Replies

1. GBA Editor
   Martin Holladay | Nov 12, 2013 11:50am | #1

   Kevin,
   For information on insulating sloped roof assemblies, see How to Build an Insulated Cathedral Ceiling.

   To insulate the flat ceiling above your second floor, it's hard to beat cellulose insulation.

   You can cover the gable-end vents if you want.

   For more information, you might want to read: Two ways to insulate attic kneewalls.

2. 
   cootershouse | Nov 12, 2013 04:00pm | #2

   Thanks for the reply Martin. I've read both of those several times over the last several months, and both have some key points that got me to where I am today. However, I question the wind washing concerns in my setup (I am drawing directly from the roof through a media/filter within the edge vent. It seems if a baffle was needed, perhaps only a small one right below the edge vent exit into the rafter bay). Also, those links point towards the foam under the rafters (for my situation, and is where I heard of the concept) but I am looking of alternative boards. I'd like to not have to drywall in there, be done in one "board" application and not use foam. Any other suggestions?

   Lastly, your point to cellulose is well taken. My ceiling there begs for something blown-in as it is mostly this old press board wood (not asbestos) particle board tile mounted on furring strips. Blown-in would fill the voids the furring strips create, but I fear the additional weight. Batting would be somewhat more supported. Thoughts here? Any other ideas?

   Thanks again,
   Kevin

3. GBA Editor
   Martin Holladay | Nov 12, 2013 04:08pm | #3

   Kevin,
   Your original description is confusing: "Replace the portions of insulation between the rafters behind the knee-walls [this is a sloped assembly, right?] with 3.5"-4" roxul or fiberglass unfaced batts thereby creating a 2" ventilation space. This area would also receive some rigid foam under the rafters to reduce thermal bridging and get a more sufficient r-value. Then, stick in rigid foam above the sloped portions." But you were already describing the sloped portions. So the sloped portions behind the kneewall will be dealt with differently from the sloped portions in front of the kneewall -- right?

   3.4 to 4 inches of fiberglass or Roxul is almost nothing. That won't meet code minimum requirements anywhere in the U.S.

   What does "stick in rigid foam above the sloped portions" mean? Above the ceiling? Between the rafters? Somewhere else?

   If your horizontal ceiling consists of "mostly old press board wood (not asbestos) particle board tile mounted on furring strips," you have a problem. That means you don't have an air barrier. I suggest that you finish your ceiling with drywall to stop the air leakage.

4. 
   cootershouse | Nov 12, 2013 04:30pm | #4

   Thanks again for the reply Martin.

   Yes, sloped assembly for about 13 feet, then flat across floor of top attic, and then sloped again.

   And, yes again. Only about a 4-5 foot run of the sloped portion is also a finished sloped ceiling, so I was going to do these two areas different because I have access. There I was going to do a board of sorts to get a higher r-value in the bay that is only 6 inches deep (true 2 X 6).

   The 3.5 - 4 inches of Roxul was only in the sloped portion behind the kneewall, and I am looking to add a board below it (below the rafters) to help with R-value.

   The sloped ceiling shorter run (above the finished sloped ceiling, or not behind the knee wall) run was going to have the thicker board between the rafters.

   I was hoping to not have to drywall (more wishing I had already done it before moving kids up there). Is there another option? I did paint the press board a couple years ago, but I understand that likely doesn't help much.

   Thanks,
   Kevin

5. 
   cootershouse | Nov 12, 2013 04:49pm | #5

   Attached is a crude typical section which may or may not help with my wordy descriptions. To try to sum it up better, from eave to top attic. I was looking to due Roxul between the rafters with a rigid board under the rafters behind the kneewall. Thicker board between the rafters behind the sloped portion of ceiling. Then two layers of batting in the top attic space.

   The finished rooms have drywall walls, but press-board tiles for the ceiling portions. I am really hoping to not have to drywall over the tiles, so looking for options there as well.

   Thanks again,
   Kevin

   
6. 
   cootershouse | Nov 12, 2013 04:50pm | #6

   I'll try to draw up a better section in the next week identifying all materials, but please keep the comments coming.

   Kevin Line

7. GBA Editor
   Martin Holladay | Nov 13, 2013 06:35am | #7

   Kevin,
   If you read my article ("How to Build an Insulated Cathedral Ceiling,") you'll notice that I advise that you need to install airtight ventilation baffles above fiberglass batts or mineral wool batts installed between rafters. You can use cardboard, thin plywood, or a commercial product like AccuVent.

   If you plan to install rigid foam insulation between your rafters in some locations, you are choosing a method called "cut-and-cobble." That method can work, but only if you pay close attention to airtightness. Each piece of foam has to be carefully sealed at the perimeter as it is installed.

   It's a waste of time to add insulation above your flat ceiling if you aren't willing to address air leaks. Air sealing always comes first, before adding any attic insulation. So you really need to install drywall on the interior side of those leaky cardboard ceiling tiles. Don't worry, however -- drywall is cheap.

8. 
   cootershouse | Nov 13, 2013 10:09am | #8

   Martin, it is not the cost I'm concerned with, but rather the timing of the tear up. You had me chuckle a little bit there as clearly you've identified my willingness to not spend money (oh well...). I appreciate your replies, and I think I'm finally getting somewhere. I read your reply to say there is no alternative and drywall must be done. It seems unbelievable that none of the 4 insulation contractors said anything about this (1 was quoting cc foam though, in his defense).

   So, let us update:

   1. Drywall (I suppose it won't be horrible, just not ideal) over the pressboard ceiling tiles. Any recommendations on minimum thickness here? (I'll try to find it in my code book as well.)

