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Community and Q&A

Replacing an Unvented Low-Slope Roof

jimmidatlantic | Posted in GBA Pro Help on

Hello GBA Community-

I have a mid-century ranch house with a Japanese-inspired roof.  It is long and relatively low-sloped 4/12 (?) with deep eaves and hip ends with large slated vented gables; there are no soffit vents.  The prevailing winds are the same orientation as the gable vents.  We are in the DC suburbs mixed-humid weather (climate zone 4A) with lots of rain storms, and every few years some large snow storms.  The roof on the northern side keeps its snow without melting and no icicles and the southern side melts completely dry from the sun almost immediately.  Summer is hot and humid.

We have two quotes for a new roof — one with a rigid Cor-A-Vent that slips under the shingles with a continuous cut in the plywood sheathing to provide venting to the attic, and the other replaces as-is with no soffit vents.

16+ years ago, a new roof was installed with a ridge vent and the former owners blew in a good amount of cellulose insulation into the attic.  When we purchased the house in 2008, the first thing we did, which now seems regrettable after reading Lstiburek’s Rules for Venting Roofs, was to install recessed lights absolutely everywhere – kitchen, dining room, living room, hallway, bathrooms, bedrooms, and art lighting – which are all on the main level directly below the attic.  There are metal boxes to separate the lights from the cellulose insulation, but our electrician said they were probably not air-tight.

Here are a few other considerations:
* It seems excessively hot in the attic in summer (but it’s DC – I’ve not taken a temp).
* Most of the plywood will need to be replaced as it is soft/spongy and from the outside, you can see the outlines of the roof trusses under the shingles.
* There are no visible leaks inside or evidence of any leaking.
* The soffits and the T1-11 siding is redwood plywood.  A vinyl or plastic continuous soffit would not look good.  In surveying the neighborhood with these similar style houses, there are no soffit vents.

Thanks for your patience through the long question…
Any advice on what to do?

Thank you.

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Replies

  1. walta100 | | #1

    I think the most important thing to do is make your ceiling air tight, this will keep the warm moist air from escaping the living space and entering the cold attic where the moisture becomes a liquid when it hit the roof deck leading to the spongy feel and failing deck.

    I am unconcerned by the attic temperatures in the summer as you should have at least R38 insulation. The temperature differential across the insulation is much smaller in the summer than the winter. The attic might get to 130°in the summer and the house is 75° or so giving you a 55° difference. In the winter you could see -20° with the house at 68° or so giving you an 88° difference. Even the best venting is not going to really change the attic temp much.

    A vent something like the one in this photo would almost be invisible when painted

    Walta

  2. jimmidatlantic | | #2

    Thank you for responding, I really appreciate the advice.

    To recap:
    The moisture is the issue.

    Question: Is it escaping out the top with the ridge vent and gable, or working so poorly that it's likely hitting the cold deck in winter and doing damage? Or the venting doesn't matter as much as eliminating escaping moisture?

    We can attempt to cover up the IC boxes in the ceiling with additional drywall boxes or see what it will cost to replace them, but it will take a while to take care of those 30 openings, digging through the insulation, etc. and will not be an easy job.

    If we don't close up the ceiling penetrations immediately, then would the soffit vents do much to help carry moisture out the top, or would it just hit the decking on this fairly low-sloped roof?

    I'm not crazy about a continuous soffit, but I suppose it's an option, although there are a few bump-outs from the original design at the garage and master bedroom that take the wall right up to the edge of the eave.

    Or

    Is the shingle vent (with a cut in the roof deck) still an option? This seems like a risk for leaks, especially if warm air is still escaping into the attic, melting snow. We might get away with a mild winter, but one big snowstorm could wreck the roof with an ice dam backing up into these vents.

    or

    Can we put in individual soffit vents every so often, based on the calculated value (1/150, 1/300- whatever - less the louvers/screen - for the net) like six louvered 16x8" vents on each side of the house? Of course, we'd still need to go up and investigate if the insulation is properly baffled, and if not, fix that too.

    Just figuring out my options. Yikes, I thought getting a new roof was a routine slam-dunk...

    Thank you again, you've been very helpful.

  3. mr_reference_Hugh | | #3

    Jimmidatlantic

    Here are a few points to consider.

    1. You mentioned “ It is long and relatively low-sloped 4/12 (?) with deep eaves and hip ends”

    My comment: I was reading on the COR-A-VENT website that they have a specific model of vent to be used on low slope hips. I suggest that you read this Cor-A-Vent webpage and check with the installers that they are using the right product for the hips.
    Read more here:
    https://www.cor-a-vent.com/v300.cfm
    They also have a 1800 number plastered all over their website if you want to call them.

    2. You mentioned “ * The soffits and the T1-11 siding is redwood plywood. A vinyl or plastic continuous soffit would not look good. In surveying the neighborhood with these similar style houses, there are no soffit vents.”

