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Community and Q&A

Moisture barrier for attic in brick house with slate roof?

Breislach | Posted in Energy Efficiency and Durability on

I live in Zone 5a in roughly 800 sq. feet of living space in a duplex, part of a “development” of houses built in 1917 to house workers in war-time industries.

The houses are brick construction, with slate roofs. Ours has a block foundation.The attic is insulated with about 4 inches of rock wool. There’s no vapor barrier between the insulation, the ceiling rafters, and the second story’s plaster-and-lath ceilings fastened to the rafters. Also, the attic is unvented at the gables. The soffits /appear/ to be vented: there’s the standard aluminum grid material one sees on soffits, but I’m not sure the wood soffits beneath the cladding have openings. The roof is very steeply pitched, with deep eaves. The rock wool looks like it has been shoved down the eave shoots. It also looks like there’s no air channel from there into the attic.

Can you suggest how to properly insulate the attic? From what I’ve read, it seems like we should remove the rock wool (it’s damp in spots), open up channels in the eave shoots to the soffits (having opened them if needed), and lay a moisture-vapor barrier before laying new insulation. But the literature on masonry buildings seems to suggest that moisture-vapor barriers are not always a good thing — but maybe that’s just in walls?

Also, the bays between the ceiling rafters are unevenly sized (ranging from 13 – 18″ apart), so I’m thinking of using loose cellulose to fill between the bays and then laying fiberglass batts perpendicularly to the joists in an attempt to raise the R value to somewhere over 40. The house’s electrical service runs through the attic, following the path of the former knob and tube wiring, and we want access to it, so I don’t want to use blown-in products. Does this seem like a good approach?

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  1. GBA Editor
    Martin Holladay | | #1

    Your description is confusing. I'm guessing that you may not know the difference between a joist and a rafter.

    A rafter supports a roof, and usually slopes.

    A joist supports a floor or a ceiling, and never slopes. Joists are level.

    It sounds like the existing insulation is on your attic floor, between the joists. Is that correct?

    The fact that some of your insulation is damp is an indication of air leaks between your heated interior and your attic (unless, of course, there is a roof leak). The first step -- before you insulate -- is to locate these air leaks and seal them. You want a ceiling that is airtight -- or as close to airtight as you can make it.

    You don't need a vapor barrier. To learn why, read this article: Do I Need a Vapor Retarder?

    It's a good idea to establish an air pathway between your soffit vents and your attic. Here is more information: Lstiburek’s Rules for Venting Roofs.

  2. Expert Member
    Dana Dorsett | | #2

    Damp rock wool should DEFINITELY be removed, since that it just keeping the wood it's touching wet! It's an indication of air leakage from the conditioned space to the attic.

    Soffit venting should always be paired with ridge venting, and not with gable venting, since the gable venting with soffit venting short-circuits, drawing the majority of the flow from the adjacent soffit vents, and almost none from the middle. If a continuous ridge vent can't be installed or if the apparent soffit venting is really just a fake, with no real venting to the exterior, it may be easier/better to seal the attic well and insulate at the roof deck and gable ends.

    Unlike composite shingles, slate roofing is inherently back-ventilated and the roof deck is able to dry toward the exterior unless there is a vapor-tight underlayment for the slate. But whether the roof decking and slate is sufficiently air-permeable to be the moral-equivalent of a vent outlet remains to be seen.

    If you insulate at the attic floor, first you have to fully air-seal between the attic and upper floor COMPLETELY, which may or may not be difficult. If it's sufficiently air tight even the incidental leakage between the outdoors and the attic may be sufficient for keeping the attic dry. With any reasonable amount of venting to the attic, if it's air-tight between the attic and conditioned space there is no need to make it water-vapor tight. But using a flash-inch of closed cell polyurethane spray foam as part of the air sealing prior to insulating will also be adding a ~1perm vapor retarder.

    Start with big holes- it's not uncommon in houses that vintage to find balloon-framed partition walls without top plates with cavities exposed to the attic. Corrugated cardboard stapled in place with the edges caulked/foamed can block those if the span isn't large, otherwise cutting OSB/ply stock to cover the gaps works. Foam seal every electrical & plumbing penetration too. Sometimes you'll have stacks for plumbing drains in unblocked chases extending through the attic too which may require some custom cutting of OSB/ply for the rough seal, to be made tight with foam. Special consideration is required around combustion flues- you need to use sheet metal as the air barrier, and wrap the flue with R15 rock wool batting (tied with wire to keep it from unwinding) before you can insulate with cellulose.

    There's no point to going with fiberglass batts on top of cellulose. Build out the necessary clearance chutes to the roof decking with 1" rigid polyiso to at least a foot above the desired depth, pre-install a bunch of cardboard depth gauges marked to the desired depth (12" , if R40 after settling is the goal) stapled to the sides of the joists in multiple places, and have at it- blow cellulose right up to the marks, covering over the tops of the joists. Rake it flat as you go, at the desired depth. This is a fairly easy DIY with a rental blower from a box store, but pre-order "borate only , sulfate-free" goods if going through a box store, rather than using the off-the shelf goods, which usually contain sulfated fire retardents. The sulfates are corrosive to metals (and stink) when wet, which could lead to other long term problems should a roof leak go undetected for an extended period. "Stabilized formula" cellulose designed for damp spraying is almost always sulfate free, and dry-blows just fine.

    There are no issues related to blowing over wiring OTHER than knob & tube. As long as the k & t is fully decommisioned and can't accidentally be re-powered, it's legal to insulate over it. It's fine to insulate over Romex & BX too.

  3. davidmeiland | | #3

    Deborah, when you go in the attic, can you see the bottom side of the slates?

  4. Breislach | | #4

    Thanks to Martin and Dana for the very helpful replies. And yes, Martin: I meant the ceiling joists, not rafters. To David: No, I can't see the slates from the attic. They're nailed to a roof deck that might be (if I recall correctly) tongue and groove -- though I'm not positive about that. According to a roofer who's done some work up there, there's a felt underlayment between the slates and the roof deck.

  5. davidmeiland | | #5

    Attic ventilation would then be in order. I rarely deal with slate roofs, but they are often nailed over skip sheathing and there is plenty of air movement.

    I would get someone with a blower door and possibly an IR camera to find the air leaks in your ceiling (and elsewhere), and seal them. That and about 20" of cellulose will likely make a huge difference.

  6. Breislach | | #6

    David: By "attic ventilation" do you mean ensuring that the soffits are open and provide air flow, and assuming (or hoping) that the slates and roof deck will breathe sufficiently without further venting? My understanding is that we can't do a ridge vent on the slate roof. "Lstiburek's Rules..." seems to indicate that lots of soffits and no ridge would be okay in this scenario. Also, blower door / IR data is on its way. Many thanks!

  7. GBA Editor
    Martin Holladay | | #7

    I wouldn't worry too much about ventilation if I were you, and I certainly wouldn't try to retrofit a ridge vent on an existing slate roof.

    If you don't have signs of mold on your roof sheathing, you're probably OK. The important thing is to plug those air leaks in your ceiling. If you do that, the ventilation rate becomes almost irrelevant.

  8. Expert Member
    Dana Dorsett | | #8

    I'm with Martin- if it's gone nearly a century without mold/rot issues WITH the air leaks into the attic from conditioned space, it won't need it AFTER you air seal & insulate the attic floor/conditioned ceiling.

  9. Breislach | | #9

    Sounds good. Thanks again: this discussion has helped resolve a lot of my confusion, since different sources suggest different options (or /insist/ on specific but contradictory approaches!).

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