on the outside of the sheathing or, alternatively, install a minimum of R-5 on the exterior with the balance of the air permeable insulation in direct contact with the sheathing.

    Either way, you want to minimize air leakage from the living space into the "attic" space. That's also why I asked about AC. If you plan on having a system, you want to be extra careful about installing drywall using airtight best practices. (See here for more info:

    If you have an recessed lights, those ideally should be removed. Low-profile surface-mounted LEDs are a good alternative.

    You can have skylights with exterior rigid insulation. In some discussions, the experts advise building a "buck" (a wooden box) to raise the skylight so it is level with the top of the rigid insulation. Of course, there may be other options for handling this situation. Let's see what some of the other members think.

    FYI. It is important to find a contractor who understands and supports your strategy. The last thing you want is to become the designated "expert" on your own project. Trust me on that one. :-)

    1. Andrew808 | | #4

      Thank you. Unfortunately we just completed an expensive rebuild -- nothings impossible, but it would be difficult to to anything in the attic since it is so small.
      The home was build in 1961 with a vented attic. While many homes here are modified, that part remains the same. We do have the fiberglass insulation in the attic, but it is not against the sheathing, so I assume that is better for not trapping moisture under the sheathing.

      Is moisture under the sheathing a concern in this environment?

      I have read the articles, but so much discussion is 4-season based. We really only have 1 season. We do have relatively high humidity, but kept at bay by trade winds usually. Would our roof ever get "too cool" to allow condensation under the roof deck? Wouldn't higher temperatures in the attic and airflow remove this concern?

      So, for roof options -- if we go with exterior insulation (Polyiso) + capsheet, we are essentially getting back to the way the roof was in 1961 with Canec (sugarcane based 2-3" thick mats) + torch down roofing. However, we would get a higher R-value with 2-3" of Polyiso, and a reflective cap sheet would have a high SRI and help prevent a lot of heat from getting to the insulation.

      The idea of venting the Polyiso layer using Vents near the ridge sounds smart.

      But, then I read that if the roof is < 3:12 pitch, there is not enough height difference to force the drive from cold/dense air to hot/less dense (is this stack effect?). The idea that the arrows don't always go the direction we want?

      If this is the case... we would just spend more money, have a more complicated system, with more leak potential (every penetration), and a bunch of vents on our roof...

      Diagram attached -- hopefully clear
      Happy Halloween,

  3. user-2310254 | | #5

    Hi Andrew,

    I am way out of my depth here, but I will post a reply to give your question a bump.

    On the drawing, it looks to me like you have detailed a vented roof assembly and an unvented roof assembly. On your attic, how tall is the ceiling at the short end? If you have 24 inches on the tall side, you probably don't have a lot of head space on the opposite side, which means you probably cannot install a consistent depth (say 12 inches of blown cellulose @ R-3.2 per inch) of air permeable insulation.

    If you went with the exterior rigid insulation, I don't think you need to an air gap above. I'm in Zone 3, but it is my understanding that installers sometimes ventilate above the foam to avoid ice dams. Perhaps you are worried about night sky radiation and your metal roof. But I think you can address this issue by taping the foam.

    My biggest concern with your project is air leakage into the attic space. Let's see if one of the GBA experts will chime in with he/her thoughts.

  4. Andrew808 | | #6

    I look forward to more inputs and thank you for your questions.
    There is not enough room for 12" of blown cellulose -- It pitches down to almost no clearance at all.
    I am not familiar with night sky radiation effects. The roof will not be metal -- it will be a torch-down capsheet that is white reflective with an ~ SRI of 80.

    Thinking back to the home design in 1961 - the ceilings were likely very "leaky" to air -- correct? The ventilation in the attic has not changed.

    I am hoping to better understand if air leaking into the attic is really a concern in zone 1 with warm temperatures year round.

    Thank you,

  5. Andrew808 | | #7

    CONCERN -- Leak potential for Roof Vents

    When deciding to have a vented air gap between the foam rigid insulation (above sheathing) and the capsheet vs no air gap, a concern is the risk of any roof penetrations.

    How much concern is there with placing a bunch of roof vents in the roof(only to vent the air gap between insulation and cap sheet)? If done "properly," should the risk of water intrusion be low enough to warrant installing this type of system?

    Also -- in reading the article about "Night Sky Radiation" it discusses a solution of "continuous layer of exterior insulation" to keep the sheathing warm.
    Would this then favor the unvented roof assembly?

    Thank you,

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