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Community and Q&A

Cape Cod roof insulation question

andy6572 | Posted in Energy Efficiency and Durability on


I have a 1950s Cape Cod house in New York Zone 5/6. It has 2×6 rafters. Poorly insulated. The plan is to ultimately put 4” of foam board on top of the roof deck once we get a new roof. That won’t be for another ~10 years, however. 

In the meantime, I am interested in trying the following:
1) Convert the currently vented roof into an unvented roof.

2) The accessible attic slopes would be spray foamed with 2” of closed cell foam. This would be followed with as much fiber insulation as possible.

3) The small portions of plastered and therefore inaccessible slopes would be dense packed with cellulose (I know this isn’t best practice). 

I just wanted to make sure this assembly is OK, assuming that the primary goal is to put foam board on top as well. Also curious if you think I should just hold off until the roof needs to be replaced. 


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  1. BSBot | | #1

    Alright, Andy, let's cut straight to the chase with your 1950s Cape Cod in the brisk zone of New York's 5/6. You've got ambitions that are a bit ahead of your roof's expiration date, aiming to beef up insulation now and slap on some serious foam board armor in about a decade. Let's navigate this without ending up in moisture mayhem or wasting your hard-earned dollars on something you'll have to rip out later.

    1. Switching to an Unvented Roof Now:
    Going unvented is like deciding you're done with those drafty New York winters messing with your attic. Putting down 2 inches of closed-cell foam directly on those rafters? Good move. It's like wearing a waterproof, windproof jacket right against your skin – it stops the moisture and air right where they start. But remember, this jacket needs to fit perfectly – no gaps, no leaks.

    2. Adding Fiber Insulation on Top:
    Piling on fiber insulation over the closed-cell foam is like throwing on a down vest over that jacket. More warmth, less bulk. But here's the kicker – that closed-cell foam is doing all the heavy lifting, keeping the air and moisture at bay, so make sure it's sealed tight.

    3. Dealing with Those Plastered Slopes:
    Dense packing cellulose in areas you can't easily get to with spray foam? It's not what the textbooks recommend, but we're not always playing in ideal conditions, are we? It's a bit like patching a hole in your boot with duct tape. Not pretty, but it'll keep you dry for the short hike home.

    4. The Grand Plan of Exterior Foam Board:
    Eyeing that exterior foam board for the future is thinking big – I like it. It's your long-term play for cutting those heating bills down to size and giving ice dams the cold shoulder for good. Just be aware, slapping on 4 inches of foam board atop your roof deck is no small feat. It's a major overhaul, so everything you do now should be with an eye on that prize.

    5. To Act Now or Wait?
    Here's the deal – waiting a decade with lousy insulation is like sitting on a thorn bush and hoping it'll turn into a throne. Not going to happen. Your plan to upgrade now, even if it's not the full monty, is solid. It sets you up for a warmer, more efficient home in the interim and dovetails into your bigger vision down the road.

    Closing Thought:
    Andy, your game plan has its heart in the right place. Just ensure that closed-cell foam is applied with the precision of a Swiss watch and that any fibrous insulation you add doesn't mess with your moisture strategy. This way, when it's time to crown your home with that exterior foam board, you're not backtracking, you're just continuing the march towards an energy-efficient fortress. Remember, the key to this whole operation is managing moisture – do it right, and you'll be sitting pretty when it's time for the big overhaul.

  2. nynick | | #2

    I think 4 inches of foam board down the road is overkill. 2 inches is plenty with everything else you're doing.

  3. walta100 | | #3

    Have you read these three articles?

    It is almost impossible to retrofit the blocking under the floor in an air tight manor.

    The way I see it most story and a half homes leak so much air insulating them is pointless until you stop the air flow so concentrate your efforts on air sealing.


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