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Ventilation questions for 1.5 story

GBA Editor | Posted in General Questions on

I have an old 50’s style cape cod 1.5 story with kneewalls, slants etc in cold climate in Minneapolis ….I have over the last 10 years upgraded the insulation gone through 2 blower door tests etc. Yet I still get more ice damming than I feel is unacceptable. One thing that has not been thoroughly investigated is whether the ventilation is adequate and more importantly that it is balanced (intake vs. exhaust). I have the opportunity to change this as I’m getting a new roof.

At present I have a few soffit vents cut in. A wall louver on the siding behind the knewalls but only one side of the house So you have you no cross ventilation with the wall louver vents. I guess the logic was intake @ sofit vent exhaust to wall louver?

The slants which travel from the top of knewalls to the ridge area are blown full. They had the original encased rockwool insulation and then were blown full with cellouse insulation. This was all done in a retrofit insulation project.

The slants are are the week point as they are only framed with this only 2×4’s so you don’t have enough depth for proper insulation.

Above the ceiling where the slants transition to the ridge area I have gable vents on both sides of the house West and East sides.

I guess the current configuration is somewhat irrelvant. I guess my pointed question is what is the proper way to ventilate when you have

1) Knee walls
2) Slants (that are blown full of insulation) no chance for baffles to be inserted to ridge. This separates the area behind the kneewalls and the ride area as no air is able travel to the ridge area
3) Gable vents both sides above ceiling in the ridge area ( I assume this is fine as this achieves cross ventilation as gables vents are on present on the East and West side of the house
4) Wall louvers behind knee walls but ONLY on one side of house.
5) A few sofit vents cut in on the back and front of the house

I have read about Smartvent from DCI but makes me nervous with the potential in a severe ice damming situation that it would back up and have nice cavity simply to come through, if it were to back up on the roof. Perhaps these are not a good idea in cold climates due to this potential?

Any suggestions? I think Martin earlier in his career was a roofer?

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Replies

  1. Doug McEvers | | #1

    What were the results of the blower door tests in CFM50 or ACH50? The 1 1/2 story I tested before a retrofit was 10 ACH50, not much better than a corn crib. You need an infrared scan while performing the blower door test to see where the deficiencies lie. The ceiling joist area under the kneewall is generally open and a huge source of leakage, radiating cold to the upper floor and the ceiling below.

  2. Riversong | | #2

    You don't need a weatherman to tell which way this wind blows (or, rather, does not).

    1) you have woefully insufficient insulation on the slant ceilings (and likely heat loss behind the kneewalls from below)
    2) you have woefully insufficient roof ventilation

    A "few" soffit vents and one-sided wall louvers are nearly worthless. And gable vents do nearly nothing unless the wind is blowing in the right direction.

    You need to significantly deepen the rafter insulation cavity, either by building down or building up when you remove the existing roof. Remove any kneewall insulation and move the thermal envelope to the roofline, making sure to seal any air leakage points between the upper and lower levels, and making sure that the insulation is continuous with the exterior wall insulation.

    Then you need to install continuous soffit vent strips and continuous ridge vents with external wind baffles (such as Air Vent Shingle Vent II or Lomanco OR-4).

    In Minneapolis, you should have R-49 roof insulation

  3. Riversong | | #3

    Oh yeah, also continuous ventilation baffles from soffit to ridge.

  4. User avatar
    Christopher Briley | | #4

    You have a real project on your hands. As Robert points out you need more r-value up there. Nothing eliminates ice dams better. You might be a good candidate for sprayfoam, (having little depth for greater insulation) if this becomes your solution, it would mean opening up your roof assembly from below and spraying in an expensive amount of foam. Then there would be no venting involved. (Remember the mantra "vent it right, or pack it tight".) Likewise, you could dispense with venting IF you can increase the depth of your roof/ceiling assembly, pack it tight with 12-13" of dense packed cellulose AND be tenacious about sealing the sheetrock as a vapor inhibitor.

    It would behoove you to spend a hundred dollars and have an expert come over, examine what you have and advise you. (and that expert is NOT an insulation installer) This stuff can get complicated, and we (those answering your question) are not going to 'see' the full picture, including what you're willing to spend.

  5. matt | | #5

    Thx for all the replies. Many of the points are well taken especially regarding upgrading the insulation in the slant portion. Unfortunately that would involve a major remodel essentially involving tearing apart the whole upstairs which is not possible due to budget constraints. I understand really to entirely solve the problem this needs to be addressed.
    The attic with the exception of the slants DOES have close to R50 insulation and the joists that run under the kneewall have been sealed and viewed with the infrared camera during the blower door. Overlaying XPS over the existing slants WITHOUT removing the existing drywall seems like a possibility. This seems like the only realistic way to increase R value without going through a major remodel. But this questions is for another time and is not the scope of my original question

    That being said what I would like to do is concentrate on the ventilation aspect as I DO have a chance to incorporate some of venting options now as the roof is getting replaced. Take for example

    #1 Roberts suggestion of a continuous ridge vent. However because I already have gable vents on both ends. Would a ridge vent be necessary? Would this be balanced by having both a ridge vent and gable vents?

    #2 If I added a continuous soffit vent would for the portion from behind the kneewall down to the soffit I would assume we would be need to have then have the square vents all along the front and back below the slants (behind kneewalls). So that intake would happen @ the soffit vent and then exit via the square roof vents. As the slants are full of insulation so no communication would happen between the soffit and the ridge

  6. Doug McEvers | | #6

    The ice dam is right at the plate line and forms out over the cold eave. A 2 x 4 rafter with a birdsmouth allows for about 2" of insulation at that point. Major conductive heat loss from the living space to the roof is the result. This 2" space above the plate line does not allow room for insulation let alone venting. This 2" constriction should be sprayed with closed cell foam with a total R-value of 13. This will also stop the flow of warm air at that point which is helping to form ice dams.

    Some gable vents in the knee wall area and near the ridge will help to move some air in the spaces where venting is possible. A well designed roof in new construction will include an energy heel roof truss and each rafter space will have a vent. The building will be airtight and snow will lay uniformly over the roof in winter as the temperature is even over the entire roof surface.

  7. Riversong | | #7

    Roof vents need a minimum of 3' height differential for any significant stack effect convection - low intake, high exhaust, balanced, continuous. If a ridge vent is installed for the exhaust (with some form of low intake), gable vents must be sealed. Soffit and roof vents with insufficient "head" (height difference) will be a waste of effort and money.

    Just as with moisture problems, the first priority is to stop the moisture at the source, an ice dam problem is a heat loss problem. First reduce the heat flow with extra insulation, air sealing, removal or sealing of can lights and heat ducts. Ventilation will help only if the primary heat flow is arrested. The eaves (rafter to plate) are a primary culprit, as Doug indicated.

    But roof ventilation also serves other essential functions. It reduces solar gain in the summer, and it allows critical drying of the roof assembly in the event of any leakage. A well-insulated (R-38) unvented roof can still create ice dams with 10" of snow on top, as the insulating snow can move the melting point to the roof deck. I never build unvented roofs and I would always try to vent a roof during a major remodel.

    Since you don't want to insulate downwards, you have the opportunity to add insulation and venting above the roof deck by adding several inches of XPS or EPS (I would not use foil-face polyiso because it prevents drying upwards) and strapping and resheathing to create a soffit-to-ridge vent channel. This would require altering the roof trim details and perhaps the option of adding gutters if they're not already present (which are the most important element of exterior moisture control for durability).

  8. Doug McEvers | | #8

    Increased roof weight is an issue when considering adding a second layer, the 1 1/2 story roof framing is weak with 2 x 4 rafters spanning up to 18'. They are supported somewhere near midspan by the knee wall but this puts weight on the main level ceiling joists that are often 2 x 6 or 2 x 8 at best. One of the upgrade problems of older homes is dealing with load bearing, often roof loads are not carried to the foundation by 1st floor walls or an adequate center footing. It is very difficult to work with the existing roofline of a 1 1/2 story to get quality living space, energy efficiency and structural integrity.

  9. Riversong | | #9

    1½ storey capes may meet a local architectural aesthetic, but they make little sense as an enclosure for conditioned space. What may have been intended initially as attic storage eventually becomes living space. Or designers and builders trying to copy a local architectural tradition will build for "attic" living space or add a dormer or two to make it more livable.

    But, for new construction, a full second storey is easier and potentially less expensive to build than a cape with dormers, and easier to insulate, air seal, roof, trim and maintain. The upper storey of a cape is difficult to fenestrate for daylighting (without skylights) and problematic for cross- ventilation. Knee-wall thermal/air barriers are always problematic, and they can transfer loads improperly.

    Also, underbuilding roof structures is one of the more foolish building practices from our architectural past, second only to poor foundations.

  10. Alicia | | #10

    Doug,
    I am curious to learn what you ended up doing. I have a similar situation in Minneapolis and all the contractors seem to have different ideas. My roofer doubts that my soffit vents even work, yet he wants to install a ridge vent.

  11. Riversong | | #11

    Alicia,

    A ridge (exhaust) vent without balancing soffit (intake) vents can make the situation worse, since it can create negative pressure in the roof assembly and draw more inside heated air into the space, also increasing heat loss and potentially creating moisture problems.

    There is only one roof venting system that works: continuous soffit vents and continuous ridge vent with external wind baffles, as well as a continuous vent channel from soffit to ridge.

    And, you have to have enough ceiling insulation to minimize heat loss and seal any air leakage pathways from the conditioned space.

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