I have just finished demolition ( down to the stud's ), on my 11'6" by 13'6" sun room in Minneapolis. I replaced the seven windows with Andersen 400's four on the west side three on the south (about 62 SF of glass) and plan to replace the fiberglass insulation with 3 1/2"of iso by cutting it small and filling the gaps with Great Stuff or Froth Pack and then adding an additional 3/4" of iso over the studs as a thermal break (that's how much the extension jambs on the windows give me after factoring in the 1/2" sheetrock ). I would like to seal off the attic (if that is what it's called. There will be no access after I sheetrock )with iso as well if that is permissible. I would then add back the fiberglass that is already between the 16" OC rafters one layer between the rafters and another running perpendicular for a total of 12". The roof is shallow pitch maybe 4x12. The siding is 10" cedar lap. One of the reasons for the remodel was the floors slant at about 1/4" a foot and heat was provided by 4 electric baseboard heaters, (the rest of the house built in 1940 is boiler with radiators). I was hoping to find wood sleppers under the floor but instead found 1/2" plywood on top of 3/4" XPS on top of a concrete slab. I would like to add radiant by using the plywood rip system with heat transfer plates with ceramic tile over that. My question would is 3/4" XPS be enough insulation over the slab and does anyone know of a good way to level this thing. I can't believe they couldn't get a slab level. I have come to the conclusion this is an addition probably built in the 80's by a very shoddy crew. Retirement has begun, Tom

Does your slab look like it started out life as a patio? No one is going to pour a patio without a slant - about 1/4" per foot is what they like.

Here's what I'd do. Schluter Bekotec is a foam panel system for radiant installations that comes in 2' X 4' panels. I presume the slab slopes away from the house. I'd pull off the plywood and XPS that is on the slab and lay a single 2' course of Bekotec parallel by the house. Then, I would put a single piece of 1/2" foam insulation under the next 2' course and another Bekotec on top. The next course would have 1" foam underneath it and so on with successive 2' courses until you got to the outer edge. By the time you got out there, you would have 2.5" of foam underneath the outer course. Then, you tube it and pour or place your mortar screed and make it level over the top. The slight slope to each piece of Bekotec might take some noodling to get the radiant tubing to cooperate, but that is what barbed staples are for.

I have also heard that you can buy EPS or XPS cut custom tapered. If that is the case, the first piece on the bottom of each course can be tapered to the slope so that there is no surface irregularity at all.

A cut'n'cobble iso & FrothPak between the studs buys you very little over installing R15 rockwool or high-density "cathedral ceiling" fiberglass batts if to you take the care to trim & install perfectly (which takes less time than the cut'n'cobble solution.)  At a typical 25% framing fraction it's only buying you about ~R1.3  in whole-wall performance due to the severe thermal briding of the framing.

To get good performance out of batts, start by taking the time to caulk every stud & plate to the sheathing (preferably with an acoustic sealant type caulk) as well as the seams between doubled-up plates, and foam-sealing where electrical runs penetrate studs.  Take care trimming & fitting in some either R15 rock wool batts (Roxul batts are available at most Lowes, and some Home Depot stores) or high density fiberglass R15s splitting them over the wiring, etc and avoid compressing them- tug them out a bit if they appear to not be proud of the stud-edge plane before putting up your interior iso. I personally find rock wool a lot easier to trim & fit well than fiberglass (YMMV), and there isn't as much of an air-suspended particulates issue either (but still wear some sort of air-filtering mask when installing them.)

Whereas iso cavity fill buys you little, the 3/4" foil faced iso on the interior buys you a LOT, and if you have space to accomodate a full inch it would be even better. (With R15 batts and 1" iso it would exceed the performance of what you were proposing, and it would cost less & go up quicker.) Air-seal the iso to the framing as you go with acoustic sealant, and seal the edges with  can-foam or FrothPak, then seal the seams with 2" FSK tape.  At 3/4" it adds about R4 for whole-wall performance, which is about a 35-40% boost in total R value (a bit less, if you punch 1000 thermally bridging nails & screws through it so be sparing). The foil facer puts a very strong vapor retarder on the interior, which is appropriate for a Minneapolis climate on a plywood sheathed clapboard-sided 2x4 framed house.

Air sealing the attic from the conditioned space is a critical first-step before insulating up there, and as long as there is at least some ventilation between attic & the outdoors it should be fine once you've insulated and put up foil-faced iso as the interior vapor barrier.  Better than adding fiberglass over the top of the pre-existing fiberglass, blowing in cellulose fills in the gaps far better and stops it from losing performance at the cold-temperature extremes, due to the much lower air-permeability of the cellulose (even at 1.5lb open-blow density).  It's not rocket science as a DIY with a rental blower.  Draped batts ALWAYS have open seams that leak heat, wheras blown goods don't.  Install chutes to keep it from clogging soffit vents, and staple some depth gauges around to be able to rake it smooth and level, and you'll have a seam-free low-permeabilty cap to the joist-batts that prevents convection through the fiberglass to the cold attic air (which becomes a serious performance hit to standard density fiberglass batting when it's 0F in the attic.)

The 3/4" XPS on the floor is way below what makes sense under a radiant floor in your climate (or any climate, for that matter).  Adding a 1.5" or even 2" layer of XPS would not be insane, if you have the headroom to work with.  You may be able to level up the floor with dry sand under the new foam layer, and TapCon the new subfloor to the slab to keep it from moving around on you.  If the headroom is squeaky-tight you might buy yourself a half-inch to an inch at the same R going with Roth panels or similar.

Thanks for the great reply's. Sorry for the late response, I have been up to my elbows ripping out the ply and foam from the floor. To address the sloping floor issue I have decided to have Home Depot rip some 2x6 tapered from 2" to 4 3/4" and glue tap con to the slab 16" OC for a nice level platform. Then I can use xps between for the 2" of xps Dana recommended, then cover with 3/4 ply and then my radiant. Dana, on the attic response is it not ok to seal the roof deck with iso? Do attics have to be connected to outside weather? Tom

Roof decks need to be able to dry. The vapor permenance of a stackup of 30# felt + shingles is under 0.2 US perms, and it's often covered with snow/dew/rain- it literally CAN'T dry toward the exterior. Foil facers on iso are under 0.1 perms. Any water that finds it's way to the roof deck will get soaked up by the wood and take years to get out (if ever.) An absolute minimum permeance to give the roof deck at least some resiliance to wind-driven rain leaks or minor seepage around shingle nails would be 0.5perms, and 1+ perms is better.

With a 2" vent gap and soffit-to ridge venting, the convection with the outdoor air will allow the roof deck to dry. The roof deck itself is vapor-retardent at about 1-3 perms, depending on material and moisture content (it's more permeable when wet), but with nothing between the deck and the ventilation channel it dries from minor wetting events before it rots.

So my roof rafter's are 8" deep. I could put lets say 5" of xps and a final layer of 1" iso between the rafter's leaving a 2" air space behind that would be vented leaving a sealed interior attic space. would that be OK? ( I would still blow cellulose on the ceiling deck ). Tom