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Converting porch to living space — insulation plan

humm9er | Posted in Energy Efficiency and Durability on


Zone 5. 

I have a 13×26 porch with glass sliders that’s on 6×6 posts about 18″ – 40″ off ground (ground slopes away from house) connected by glass doors to main living area. Roof is unvented cathedral ceiling, open to porch below. Currently about 6″ of soffit overhang on exterior, so not a ton of room to build out exterior with rigid foam if I want to preserve roof overhang.

We want to turn it into conditioned space (4 season) with 14 windows (sigh), and I have some questions on my insulation plan. I have read the FHB article on insulating a cold floor.

So, here are my ideas and questions:

Ceiling is currently cathedral, uninsulated with 2×8 framing. Only real option is closed cell spray foam to meet code is my understanding — correct? and is 7.25″ sufficient?

Walls are currently 6×6 posts. Option 1: 2×6 framing, with fiberglass bats, 1″ rigid foam on exterior, zip system. Option 2: 2×4 framing flush with interior side of 6×6 posts, fiberglass bats, 2″ rigid foam on exterior, zip system (this will leave exterior of my 6 6×6 posts flush with zip, and not insulated). Or another option?

Floor is currently 1×4 PT decking on 2×10 joists. Option 1: 3/8″ ply subfloor over existing PT, 4″ rigid foam underneath PT in joist bays, then 2-3″ of closed cell spray foam (I realize this will leave some thermal bridging of joists). Option 2: remove PT floor (a lot more labor), r-30 unfaced fiberglass bats between joists, 3″ thermax across bottom of joists.

Thank you in advance for your feedback!



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  1. aaronbeckworth | | #1


    I’d consider a slightly different approach.

    Starting with the floor, consider how leaving or removing the PT decking will affect the height of the finished flooring relative to the adjacent room(s).

    Then 2x6 framing between the posts and cut back the rafters flush with the framing. Continue the sheathing up the walls and onto the roof and tape seams for a continuous rigid air barrier. Also continue the sheathing below the floor assembly to protect the riding foam and detail as the air barrier. Then install the correct thickness of rigid foam over walls and roof for your climate zone.

    Finally, add secondary roof framing over the foam (vented “cold roof”) with rafterlength designed to shade the windows during the summer months. Of course this idea could be more complicated depending on if and how the current porch roof ties into the house roof.

    Of course there may be details/constraint that would lead you to other options.

  2. Expert Member
    Dana Dorsett | | #2

    If it's possible to re-roof adding 4.5-5" of height above the roof deck, adding 4" of exterior polyiso and installing /R30 rock wool in in the 2x8 rafter bays is cheaper than 7" of HFO-blown closed cell foam ( and higher performance too). In zone 5 it only takes R20 on the exterior to have sufficient dew point control at the roof deck with R30 cavity fill at the roof deck in zone 5. Derating polyiso to R5/ inch for temperature and age is safe, and used roofing polyiso is ubiquitous & cheap.

    7" of HFO blown foam in my area cost about $9 per square foot. In my area used 2" roofing polyiso is typically $10-15 per 4'x8' sheet, worst-case call it a $1 per square foot for the polyiso, a $1.5 /ft^2 for box-store R30 rock wool, plus a nailer deck, some flashing, #30 felt & shingles, and a facia board, it'll still be less than just the HFO foam.

    For the walls, ZIP is about the most expensive options. A 2x4/R15 rock wool + 2' reclaimed polyiso solution would be cheaper and higher performance. Performance-wise setting it up with the 2" polyiso on the exterior side of the 6x6 posts would be preferable to setting it up with the foam flush with the posts. Since the 2x4 wall isn't structural, 24" o.c. spacing would be fine.

    On the floor, if there is already bottom sheathing blowing it full of cellulose or 1lb fiberglass would be enough, performing at about R32-R35, or even low density R38 batts compressed would hit R32. Rigid foam between joists is a waste of good foam, since it's substantially robbed of performance by the thermal bridging. If putting continuous foam underneath the bottom sheathing ideally it would be at least 25% of the total-R , 28% even better for dew point control. It would be fine to use 2" of foil faced polyiso or 2.5" of fiber faced polyiso there, roughly doubling the R-value of the framing fraction.

  3. humm9er | | #3


    Thank you both for your input!

    Porch floor is about 8" lower than main house. There will be a stepdown no matter what, so leaving or removing PT won't change that aspect.

    I am reluctant to's a hip roof off the north side of house, raising it would affect roof proximity to existing house windows, and the shingles are relatively new. I hear you on cost of spray foam. Aside from cost, any pitfalls with spray foam? Alternatively, I suppose I could remove ridge shingles, add ridge vent, install soffit vents, frame a lower ceiling, and create an attic to insulate with cellulose? Increases need for ceiling air-sealing perfection to limit air leaks.

