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Community and Q&A

Attic condensation

user-3168847 | Posted in General Questions on

I recently discovered a condensation issue in my cape cod style attic that I’ve unfortunately recently finished re-insulating.  I live in Prince Rupert, BC (close to Alaska), which is on the northern BC coast right on the border of zone 5/6.  Fairly mild, maritime climate.
I furred down the existing 2×4 cathedral rafters with a 2×4 for a rafter depth of 7″ and installed a 1 1/2″ vent chute using R5 ridgid foam (spray foamed in place), then filled the remainder of the cavity with R22 Roxul for a total of a little less than R28 since the batt was compressed an inch.  Ideally I would have furred down the rafters further to create a deeper cavity to insulate, however, I also had headroom issues to deal with.  Kneewalls were insulated with R22 roxul and the floor joist area behind the kneewall was insulated with approx R50 blow-in fibreglass with ridgid foam blocking between joists below the kneewalls.  I was able to get close to R50 with the blow-in insulation right to the exterior wall, since the rafters oddly ran right out to the soffit, not sitting on the exterior wall, so I framed a small pony wall supporting the rafter above the exterior wall. The walls and ceilings in the attic all have a 6mil vapour barrier installed as per our code requirements in BC.  The main floor ceilings below do not have a vapour barrier installed as far as I can tell and are a mix of fibre-board ceiling tiles, lathe and plaster and drywall depending on the room.

I’m also in the process of re-doing the old cedar shake roof, stripping off the cedar shakes, sheeting with 1/2″ plywood and installing asphalt shingles.  Half of the roof is currently completed.

We’ve recently had some cool weather, down to approx -5C and shortly thereafter, I noticed staining on the living room ceiling tiles and my first thought was a roof leak as it was on the side of the roof not yet fixed, however, after going into the attic I discovered that it wasn’t a roof leak, but the underside of the roof appears to be either iced up, or wet, which would suggest heat loss.  This is on the underside of the roof, both behind the kneewalls above the blow-in insulation and from what I can see, continuing up the underside of the roof into the vent chute cavities as well.  My suspicion is the heat loss is getting up through the blow-in insulation and continuing up the cathedral ceiling, since I don’t know how the heat loss would be occuring through the cathedral ceiling itself(vapour barrier, Roxul and air sealed vent chute).  This condensation issue seems to be consistent between all areas of the attic regardless of the room(ceiling type) below.  The attic was initially insulated with a small amount of R12 fibreglass batts and vermiculite and didn’t appear to have any condensation issues. Any thoughts/suggestions would be appreciated.  Thanks in advance.

Tyson Hopkins
Prince Rupert, BC

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  1. Expert Member
    BILL WICHERS | | #1

    This is probably a problem of moisture migration and air leaks more than a problem of “heat loss” through the insulation. It looks like you have frost on the nails coming through the roof sheathing.

    I would suspect one of the following:
    1- you missed some things when doing air sealing work (you did air seal everything before installing insulation I hope). I would check top plates, especially electrical and plumbing penetrations. This is what caused this same issue in my own house.
    2- you have something leaking, or a bathroom or dryer vent located in such a way that a portion of the moist exhaust air is getting into one of your soffit vents and entering your attic.
    3- you don’t have enough attic venting (this is a last step or a band aid if 1 and 2, above, can’t be fixed).


  2. walta100 | | #2

    I am sorry to say this but the real problem is someone built a story and half house, by their very nature it is almost impossible to keep the warm air inside the house. The next problem is someone bought it and tried to improve it.

    You may find the articles on the devil triangle useful.

    In my opinion you need to air seal your house to less than 2 ACH50. This will almost certainly require you to remove the new remove your new insulation, if you can’t get a good blower door test do not put the insulation back as you need the heat in the attic the keep it dry.


  3. Expert Member
    Akos | | #3

    If your ceiling is only ceiling tiles as you mention in your post, then that is your problem.

    Ceiling tiles leak like a sieve. You need a proper air barrier above them. Blown fluffy insulation does nothing for stopping air flow.

    If this is actually the case, fixing if from the attic now would be pretty hard. You would have to rake all the insulation away and seal it up somehow (1" of spray foam is the easiest, you might be able to detail 6 mil plastic but it will be tricky).

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