Hello, first post here. I apologize in advance for the lengthy post. My name is Jeremy, I own a remodeling company in NJ and I am working on a sunroom project. I belong to JLC and FH and have tried to search but I can’t come up with much. Normally we have our arch or eng draw things up from scratch, but in this case we are trying to come to them with the details we are looking to achieve, then have them pick it up from there.

I am working on designing a 26W x 20D deck, with a “sunroom” (really an enclosed porch) sitting atop it at 12×20, with the remaining 14×20 being open deck. The front gable wall, and side wall of the room will be sitting flush with the end of deck. I have attached a rendering for a visual. Everything is to scale except the deck height to grade, which will be about 5ft at sunroom side of deck.

I designed the deck framing to be a continuous frame with (2) 2ply 2×10 girders running parallel to the home, and the joists running out and cantilevering over the front beam 12″ Standard deck frame, or at least what I am used to. I planned to do the joists under the open deck 12″ OC for the PVC decking, and 16″ OC under the sunroom since that will be getting a subfloor and finished flooring. I planned on doubling or tripling the joists that would directly under the sunroom wall framing. I have also attached a screenshot of my deck framing plan. 

The sunroom portion of deck will be insulated from underneath and airsealed, either sprayfoam or rigid foam between joists + sprayfoam to air seal (think rim joist insulation) and finally sheathed with 7/16″ ZIP as a solid soffit. I will be using ZIP on the walls and the roof and it will be a cathedral ceiling. There will also be solid skirting around, with 3/4″ stone gravel on grade under the entire deck. Wall framing will be 2×4 with 3.5×9.5 LVL headers on all 3 sides. Roof to be 2×8.

This will be a conditioned sunroom, with a Daikin MiniSplit unit. Windows are Andersen 400 series DH, except the gable glass which will be made custom to fit by my glass contractor, which will be insulated and made to match the 400s. The goal of this room is to be an extremely well built, comfortable and functional space all year round, providing an area to escape the heat in the summer during parties and BBQ’s, a place to hang out and enjoy the snow in the winter (think X-mas tree against the gable wall, with snow on the trees in the backdrop, beautiful). I also want to build this to my highest potential (within budget), maximizing dollars in areas that make a difference, to up my game as a professional, and to showcase to future clients what our company is capable of.

There are multiple questions I have been mulling over in my head for a while in preparation for this project, and I’m sure some of these topics can be discussed ad nauseam so bear with me…we are starting this soon so I need to finalize my decisions and I am hoping the fine people of FH can help me out:

1). Should I frame this as 2 “separate” decks, keeping the sunroom deck separate from the open deck? One thought is I worry about the movement of the PT lumber (more on this below) and keeping them separate at least in my head may help with this? Secondly, I can then build the sunroom deck level while slightly pitching my open deck away and not worry any runoff issues.

1a.) IF SO, should I be changing the joist direction under the sunroom? This would eliminate the 2ply or 3ply joists I planned on under the sunroom walls, and I imagine provide better bearing support for the longer side walls? 

2.) Regardless if I separate the deck framing or not…taking into consideration my plan for conditioning the space, skirting around deck, adding a gravel base, air sealing, and insulating well, etc….Would I be able to change the joists under the sunroom from PT to DF, or even I-joists? I am hesitant to use PT because we want to tile the floor in the sunroom and I see this being catastrophic between the movement and inconsistency of PT lumber.

3.) I have only created a few cathedral ceilings in the past during some renovations. They looked great but we had to be cautious with insulation and ventilation. We used foam baffles, fiberglass batts, and added soffit and ridge venting. In one case we had moisture build up in the ceiling, needed to create even more venting in soffit. However, in this case, I want a higher R-Value and also do not want any venting through the ceiling. I plan to sprayfoam the sunroom walls and ceiling, or rigid foam between bays and air seal around (again, think rim joist insulation). I plan to air seal my sheathing around the rafter tails with a liquid flashing, build solid soffits and air seal there as well. All this being said, so long as I air seal properly, I should not have any issues with moisture in the ceiling correct?

Thank you for reading and I appreciate any feedback!