   2. Pull old insulation.

   3. Site built Ventilation chutes from rafter blocks up sufficiently past flat ceiling. I'm thinking 1.5 inch vent space and something thin like cardboard or lauan with caulk. (All except one dormer...more on that later.)

   4. Seal all chases and other holes as seen from the attic with can foam, boards, or caulk.

   5. Roxul Boards (Just a follow up here, do you think these are better than rigid foam from a potential health risk point of view? That, and I really like that I wouldn't have to drywall under them as these spaces are just storage, but let me know which you prefer for this and make me do more drywall if you have to!) under the rafters behind the knee walls to boost R-value there.

   6. Roxul or fiberglass batts in the gable ends behind the knee wall and in the short exterior wall that is adjacent to the soffit area. Cover all that with Roxul Boards (or foam and drywall).

   7. Blown-in cellulose to fill each sealed cavity from the outside wall up to the flat ceiling with a nice top coat over the flat ceiling with it's new found drywall support. No cut and cobble in the sloped portions (small sacrifice). I'll slip in flimsy chutes for the one 8' wide shed roof dormer at this point. (Slight change of pace here, but in doing the drywall I would have to get all whatever else dialed-in up there, so I might as well go cellulose at this point as I would now never intend to go up there again, and it'd be nice to save some labor as much will be spent in labor doing the chutes and drywall.)

   8. Relax, and watch my propane usage plummet.

   Last bit. Whole house fan. In interests of never going in the attic again (other than periodic checks on things), how do you feel about whole house fans in my climate? Also, if I were to add one. It seems it would be best to plumb it's exhaust directly to a gable vent, yes?

   Thanks again for your replies Martin. If you (or any one else) see anything in this updated plan where I can save on cost or labor, please let me know. It still seems like there should be a air-sealing paint of some sorts for the pressboard....

   I guess I know what I am doing this winter.

   Kevin

9. GBA Editor
   Martin Holladay | Nov 13, 2013 12:50pm | #9

   Kevin,
   The thickness of the gypsum drywall you need is 1/2 inch.

   I see no reason to be concerned about any health risks from either rigid foam or Roxul. If the fact that formaldehyde is used to manufacture Roxul bothers you, use rigid foam. Polyisocyanurate is considered to be the most environmentally friendly type of rigid foam.

10. 
    cootershouse | Nov 13, 2013 02:43pm | #10

    One last dig at the leaky ceiling tiles. Are there any other options to seal them?

    Thanks,
    Kevin

11. GBA Editor
    Martin Holladay | Nov 13, 2013 03:04pm | #11

    Kevin,
    Trust me -- drywall is cheap. If you can't afford drywall, the usual low-income emergency solution is polyethylene sheeting. But exposed polyethylene is a fire hazard.

12. 
    cootershouse | Nov 22, 2013 12:20pm | #12

    Martin, thanks for your replies. So, in regards to Vapor Barrier. Is the Drywall going to be my only Vapor Barrier in the system/steps mentioned above, or am I missing a component?

    Thanks,
    Kevin

13. GBA Editor
    Martin Holladay | Nov 22, 2013 12:42pm | #13

    Kevin,
    What you need is a good air barrier, not a vapor barrier. Once you have installed your new drywall, one of two coats of latex paint will provide an adequate vapor retarder.

    For more information on this issue, see these articles:

    Do I Need a Vapor Retarder?

    Vapor Retarders and Vapor Barriers

    Forget Vapor Diffusion — Stop the Air Leaks!

14. 
    cootershouse | Nov 22, 2013 01:14pm | #14

    I think I finally have it. My painted ceiling tiles may have been an adequate vapor retarder, but can't possibly be an effective air barrier.

    I just want to be sure I am not creating any sort of trap for moisture as this assembly (as described above) will not be super easy to visit later.

    Thanks again Martin,
    Kevin

15. 
    cootershouse | Sep 27, 2022 01:07pm | #15

    So, I'm finally tackling this. Old insulation has been removed. Drywall is up under the ceiling tiles. Electrical changes are in progress. I want to run the plan by y'all again looking for any advice on improvements or cost effective changes, here's what's next:

    - Seal all top attic penetrations with expanding foam, and all chases with board and expanding foam.
    -Cover all top attic electrical with fiberglass batting.
    -Build baffles with Polyiso or EPS. I plan to use 2 inch, wedge and seal it. (Chance of mdf in sloped ceiling.)
    -Blow-in 16 inches of cellulose for top attic. This will also fill under baffles down into the sloped parts of the ceilings (3' of sloped ceiling).
    -Seal baffles at eave vents.
    -Add another layer of foam under the baffles in side attics (behind knee wall) to bring insulation flush to bottom of rafter.
    - One more thinner layer across rafters for thermal bridging.

    That all is pretty set. Let me know any good suggestions, but I'm staying vented and doing the labor myself.

    Part I hadn't completely brought up is gable ends behind the knee wall (these side attic spaces are in the conditioned envelope). I'm thinking just doing unfaced batts here to allow drying to the inside. I was going to do unfaced Batts with an inside layer of Polyiso on the studs, but that seems like a moisture trap.

    Any suggestions for improving this plan greatly appreciated,

16. 
    severaltypesofnerd | Jul 01, 2024 04:27pm | #16

    Solid sheets of Rockwool can take place of the solid foam, in many cases. And that stuff won't burn if you use flamethrowers and furnace!

1. Expert Member
   MALCOLM TAYLOR | Jul 01, 2024 05:05pm | #17

   seversaltypesofnerd,

   It depends on what the insulation is being used for. In many situations (like the interior of foundation walls in basements, or many cathedral ceiling assemblies) the continuous insulation needs to be impermeable, so mineral wool can't be substituted.