Replies

1. 
   Malcolm_Taylor | Aug 27, 2024 03:58pm | #1

   James,

   There are two types of roofs: vented and un-vented. With the exception of zones 1, 2 and 3, (you are in climate zone 4 marine, or 5), if you remove the venting you can not use fiberglass batts in the bays, you would have to use spray foam or foam board.

   This article shows five common roof assemblies . I would choose one of them:
   https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work

1. 
   jamesSeattle | Aug 29, 2024 09:46pm | #2

   I am trying to minimize upfront cost for an over budget project. My understanding for the code is that it is designed to save power and the amount of insulation needed to prevent condensation and ice dams is much lower. Do you know what those values are?

1. 
   Malcolm_Taylor | Aug 30, 2024 01:22pm | #4

   James,

   There is a huge difference in how un-vented roofs which have to rely on impermeable insulation, and vented ones which use batts or cellulose work.

   Vented roofs have no minimum amounts of insulation necessary to prevent condensation. As long as it was air-sealed, a completely uninsulated roof would not experience any moisture problems.

   If you close the vents you cannot use batts or cellulose, as moist interior air will move through that permeable insulation and accumulate in the sheathing with no way of drying. Un-vented attics rely on spray foam or foam board to stop that moisture movement, and that layer is what needs to be thick enough to stop moisture accumulating on it's inner face.

   So the first choice you need to make is whether to go vented or un-vented.

   If you keep the vents and use batts under proper baffles, you can then decide whether you are content with the amount the rafters can contain, or whether you want to fur them down to up the R-value.

   If you remove the vents you will need to install a layer of spray foam, or foam board against the sheathing, and can them fill the rest of the cavity with batts. The thickness of foam necessary is based on your climate zone and the ratio of foam to the batt insulation you use.

1. 
   jamesSeattle | Aug 30, 2024 02:40pm | #5

   Thank you for the thorough answers

2. 
   jamesSeattle | Aug 30, 2024 02:55pm | #7

   One more question - is it worthwhile to install 0.5-1" of rigid foam board behind the drywall to stop thermal bridging? Or would this cause condensation issues in that space?

   I don't want to scab on 2x4 to the rafters due to space limitations, work, and figuring out how to engineer them on. But that only leaves me with 1.5" air gap and 6" of insulation space in the rafter bays, which is R20-R36 depending on materials used. I know thermal bridging can make a huge difference so I'm wondering if that extra 0.5-1" of board would be worthwhile investment, even if I don't hit the code recommend amount of insulation.

1. 
   Malcolm_Taylor | Aug 30, 2024 05:23pm | #8

   James,

   It it's a vented roof them yes .5" to 1" foam on the interior is a good idea and works well. If it's an un-vented one then it can end up with no drying path either to the inside or out.

2. 
   Uiloco | Aug 30, 2024 08:20am | #3

   Definitely keep the baffles! I went through this when I converted my attic. Even if the plastic baffles don’t cover everything, they’re still important for airflow. I tried sealing the vents once and ended up with condensation issues. Using foam and R30 fiberglass should help, but good venting is key to avoid problems down the road. Trust me, it’s worth the extra effort to keep things dry and mold-free.

1. 
   jamesSeattle | Aug 30, 2024 02:41pm | #6

   Thanks for sharing your experience