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Insulation board over drywall in a dome

SaraBlue | Posted in General Questions on

I live in a geodesic dome in windy, cold Wyoming. It was built in 1985. The original ceiling is sheetrock which, in a dome, only lasts about 10 years before it begins to crack at the seams do to the natural movement of a dome. I had the sheetrock replaced in the loft/upstairs and because of the triangles and tight space, it was quite the ordeal. The contractor said he’d never do it again.

I want to insulate the ‘vaulted’ ceiling over the living room and I know that it needs new insulation. The cracks between the triangles are ugly. I am avoiding tearing down the existing sheetrock, and I think I may have come up with a solution if it is something that would actually insulate, that I can do myself (I’m a 60 yr old woman), and that won’t require the contractor, the cost, and the mess and waste that tearing down the sheetrock would make….

How about cutting insulation board into triangles, covering it with a lattice ‘wallpaper’ type material I’ve found (so it looks like wood) then glue the insulation board over the top of the sheetrock, and spray a low-expanding foam between any cracks and trim with wood (screwing into the framework) on all sides of each triangle? It would certainly look pretty (I’ve noticed lots of dome ceilings are wood – can’t afford that one), but would it actually add insulation value to the ceiling as it would be inside the dome?  Is there a glue other than spray foam that would defy gravity, or would the wood trim do that work?

I had to make boxes for artwork 20 years ago and I needed them to be lightweight but strong. I used roofing board, spray foam, and 1×2’s all around. Last week I tried to break them apart but couldn’t! I had to take a saw to them. They were very strong and built much like I am considering for the ceiling. But they didn’t need to insulate.  Will it do the job on top of sheetrock? 

Thank you much in advance for any and all responses!! Photo shows a small portion of the dome’s ceiling with a corner of the large skylight.

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Replies

  1. Expert Member
    Zephyr7 | | #1

    That doesn’t just look like cracking, it looks like the tape is separating from the drywall. That’s often a sign of moisture issues. Are you sure you don’t have moisture issues in your walls/ceilings? If you have moisture issues, using rigid foam board as you describe may make things worse.

    Bill

  2. Tim R | | #2

    Most rigid insulation requires a thermal barrier if used in a living area. Think of foam as solid oil - it burns quickly. I think there is a brand that has the thermal barrier built in. I can't think of the brand right now, but you could cover that with material or paint it
    Gluing the sheets may be problematic -perhaps best to screw them to the wood framing then trim out the seams after spray foam.
    Check out the possible water or moisture issue
    You could make cardboard templates to get the triangle shape for each panel for the best fit.
    Rent or buy some scaffolding to make the job easier.
    Tim

    1. Expert Member
      Zephyr7 | | #3

      Dow Thermax is rated to be left exposed. It has both white and silver opinions. Johns Manville also makes a polyiso product that is rated to be left exposed.

      Bill

  3. Expert Member
    Akos | | #4

    There is no way to vent a geodesic dome roof, I'm with Bill here, I think most of the damage is from condensation. Since your current roof relies on drying towards the inside, putting foam over it would make the problem worse.

    Probably not what you want to hear, but your best solution is either re-roofing with exterior rigid insulation or spray foaming the ceiling. Both are very expensive options. I would wait until the roof need replacing and insulate from the outside.

    In the mean time, if you can get a taper to re-tape the seams with mesh and setting mud (drywall guys sometimes call this hot mud) instead of regular mud, it will last longer, will handle a bit more moisture before separating.

  4. SaraBlue | | #5

    Thanks all. I'm not sure it's moisture. I know that domes have problems with that, but Wyoming is so very dry... Don't know if this makes a difference, but in 2008 when we bought it, we put in 2 smaller skylights that open, and like typical skylights that open, don't seal as well as I'd like. (I'm guessing we get a lot of venting this way?) How would I find out if it's just an old, badly done tape/mud job or if it's a moisture problem? Maybe a test strip of some sort?
    Tim, thank you for letting me know about the possibility of a built-in thermal barrier in a foam board and the suggestion of the cardboard template. If it isn't a moisture issue, would this do the job of insulating?
    Sadly, I got a new roof put on just a few years ago.... It didn't even occur to me to do the insulating then! That would have been the smart thing to do, then have a taper (and scraper to get that nasty popcorn stuff off) would have been perfect. Live and learn.

  5. Expert Member
    Akos | | #6

    Sara,

    The problem with venting isn't outside air, but inside air. Even relatively dry indoor air (say 30% rh at 70F) will condense when it hits colder sheating in the roof. No amount of fresh air inside the house will prevent this. If the celing is well sealed (no potlights and such), this is a slow process and if you are lucky, the moisture doesn't accumulate to the point of causing a lot of damage and can dry during the summer. This is why you don't want foam (more specifically non permeable foam) which would prevent this drying.

    Usually you see most of the damage from this towards the roof ridge, I'm at loss as where that would be a with a geodesic structure.

    You can get one of those pin type moisture meters and check the drywall at a a bunch of spots near the top. You can compare the reading with a piece of similar drywall that you know is dry lower down.

    1. SaraBlue | | #7

      Thank you Akos. I now have a better understanding. Wherever you all are, be cautious and careful please. Wash hands with soap and water. Don't fall victim to panic buying or hoarding. Take care of each other. And thank you all for your answers. S.

  6. Tom May | | #8

    I'm sure there is plenty of movement of each of the panels which led to the cracking. Plus they probably just used regular drywall tape rather than mesh tape, which means no mud in the gap between the panels if they didn't mud / fill them first. Probably best to peel off the old tape and see what's underneath, and replace with mesh and re -mud. Or if you find that the only problem after you remove the tape is in the joints or seams and the rest of the panel is fine, which looks to be the case, you may be able to get away with just using caulking which would be more flexible and can be painted over, and a lot less work and mess.

  7. Walter Ahlgrim | | #9

    You may want to cover the drywall with expanded metal lathe and plaster over the dome. It is a doable one person job. It is a lot of ceiling but you could do it little by little and with that much practice you will be very good by the time you get done.

    Walta

  8. SaraBlue | | #10

    Thought you all might be interested in the outcome of my quest to fix my dome ceiling. I moved away from the idea of just covering everything over and decided to have it done professionally. I decided to have the sheetrock on the ceiling AND outside facing risers/walls removed. Good thing too. Apparently what was happening to cause the ceiling cracks was a lack of adequate sheetrock screws causing the rock to sag (hence the cracking). Also, the original builder in '82 had put in particle board rather than exterior grade ply over 2 sections of risers on the windy side of the dome which had gotten soaked over time (of course) and were moldy. I knew about the mold as I had torn out the baseboard when I removed the carpet. At that point, I found the wind would LITERALLY blow in under where the baseboard once was. (In Wyoming, it is 'normal' to have 30 and 40 mph winds. We've felt winds as high as 60, and in certain parts of the plains it has been clocked at nearly 100!) Thankfully the contractor found the dissolving particle board and will replace it so the wind will no longer be a problem. The interior will be sprayed with open-cell insulation, then new sheetrock and paint. I'm betting the cost of heating the place will be considerably lower which should off-set the cost of the renovation.

    You all were SO helpful and offered so many great solutions, so I wanted to update you, and for anyone interested, I've added pics of the dome structure. You will notice areas of zero insulation (by the front door, for instance, and in a few of the smaller triangles). This dome certainly had much bigger problems than a few unsightly cracks in the ceiling!

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