    For the walls, if I frame flush to 6x6 with exterior and add 2" of foam, and dont change my roof, wont that leave me too little overhang? Also, how would I fur out the 2" interior space that would leave me between walls and 6x6? Or I suppose I could just box-in bump-outs around posts...

    There is no bottom sheathing on floor at present. I suppose I could add polyiso across bottom of joists, airseal, then sheath (or use thermax xarmor...tough enough to leave as exterior sheathing on floor bottom?), then remove several PT boards and blow in cellulose?

    I plan to trim out the entire exterior with PVC (paneling-style) and use no wood, any need for a rain screen?


  4. humm9er | | #4

    Ok, I've taken everyone's comments and done more research and thinking.

    How does this sound?

    Ceiling/underside of roof: closed cell spray foam to r-49. Raising the roof deck is not in the cards for me. This doesnt solve thermal bridging of roof rafters, however...I guess life is full of compromise?

    Floor: 2" dow thermax xarmor across bottom of 2x10 floor joists, carefully airsealed with foam and tape. r30" unfaced batts or cellulose between joists, then subfloor.
    - Floor bottom is 2-3.5' feet of you think Xarmor is safe exposed (animals etc), or should I sheath with ply?

    Walls inside to outside: 2x4 framing flush to interior of 6x6 posts, fiberglass or rockwool between studs, then 1/2" ply sheathing, then 2" rigid foam (EPS or Polyiso?) and 1/2" rigid over 6x6" posts to flush that up the 2" sections, then WRB (felt or tyvek) over it all. In this scenario, sheathing is over walls, but not the existing 6x6 posts.

    Siding: Given the number of windows and relatively small amount of solid wall space, I plan to trim out with wood...should I still add rain screen to allow drying of materials behind siding or not necessary?

    Thank you!!

  5. Expert Member
    Dana Dorsett | | #5

    If you have sufficient headroom add 2" to the underside of the rafters with Bonfgilioli strips you can install ~3.75-4" of HFO blown foam (R27-28) and 5.25-R5.5"/ R23 rock wool, which would be higher performance and lower cost than 7" of closed cell foam. With 7" of foam the framing fraction is about R8.2. With 1.25" polyiso + 0.75" furring that more than doubles.

    Even with 1.5" Bonfiglioli strips and 3.25" of foam (R21-ish) and R23 rock wool would be higher performance despite somewhat lower than code R44-ish at center cavity due to the thermal break on the framing fraction. With a full fill 7.25" of rafter, 0.75" furring and 0.75" polyiso the framing fraction would be about R14.6, nearly doubling it's performance.

    I'm not sure if Xarmor sheathing for the underside of the porch would cut it on it's own or not. Using 2.5-3" reclaimed roofing polyiso + half-inch OSB would probably be cheaper and more rugged.

    The wall stackup works. Take care to detail the air sealing well where the studwall meets the posts.

    Even 1/8" of air gap can be an effective capillary break and drain space- no need for a 3/4" furring type rainscreen. (Even in foggy dew parts of B.C. code only demands 10mm, or ~3/8".)

    1. fourforhome | | #6

      As an aside, on reclaimed poly iso, would you spray can foam whatever holes are in the foam? What about the divots left by the old fastener washers?

      1. Expert Member
        Dana Dorsett | | #7

        A lot of reclaimed roofing foam has no holes, or dings, having been dry-stacked on flat commercial roofs under membrane or torch-down without benefit of fasteners. Leave or just tape over the divots & dings, fill any through-holes (can foam is fine) or just tape over them with housewrap tape. Air tightness is more important than R-value on the hole patches.

        The foam can be initially hung with cap nails. Seal the seams of the foam with housewrap tape, stagger the seams of the bottom sheathing with those of the foam board. Use pancake head timber screws 16-24" o.c. long enough to penetrate the joists by 1.5". (eg: For 2.5" foam + 0.5" OSB use 4.5" pancake heads, minimum. For 3" foam it'll take 5" screws.)

        1. fourforhome | | #8

          Thanks Dana.

  6. humm9er | | #9

    Thank you Dana!

    For the ceiling, the idea of Bonfgilioli strips is intriguing. It does cut costs, but adds significantly to labor time. I will have to think on that.

    For the exterior walls, is foil-faced polyiso doable? I will stagger 1" sheets to get to 2". And given how vapor-impermeable that is, NO vapor barrier on inside of wall correct? Not a fan of poly-sheeting...but also no kraft paper, nothing on interior, correct? Do I need to be careful with paints etc? And what is consensus on best WRB over the rigid foam? I see the case for 15lb felt over Tyvek when clapboard is involved, but given this will be trimmed in PVC does it matter?

    For floor, it sounds like foil-faced versus non foil faced polyiso across joist bottoms makes no difference as long as I skin in OSB or ply. Fair to say that cellulose will do a better (less prone to voids) job between the joists than fiberglass batts? And in terms of vapor movement, do I need anything beyond 15lb felt between the plywood subfloor and the pine finish floor?


  7. Deleted | | #10


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