GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Vapour retarder – main floor ceiling behind the kneewall

Mill_house | Posted in General Questions on

Climate zone 7 Canadian prairies, unvented unconditioned attic, cape cod
I’m gathering that any place I would think of adding 6 mil poly, I can add membrain smart vapour instead.  How would you assemble the main floor ceiling behind the kneewall?  (Here’s what I’m thinking of doing.  From the main floor room working up into the attic behind kneewall: drywall, membrain or poly, mineral wool batts.)  Membrain just seems so delicate. 
Please note the attic side of the kneewall will have 3″ EPS type II rigid board insulation with mineral wool batts in 2×4 studs of kneewall.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    If you are talking about the triangular attic behind a kneewall, it's always best to include the triangular attic within the home's conditioned space. That means that you don't want insulation in the kneewall or the attic floor; instead, the insulation should follow the sloped roofline.

    For more information, see “Two Ways to Insulate Attic Kneewalls.”

    If you ignore my advice, and install insulation on the floor of the triangular attic, it can be insulated like any attic floor. You don't need polyethylene or MemBrain under the floor insulation -- just pay attention to airtightness (and especially air barrier continuity -- which will require blocking between all of your floor joists).

  2. Mill_house | | #2

    Thanks. After assessing my house, my budget and after reading and asking questions, I will be going with the traditional approach as outlined in the article you link to ie) I will be insulating the kneewall and the attic floor behind the kneewall.
    Two questions:
    1. There's a few pieces of duct work in the kneewall stud bays that heat/cool the upstairs. Because I will be installing 3" of EPS rigid foam on the attic side of the kneewall that will come into contact with the ducts, how should I prepare the ducts prior to mounting the foam? ie) i'm asking about shielding the foam from the hot/cold of the duct.
    2. Your recommendation of not installing a vapour retarder on the ceiling of main floor when it's demo'd - which in turn is the attic area on the second floor behind the kneewall - is not the answer I was expecting. Just to clarify, the walls on my main floor will have mineral wool batts in the stud bays, Membrain and drywall...but the ceiling won't have a vapour retarder, per your advice. (The ceiling will just be drywall and mineral wool batts.) Why smart vapour on walls but not on ceiling?

  3. GBA Editor
    Martin Holladay | | #3

    You're making the wrong decision, especially if you have HVAC ducts in the attic. Whenever there are HVAC ducts in the attic, the right decision is to locate the insulation along the sloped roofline, so that the ducts are brought inside the home's conditioned space.

    In the U.S., there is no code requirement for a vapor barrier or vapor retarder on the interior side of the ceiling insulation when the ceiling insulation has a vented attic above it. Needless to say, if your job will be inspected by a Canadian code official, you need to verify that the Canadian code official will approve of your details. From a building science perspective, you don't need an interior vapor retarder under your ceiling -- just attention to airtightness. MemBrain would be unnecessary but harmless.

  4. Mill_house | | #4

    It's not an HVAC duct. It's a tin duct connected to the furnace in the basement. It's set against the studs on the attic side of the kneewall and is only 3inches deep and about 1 foot high. I only have two of them. Otherwise there is nothing in the attic portion behind the kneewalls. Just wondering what to put against them prior to putting the foam against them.


  5. Expert Member
    Peter Engle | | #5

    Since there's only two of them and they're small, you can probably insulate them individually. First, make sure they are well air-sealed at all seams and at the penetrations through the wall and attic floor. stuff your fluffy insulation around them to fill the wall cavity, then use a layer of 3" foam behind the ducts to isolate them from the attic. Since EPS is cheap, you might want to use two layers, extending out 6"-12" beyond the ducts to reduce edge-effects in the insulation.

    If you are insulating the stud cavities of the kneewalls with fluffy insulation, you also need to install a vapor permeable membrane or rigid air barrier layer on the back side to keep the cold air out of the fluffy insulation. The air barrier can go in front of or behind the EPS you're using to insulate your ducts.

    This approach should work in the Canadian prairies, with a short (or nonexistent) air conditioning season. It won't work in hot/humid climates because you would still get condensation on the ducts in cooling mode.

  6. GBA Editor
    Martin Holladay | | #6

    You wrote, "It's not an HVAC duct. It's a tin duct connected to the furnace in the basement."

    That sounds like an HVAC duct to me -- either a supply duct or a return duct that is part of your space heating system.

    "HVAC" refers to any duct connected to a forced-air heating system or a forced-air cooling system. It also refers to ducts used for mechanical ventilation.

  7. Mill_house | | #7

    Sorry, my mistake. I meant to say it's not an HVAC system in the attic. Just two tin ducts about 3inches thick (max) and about 1 foot high. They are both in a kneewall stud bay. Otherwise there's nothing in the attic areas of the cape cod. Just had concerns about putting the EPS rigid board directly against it.

  8. Expert Member
    Peter Engle | | #8

    There is no problem putting EPS directly against the ducts. They don't get hot enough for any issues. Just make sure the ducts are airtight at seams and building penetrations before covering them up.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |