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Community and Q&A

Cathedral Ceiling Retrofit with New Addition

bradesp | Posted in General Questions on

Ok everyone, I need your most creative / science backed insights on how to deal with a VERY challenging 1969 Cottage / Cathedral Roof challenge. NOTE:  The Cottage is located in the 4A climate zone – Straddling the NC/VA state boarder one hour north of Raleigh. I have attached two pictures that illustrate my roof / ventilation / cathedral roof challenge.   We want to add an addition that will eliminate all possibility of ventilation to once large side of the current roof / cathedral ceiling… and it is THIS CHALLENGE for which we need a solution 🙂 First some background Cathedral Roof / Ventilation “As Is” Situation In Picture #2 you will see a cutaway detail that shows how the roof is currently configured.  Many of you will likely ask… “Why is the roof built this way”… Here’s the answer.  When I purchased this cottage 20 years ago, the interior underside of the roof rafters was original T&G that we did NOT want to remove… Also, the rafters were filled with 4″ of fiberglass insulation filled with moisture.  So instead of tearing out the T&G on the interior, we literally peeled the shingles and plywood decking off the top and we sprayed 3.5” of closed cell foam from the outside / topside of the house.  We then placed new plywood decking and shingles on top. Recent Improvements / Additions to Underside of Cathedral Ceiling We did an extensive interior renovation on the home last year… In doing so, we decided to “tighten” up the interior layer by adding a 3/4″ foam layer that was well taped and sealed at the perimeter.  We then added a new layer of decorative 5/8″ T&G to the underside. New Addition  We now want to add 1,000 square feet to this small cottage.  The new addition will connect to the existing cottage in the area highlighted in Picture #1.   Of course in doing so we’re now introducing a ventilation problem for this entire roof… hence my big challenge! Possible Solutions / Ideas? In no particular order, here are some solution “ideas” I’ve been thinking about… I am hoping all of you experts can tell me if any of these ideas have merit, or if you have better approaches I should consider! Convert Roof to Unvented Dense Pack? Could I dense pack the airspace in the existing roof, eliminating entirely the need for ventilation? Foam-Fill? Could I have foam “poured” into the roof cavities to fill the airspace as an alternative to Dense Pack? Nail-Base? If I dense pack the airspace in the existing rafters, would I then need to add a 1.5-2.5″ inch thick nail base to the TOP of the roof as the final Could I add a Nail Base Layer on top of the existing roof to avoid excessive condensation in the dense pack layer? One More Detail – New Metal Roof? We *might* put a metal roof on this house to replace the existing asphalt shingles… Does this offer us any additional ideas we might purpose?  For example, would installing purlins / airspace under a new metal roof offer us any potential benefits as part of an overall solution to my challenge? OK everyone, what do you think! Thanks!

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  1. Expert Member
    Akos | | #1

    If the gabled addition has a flat ceiling, you can use the gable vents of the addition to feed the existing vent channels. All you would have to do is cut some holes in the roof sheeting in the original roof there to connect the two.

    If it is a cathedral ceiling, it would be a bit more challenging. The simples would be to cross strap the new rafters with horizontal 2x2s on top while keeping the vent channel between the rafters. This allows for air flow in two directions allowing for vent flow through the rafters bays that would otherwise be blocked.

    A further option it to do nothing and see how things are a couple years down the road. Since your ceiling is well sealed with SPF, there is a good chance that the ridge vent is enough to keep moisture at bay without any intake vents.

    1. bradesp | | #3

      One other comment / question... Not sure I visually understand your description of "cross strapping the new rafters with horizontal 2x2's"

      1. Expert Member
        Akos | | #4

        Normally with a lumber roof, your vent channel is formed in the space between the two rafters, sheathing and insulation.

        For this type of venting to work, you need this channel to be continuous from soffit to ridge, so it only really works well for a gabled roof.

        To fix this issue, you would install 2x2 above and perpendicular to the rafters but still keep the vent gap between the rafters. This now allows air to flow not only along the rafter but also across them, which means you can vent more complicated roof shapes such as the valley area in your case.

        You would still have to connect this vent channel to your old roof as well, so you have a couple of areas to watch there that you don't block vent flow.

        In your case, I think the simpler approach would be to go with scissor trusses and a combination of soffit, ridge and gable vents. This would let you simply connect the old roof to this new attic space but still give you the cathedral ceiling you want. It would also eliminate the ridge beam and associated support. As long as you keep the ceiling side slope bellow 4:12 you can also loose fill the attic for cheap high R value roof. Just make sure to install sufficient baffles that the insulation won't block your soffit intakes.

        1. bradesp | | #6

          Hi @Akos, I'm trying to visualize your suggested approach... Any visuals that might help me better understand are appreciated!

  2. bradesp | | #2

    Interesting idea regarding venting the roof into the roof of the connecting addition... We are 75% leaning towards a cathedral ceiling in the new addition thought... so venting into the new roof structure might not be an option.

    Waiting doesn't seem like the right thing to do because the larger current roof will be inaccessible once we build the new roof on top of the existing roof.

  3. bradesp | | #5

    Any other ideas / suggestions?

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