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Turning three-seasons room into full-time living space

Evharris7 | Posted in General Questions on

I have read a lot of articles on this site, and the amount of knowledge is fantastic.

What it currently is: Zone 6 southwest facing 15FT X 17FT enclosed 3  seasons room built on top of a section of deck(~8ft off the grade) in the late 1990’s, when we purchased the home two years ago, we knew this

area has a roof leak that was getting into an area of the wall. It is closed off from the main house by a glass slider. We got to the point of opening everything up to inspect it this spring and are getting ready to dig in. It is a 3/12 pitch shed roof terminated into a vertical wall on the lower area of second story of the main house. It has asphalt shingles, 15/32 sheathing, 2×8 rafters, unvented and uninsulated currently, it had caulked and latex painted plywood on the inside roof for the ceiling, and it was a heated space. The room had multiple issues that cause the roof failure in my opinion (heating this roof setup, terminating a gutter from the second story onto the roof surface, and installing the flashing at the wall joint under the shingles). I am amazed it lasted this long to be honest. The walls are 2×4 construction, T1-11 siding on the outside to the studs, foam board insulation, and wood paneling on the inside. The floor is over a “shed space”, it is enclosed with T1-11 siding on 3 sides, the 4 th side is the basement block wall for the foundation and a
dirt floor. They put foil backed poly insulation board under the joist’s(joints not taped).
The goal: To bring the room up to code to be an inhabitable space and remove the sliding door in between this room and the rest of the house (1984 Chalet style home, well constructed 2×6 outer walls, proper insulation and roof venting).
The plan: Redoing shingles, any rotten sheathing, Ice and water shield on edge, tar paper higher up, soffet vent, and flashing vent at the top wall joint (Lomanco OW-4 or similar), with site build baffels out of 2”
R13 RMAX Thermasheath spaced 1” below roof sheathing connecting top and bottom vents and seal the edges against rafters with great foam. I have the space to add 2×6 to the bottom of the rafters, so I plan on putting R38 compressed fiberglass batting in to get R49, and then finish the ceiling with gypsum drywall.

Walls: Add 2” to the inside of all framing in the room to make it 2×6, and put R21 fiberglass batting inside, and board and batton wood on the inside. Side note the house was built before wrap was standard, we will be residing in a couple years, the plan is to wrap(Tyvex or similar) over the T1-11 and
then install Board and Batton Siding over the old T1-11. There is currently no sheathing on the house, just T1-11 to Studs on whole house.

Flooring: There is currently a 5” stepdown into this room. We want to raise it up to be level with the rest of the house for continuous flooring. I believe we need R30 in the floor if it is “cold/outside exposed” below. This is where I am open to options, it is currently subfloor, pressure treated deck boards, and
joists. Subfloor needs to come up anyway and be replaced, I am open to leaving or ripping up the deck boards, and building on top of joists to get correct height. Is this better to be rigid foam board insulation,
or fiberglass batting?
One other thing is we plan on installing a wood burning fireplace insert into this room for the winter months. (Not sure if this is a factor at all)

Is there anything in there that is a glaring mistake, I am hoping to not have to open anything back up in the next 10-15 years due to moisture?

I would really appreciate any critiques or insights. Sorry for the long post…..


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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Evan,

    Not seeing anything inherently wrong with the plan for your ceiling or walls, except that you didn't make mention of any interior air barriers, which both should have in your situation, and there are many options for. For example, you could detail a variable-perm membrane like Intello as the air barrier, which would also give you some vapor control, or you could install airtight drywall on the ceiling and walls, before the paneling.

    Also, if you are deepening the walls studs, considering using strips of rigid foam to help mitigate thermal bridging, like this: Breaking the Thermal Bridge.

    As far as the floor goes, I suggest you read this article: Insulating a Wood-Framed Floor Assembly.

  2. Walter Ahlgrim | | #2

    Before you get to deep into this project have a look at the foundation. All too often the foundation built under a deck is just enough for a deck. The next owner sticks a roof over the deck. The third owner replaces the screens with windows and finely someone else someone wants to add insulate and add HVAC and it is just to much.

    Given that the current roof is very low slope 3/12 and that you want an R38 ceiling I doubt you will have enough headroom to remove the step down.


    1. Evharris7 | | #3

      I did check that first, they are 6x6 into concrete that is at least 36” deep(I stopped digging at 36”, it seemed like it was still going) I will check the calculations, but I know portions of the deck were redone in ~2005, and this matches those posts. There were some old sections that are still 4x4 in another area by the stairs on other side of the house. Still worth double checking the load calculations, but I also have permits from when they did it that were included with the house.

      I thought about going unvented and 4” of foam on top of the roof deck (R25/R25 Fiberglass below sheathing) but venting seemed better.

      1. Expert Member
        Akos | | #4

        Going unvented with 4" of rigid above the roof deck would let you meet code on U factor bases which means a whole assembly R value of R38. You can then just insulate the ceiling with R19 batts and not have to worry about furring down your rafters. You can probably even go down to 3.5" of foam. Run your roof through the calculator here to see what you need:

        This would give you much more headroom bellow and would preform better than your R49 roof because of less thermal bridging from the rafters.

        1. Evharris7 | | #5


          Thank you for the input, the ceiling will be dry walled as an internal air barrier, the walls will either be dry walled under or Intellio will be used. I have changed my roof plan, and will be doing 4” of foam (~R26) on top of the sheathing, and R28 in between the rafters which should hopefully be close enough to the 50/50 to keep the sheathing from getting condensation. The one thing I am unclear about is the air barrier some say to put over the sheathing, but under the foam. Is just taping the seams of the sheathing enough to be an air barrier? Or do I need another product? Then do something like Gracie Ice and Water around the perimeter edges on top of the final sheathing(over the foam) and wrap it around the edges of the foam and bottom sheathing? Then felt the rest and shingle over?

          1. Expert Member
            Akos | | #6

            Taped plywood is an excellent air barrier. The one area you'll have to do a bit more detailing is around the perimiter on the inside. You want to run a bead of caulk around the edge to seal the deck to the top plates. Much easier than trying to wrap peel and stick around your overhangs.

            Your drywall will be your primary air barrier, so you don't have to go crazy on these details, just do your best to seal it up.

            Felt is fine under the shingles, you might want a strip of I&W by the gutters.

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