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Community and Q&A

Ceiling Vapor Barrier

Chrisroche | Posted in General Questions on

I am in the process of building a 12 inch double stud wall assembly with dense pack cellulose insulation. I am planning on wrapping the inside wall with a smart vapor barrier of either Siga Majrex or Intello Plus. I have heard Siga’s Majrex is a superior product in that Intello has had some issues with trapping moisture from fall months when the barrier is mostly open, due to the transition into winter when the perm rate shrinks and the water has not had enough time to transmit out of the wall cavity. (This is advice I received from a retailer of Siga so I am taking it with a grain of salt)

In any event, I am thinking about taking the same membrane and applying it to my ceiling to prevent moisture from intruding the attic from the home, but am questioning whether it is necessary. I am building a raised heel truss attic with r-60 blown cellulose and plenty of ventilation.

Since I am planning to install some low profile canister lights throughout the home, my insulation contractor suggested that instead of applying a vapor barrier, a better approach would be to do 1-2 inches of spray foam followed by r-50 cellulose. His reasoning is that the spray foam would both act as a vapor barrier and at the same time would seal the canister lights which are notorious for being leaky.

Has anyone had any experience with either spray foaming an attic for this purpose?


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  1. Chrisroche | | #1

    Just to clarify, I live in NH where the climate tends to be very mixed. We go from having -15 F degree days that are super dry, to 2 days later it can be 45 degrees and humid.

  2. charlie_sullivan | | #2

    The story you got about the advantage of Majrex doesn't sound very accurate, but Majrex is a very interesting and very new material. The brochure mentions some studies at TU Dresden that demonstrate its advantages, but I haven't located those studies yet to understand what conditions it would actually offer advantages over the other smart membranes. It claims to have different permeability in different directions.

    In any case, I think it's unnecessary on the ceiling given that your attic is well ventilated.

    I would not opt for the spray foam option. You have to either find someone who offers the new HFC-blown foams with low global warming potential, or you have foam blown with gases that have 1400X worse global warming impact than CO2. And even then you have a small risk of problems with incomplete curing and lingering odors, and you have a summer moisture risk on the top side of the foam if you air condition inside.

    I'm not sure what you mean by low-profile canister lights, but you can now get surface mount LED lights that require nothing but a junction box in the attic. I'd recommend that route for easier air sealing and flexible swapping out of different styles in the future.

    Is your insulation contractor experienced in dense packing 12" thick walls, and are you confident of their skills there? That takes a lot more skill than I realized going into our project, and our first insulation contractor was a disaster. I'm in central-west NH, so I'm hoping you aren't dealing with the same people. I eventually found someone excellent, from VT.

    What's the outside of your wall--sheathing, WRB, rainscreen, and siding?

  3. Chrisroche | | #3

    Hi Charlie,

    Thanks for the response!

    Your thoughts on spray foam do resonate with me as well. I have been somewhat opposed to going that route from the beginning. I also have a walk out basement that sits in a somewhat damp area, so I am anticipating some moisture vapor making its way into my basement (even though I waterproofed the outside wall) In anticipation of this, it seems that spray foaming my basement walls is the best solution to prevent vapor transmission from the concrete walls into the house. This is what led me to considering spraying the attic as well while they were on the job.

    I was definitely planning on going the LED route, I was hoping to find something that mimics the look of canister lights. Your suggestion to go with standard electrical boxes does seem like a good option, especially considering I was already planning to use Airfoil receptacle boxes throughout my wall assembly.

    As you mentioned, it is difficult to know exactly how qualified the insulation installer truly is at dense pack. I live in the Lakes Region, so we are somewhat central to the state. As of now, I have spoken with Quality Insulation out of Meredith and Turn Cycle Solutions out of Nashua. Both seem to be confident in their ability to do dense pack.

    I purchased my Intus windows through Performance Building Supply out of Portland Maine and they have been great to work with so far, so I am considering purchasing air sealer/house wraps through them as well.

    My single story 1,700 sq ft wall assembly is as follows: siding material (possibly hardi plank or vinyl shake), Majvest house wrap from Siga (or possibly Tyvek), Fentrim for window flashing, and wigluv tapes to air seal all the sheathing and windows (not yet sure if I want to go plywood or OSB). The sill plate is sealed with Conservation Technologies EPDM gasket.

    I am trying to make the house as airtight as possible with the new Panasoinc Intelli-Balance 100 ERV to ventilate.

  4. charlie_sullivan | | #4

    The crew we had a bad experience with was from a different branch of "Quality" Insulation, so that's not very encouraging, but the reason we initially went with them was largely based on a recommendation from someone who had actually worked with the Meredith crew, so you should be OK with the Meredith crew. I don't know anything about the Nashua company.

    I don't think you should feel like spray foam is the only good choice for your potentially damp walkout basement. EPS foam should work fine. But if you do go with spray foam, here's the article that lists the companies supplying the low-global-warming potential HFC-blown foams: Demilec Heatlok XT HFO and Lapolla Foam-Lok 2000-4G are the specific products I know of but probably others will have them soon if they don't already.

    You didn't mention a rainscreen gap between the sheathing and the siding. I think that's a key feature to include. You can do it with furring strips or a plastic mesh product like "Slicker" from Benjamin Obdyke. It's really a judgement call of which measures to take to avoid moisture problems in double-stud wall sheathing, the possibilities including:

    1. Rainscreen.
    2. Plywood instead of OSB for sheathing
    3. Board sheathing instead of plywood or OSB--rare in most of the country but worth considering in this region since you can buy locally milled pine boards inexpensively.
    4. Smart membrane on the interior.

    We did 1, 3, and 4, which is the equivalent of belt, suspenders and elastic waist-band. Probably just belt and suspenders is plenty. But I think the rainscreen might be the most important of the four, so I wouldn't recommend skipping that.

    Yes, you can definitely mimic the look of can lights with surface mount lights. The ones that do that best are the Philips slimsurface ones. Personally, I prefer ones that are a little more domed so that some light is spread across the ceiling. But overall, my favorite is the Home Depot/Commercial Electric cheap one because the light quality is fantastic, if you like warm white. It's one of many rated 90+ CRI which should all provide excellent color quality but for reason I don't understand I really like the color of this one. Comes in 4" and 6". The look cheap in the store but look fine installed.

  5. Ken Levenson | | #5

    Hi Chris,

    Delayed input here, but sounds like a great project and would like to second the sentiments of Charlie Sullivan.

    It’s unfortunate to hear of misinformation being circulated regarding INTELLO.

    Briefly, INTELLO has over a 10 year track record and has been installed for over 6 years in northern New England, with monitored assemblies in difficult conditions, demonstrating performance as expected – building drying reserves and protecting highly insulated assemblies from moisture damages. As we’ve not heard of any problems, I suspect such claims are being misconstrued from misinterpreted WUFI analysis – which we’ve experienced before - as WUFI is a complicated program and can easily/accidentally show very misleading results. We have a good deal of information on INTELLO on our website in blog posts and encourage you to review them. (

    From everything we are experiencing, seeing and reading, INTELLO is clearly the most robust smart vapor variable membrane on the market.

    I hope you also consider our SOLITEX Mento 1000 house wrap (with battens, for vented rainscreen as Charlie suggests or just SOLITEX UM with integrated mesh) and TESCON Vana, CONTEGA Solido EXO and EXTOSEAL Encors tapes – as they are also second to none.

    And if we can still be useful, consider reaching out to Oliver Klein, our representative for your area. Oliver (at) foursevenfive dot com - he has experience working with dense pack installers in NH and should be able to provide some help in that regard as well.

    Good luck!

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