    Response: These days there’s are so many alternatives for soffit vents. Searching online you should find a product that fits your home’s architectural style.
    See 2 examples here:
    https://lpcorp.com/resources/product-literature/installation-instructions/lp-smartside-soffit
    AND
    https://www.jameshardie.ca/products/hardiesoffit-panels

    3. I agree with - almost - everything walta100 said and if you read a related GBA Q&A you will see that Martin Holladay agrees -fully- with walta100. (See link below)

    Where I disagree: I would not use drywall boxes to cover the recessed lighting metal boxes. Why? because drywall will only stop the airflow (if they are fully sealed) but they can’t stop the movement of humidity into the attic. Drywall is not a vapour barrier and most drywall is prone to mould. Martin Holladay does suggest rigid foam instead or drywall. The right type and thickness of rigid foam would create air and vapour barrier.

    I have seen where some builders build boxes using Huber ZIP OSB. But all these boxes need to be sealed air tight regardless.

    Note that yet another contributor to the linked GBA Q&A said this “ They either cover the light with a foam box or wrap it in batts, and then shoot about 2" of [spray] foam (…), over the fixtures,” Using spray foam as an air and vapour seal (NOT from a small can please) is a solution that would be easier than building and installing 30 individual boxes.
    Read Martin’s and other response to related Q&A here: https://www.greenbuildingadvisor.com/question/recessed-can-lighting-airtight-housings-airtight-baffle-trim-or-air-seal-from-attic-with-boxes

    4. I would suggest reading this pamphlet I found on the COR-A-VENT website. It explains how those vents work. It basically tells you what type of soffit vent is required for the Cor-a-vent to work as designed. Super important.
    Read more here:
    https://www.cor-a-vent.com/uploaded_files/files/Tech%20Drawings%20PDF/balancedventilation.pdf?1429120540222

    5. You asked:
    Question: Is it escaping out the top with the ridge vent and gable, or working so poorly that it's likely hitting the cold deck in winter and doing damage? Or the venting doesn't matter as much as eliminating escaping moisture?

    Response: Both venting and moisture control are critical.
    1st Venting - venting gets rid of heat escaping into the attic through the insulated ceiling. To have heat move from living spaces into the attic is normal and venting is the solution.

    Of course, venting will carry away some humidity but the humidity moving from the warm interior spaces through the recessed lighting boxes will tend to condense when it hits the cold winter air in the attic. It can condense on the the underside of the roof deck or anywhere else in the attic.

    There is of course humidity in the cold winter air that comes in through soffit vents but that is another topic and it is not a problem with a properly vented roof.

    6. You asked:
    If we don't close up the ceiling penetrations immediately, then would the soffit vents do much to help carry moisture out the top, or would it just hit the decking on this fairly low-sloped roof?

    Response: Yes soffit vents are important regardless, perhaps more important to improve air flow and move more moisture outside faster. But you still should seal those lights eventually (reference Lstiburek’s Rules). Again, Google the most recent options for nicer looking soffit vents.

    Also think about this. Rights now. Your ridge vent allows a minimum of air to escape from the attic (but we think it is not enough). How does the air that is escaping get replaced in the attic? I expect that the air that is escaping the attic is in part being sucked up from the living spaces through all kinds of gaps and holes you can’t see it the ceiling…. And I’d course likely through the 30 recessed lighting boxes.

    7. You asked
    Is the shingle vent (with a cut in the roof deck) still an option? This seems like a risk for leaks, especially if warm air is still escaping into the attic, melting snow. We might get away with a mild winter, but one big snowstorm could wreck the roof with an ice dam backing up into these vents.

    Response: if you are currently getting ice dams, your attic venting is almost definitely not adequate. If you need to replace the roof decking, it is likely a pretty serious venting issue.

    You would need to have some sort of venting at the soffit and venting at the roof ridge (or using something other types of vents near the top of the roof). You should read about ridge vents a little more. In my past reading, I understood that they work best when prevailing winds move perpendicular to the roof ridge. In central/Eastern Canada, we use lots of vents called Maximum, combined with soffit vents. Link here
    https://ventilation-maximum.com/en/

    I can’t speak to the design of Cor-A-Vent but I can’t see why they would leak if you have soffit vents and stop the ice damming.

    8. You asked

    Can we put in individual soffit vents every so often, based on the calculated value (1/150, 1/300- whatever - less the louvers/screen - for the net) like six louvered 16x8" vents on each side of the house? Of course, we'd still need to go up and investigate if the insulation is properly baffled, and if not, fix that too.
    Response: it depends on the exact product you are talking about. I would contact the manufacturer and ask them if they think it would work. You only need a certain amount of air coming in from the soffit so you do need to hit that number. I still like the LP soffit vents myself. It would provide a seamless look on all the soffit and would be more visually appealing that multiple soffit vents but cost/budget and personal preference is going to rule the day if the amount of venting is the same for both products.

    9. Are there other vent options?
    Response: yes. The GBA article (link below) mentions “ There are many types of roof vents on the market: louvered vents for gable ends, soffit vents, continuous ridge vents, mid-roof vents designed for buildings that don’t have soffits, vents for hip roofs, (…)”
    Link:
    https://www.greenbuildingadvisor.com/article/a-vent-for-every-roof

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