Raising and insulating floor for four-season room
(Edited to correct some of my numbers and add some diagrams)
We are planning to turn our screened, roof-covered, screened porch (approx. 22’ x 14’) into a four-season room. I would love to hear people’s thoughts on our plans for the different areas of the project. Yet, here, I will address only my doubts about the floor.
ANY feedback, even if not to all of my questions, would be very much appreciated!
The floor will need to be framed, as the current floor (a tiled cement slab) needs to be raised, by approx. 9 inches, to the same level as the adjacent kitchen (currently, there is a 5-foot double, French door between the kitchen and the porch). The floor will be a continuation of the kitchen’s hardwood floor.
For the framing and insulation of the floor, I’d like to follow some of the suggestions given by Steven Bazcek in the Journal of Light Construction (I can give the more precise reference if needed). However, there, Bazcek gives suggestions for a floor that needs to be raised by 5 inches in total. His scheme is:
– a 4-mil cross-linked polyethylene wrap covering the existing cement floor (tiled over cement in my case);
– 2-inch x 4-inch sleepers
– 1 1/2-inch EPS foam board between the sleepers
– 2-inch EPS foam board
– 3/4-inch subfloor on top of the rigid insulation, driving 3 1/2-inch screws through the 2-inch rigid insulation and into the sleepers. The diagram enclosed with the article shows two screws, side by side, which of course work very well when joining two sheets of subfloor.
The questions I am struggling with are quite a few unfortunately:
1) The sleepers option, with 2-inch x 4-inch boards, does not seem viable for me, as I have 4 more inches to make up for, in height. I am not sure whether having 8-inch x 2-inch joists (hence placed vertically) can work, as such joists would be only 1 1/2-inch thick, hence perhaps too narrow to receive two screws side by side, to attach the subfloor. Would using even lower joists, 6-inch x 2-inch (sistered to make them stronger?), and screw 2×4 sleepers on top of the joists, forming a T, be structurally sound? Any other ideas I should contemplate, which would not be cost prohibitive? Incidentally, as the existing floor is slightly sloped down away from the house (by approx. 1 inch), the joists will have to be ripped.
2) For sleepers/joists, should I use pressure-treated wood? Should that be used at least at the perimeter, for the three sides at the outer walls?
3) If any of the framing touching the floor will be in the form of sleepers, I read that, even for a 14-foot length, it would be structurally better to off-set them; that is, to have a 6-foot board from each facing wall, and a 6- or 8-foot board joining and overlapping them in the middle. Is that really better?
4) Steven Bazcek suggests using EPS boards. Yet, in storage I have five brand new 2-inch boards of XPS (pink ones, Owens Corning I think), which I bought years ago. So, it might make sense to just buy five more and use XPS. The compressive strength, if anything, is better. So, using XPS instead of EPS should be OK, right? I know that unfortunately XPS is so much worse for the environment, and I will try to get salvaged boards if possible. Incidentally, Bazcek’s article does not mention whether the board that sits on the sleepers and is then covered by the subfloor should be taped at the seams; yet, that would be good practice I imagine; right?
5) Still on insulation, I should be able to adapt Bazcek’s setup so as to use open-cell spray foam between the rafters. I am thinking of having 4 inches of spray foam between the joists, and then the 2-inch XPS foam board. Are there any drawbacks of doing so? (Incidentally, I am not considering rock wool, which would have even better R value, for any parts of this project because we have too many mice in the area, and anything that is not foam would be way too risky I fear. The current plan is that of using open-cell spray foam in the knee wall which will need to be built, and the ceiling, though we also want to use polyiso boards around the whole structure and, inside, on the ceiling.)
6) Following something like the above approach, I should get to approximately R-35 (indeed, likely a little more) for the floor. Will that be enough for zone 5?
I should mention that, when looking at posts about raising floors by 10 inches or so, I have encountered mostly posts that recommend pouring an additional concrete slab over an EPS board, and install sleepers over the new slab. To me and for my project, it seems a pity to fill with concrete what could be filled with insulation. Yet, the raised floor must be structurally sound, not squeak, etc. So, if the above-described approach is not what I should pursue, I’d love to hear folks’ ideas even if they are very critical.
I am adding a couple of diagrams, one found on GBA, the other from the JLC article. They well illustrate the type of approach I would like to apply to this project.
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