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Attic insulation – R-21 and radiant barrier

msimm15 | Posted in Energy Efficiency and Durability on

Hi I know this topic has been asked a lot but I was hoping to get some info I could not really find in other posts.

I live in Queens NY – climate zone 4, I have a tall pitched non-vented attic. The attic is really a 4th floor with a smaller crawl space attic on top and closed un-vented spaces on the side these spaces get really hot and heating up the room. BTW I have no duct work in the un-vented attic spaces but we do use it for storage. The house is from 1945 – used to have a stone roof but the previous owner replaces it with a shingle roof maybe 8 years ago (only the shingles and roof paper not the sheathing were replaced) and I am in the process of redoing the insulation, the roof joists are 2×6 so I am using r-21 faced batts, not wanting to go with spray foam for various other reasons

My issue is the house is in direct sunlight all day and gets really hot just from the radiant heat. I wanted to use a radiant barrier since the sun is so strong but since its an un-vented roof I am trying to figure the best route. I tested the following scenarios today and these were my results

Outdoor temps today got up to 82 degrees, these were the inside temps:

1 – bare underside of roof – roof got up to 107 degrees !!!
2 – old foil faced R7 insulation – 88 degrees
3 – new R-21 kraft insulation – 83/84 degrees
4 – New R-21 with 1/2″ air gap and Radiant barrier – 88 degrees ??
5 – Radiant barrier alone – with 6″ air gap – 91 degrees

As you can see the Radiant barrier worked – by itself, it didnt help when put after the R-21 (even though I left a gap.

What i am proposing and think would give the best effect is to install the radiant barrier first – with 3/4″ gap on the underside of the roof, then install the r-21 faced insulation this way I am insulation 91 degrees instead of 107 degrees. My problem/concern is that the air will be trapped in that 3/4″ air gap between the radiant barrier and the roof sheathing (actual wood slats not osb board) I am concerned that the super hot air will cause damage to the roof as it has no where to go.

Interesting to note – the old R7 foil faced insulation was stapled so there was a 2″ air gap behind it but I really dont understand why since there was no place for that air to go.

Any thoughts and info would be greatly appreciated.


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  1. GBA Editor
    Martin Holladay | | #1

    M. Simm,
    Several aspects of your post are difficult to understand.

    What do you mean by a "stone roof"? Do you mean a slate roof?

    Are you talking about triangular attics behind 4-foot kneewalls, along with a cramped attic under a gable roof above a flat ceiling, as is typically found in a Cape Cod building? Or are you talking about one big attic space?

    If you are trying to insulate a sloped roof assembly that is unvented, and you are working from the inside, there is only one safe way to proceed: You need to install an adequately thick layer of closed-cell spray foam on the underside of the roof sheathing. If the closed-cell spray foam is thick enough -- most codes call for R-38 or R-49 minimum -- it can satisfy minimum code requirements.

    Otherwise, you can use the flash-and-batt approach.

    For more information, see these two articles:

    How to Build an Insulated Cathedral Ceiling

    Flash-and-Batt Insulation

  2. msimm15 | | #2

    Hi Martin,

    Thanks for the quick response. To answer your questions
    - Yes I did mean slate
    - Apparently I am talking about a gable roof, didnt know thats what they are called. Difference here is the kneewall is 6' tall so there is quite a bit of space. Its basically a 3,4,5 triangle. Knee wall is 6', floor above ceiling is 8' long and pitch is about 10'
    - I know closed cell would be the correct approach here but not looking to deal spray foam for the time being.

    I am not really looking to achieve the rated r value as I know I wont be able to without the spray foam.

    What I am most interested in knowing is if I can install the radiant barrier maybe with the solid panel foam board backing with an air gap underneath the sheathing from inside and then put the r-21 under the radiant barrier. Basically what it would look like from outside int is the shingles/roofing then sheathing, then air gap, foam board, then r-21 batts.


  3. GBA Editor
    Martin Holladay | | #3

    M. Simm,
    A radiant barrier will in fact provide limited R-value. If installed with an adjacent air space, the air space will provide an R-value of about R-2 or at most R-3. It's better than nothing.

    You should realize, however, that attempts to insulate a sloped roof assembly without following the rules can put your sheathing at risk. If interior moisture has access to cold sheathing, condensation and rot can result. Because of this possibility, I'm leery of home-brew solutions, and urge readers to follow the rules.

  4. msimm15 | | #4

    Martin, I attached 2 drawings to help you understand the situation. I read those articles but could not confirm whether I can put the radiant barrier in the location I am suggesting.


  5. msimm15 | | #5

    I totally hear what you are saying, I am not providing the radiant barrier for the R-Value it is to block the radiant heat which as my experiment from this morning showed - worked very well, but not with the fiberglass in the picture which I why I was proposing the fiberglass after the radiant barrier to deal with the conduction and convection. The barrier brought the temps down from 107 to 91.

    Regarding the moisture I am weary of it as mentioned in the article but I thought that was the point of a vapor barrier ? I was thinking about lining the whole assembly with 1" rigid insulation after the fiberglass and taping up the seams. Is moisture really going to seep through this ?? and even if so - so much that it will cause a problem ??

    Thanks for your input, greatly appreciated

  6. msimm15 | | #6

    Sorry I was a little unclear there - the r-21 is faced and has a vapor barrier, the rigid insulation would do on top of that.

  7. GBA Editor
    Martin Holladay | | #7

    M. Simm,
    Is the air gap under the sheathing connected to a soffit vent at the bottom and a ridge vent at the top? Or is this unvented?

    If it is unvented, you have to stop trying to break the rules. You can't insulate unvented rafter bays with fiberglass batts. As the articles stated, you need closed-cell spray foam on the underside of the roof sheathing.

    If any of this is unclear, re-read the articles.

  8. msimm15 | | #8

    If I take moisture out of the problem, is there an issue with heating the underside of the sheating too much due to the radiant barrier reflection the heat.

  9. msimm15 | | #9

    Ok. thanks for the answers.

  10. thermooo | | #10

    Hi Msimms, How did you measure the temperatures you reported? Did you use an IR gun? If so be aware the radiant barrier has vastly different emissivity values from what the gun was likely calibrated against.

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