Screen porch remodel floor insulation vapor retarder

[gregm]

I am in process of remodeling my screen porch into a 3-4 season room. I'm in western NY. It's above grade slab, 3 cinder blocks off ground, but I don't know what's under it.

My question is about flooring, specifically about whether to include a vapor retarder layer. My plan, based on reading some great advice to others on this forum is:
1. 2" XPS Foam board directly on top of cement slab (FOAMULAR 250 because I couldn't find any EPS near me)
2. 2 layers of 1/2" plywood (this height will match closely with the adjacent living room
3. Vinyl plank flooring

Do I need a vapor barrier/retarder layer in here somewhere?

Thanks in advance

 

[WorthFlorida]

The foam board will provide a fair amount of moisture barrier but it really depends on the final floor material. Vinyl is about a tough as it gets and it is not impacted by moisture but read the instructions for proper insulation. The problem I see is 2" foam board will have too much bounce or give. 2 1/2" layers of ply on top will help but this foam board is not designed for floor use nor carry any kind of load. Only under concrete as read in it's instructions.

You be better off installing sleepers 16" on center and install the foam board between then. Use 3/4" ply as the underlayment and top it with 1/8" lauan plywood for a smooth finish for the vinyl planks. If you place the vinyl on the underlayment, all the seams and grain of the 3/4" ply will show through. Check the installation instruction for the vinyl since it may recommend an underlayment.

Being a older home and concrete is well cured and dried out, unless there is standing water around the home, there is little chance of a moisture problem.

 

[Sponsor]

 

[Dana]

I am in process of remodeling my screen porch into a 3-4 season room. I'm in western NY. It's above grade slab, 3 cinder blocks off ground, but I don't know what's under it.

My question is about flooring, specifically about whether to include a vapor retarder layer. My plan, based on reading some great advice to others on this forum is:
1. 2" XPS Foam board directly on top of cement slab (FOAMULAR 250 because I couldn't find any EPS near me)
2. 2 layers of 1/2" plywood (this height will match closely with the adjacent living room
3. Vinyl plank flooring

Do I need a vapor barrier/retarder layer in here somewhere?

Thanks in advance

XPS is the most environmentally expensive insulation in common use today. Going with Foamular 150 would be modestly less damaging than the higher density Foamular 250, but it's really pretty bad:

The Lowes box store at 3150 West Ridge Road in Rochester carries 1" thick Type I EPS, which would be appropriate in two layers (72 sheets in stock as of 30 seconds ago). It can be ordered for delivery from their store in Canandaigua, but not for pick-up.

Tape the seams of each layer with housewrap tape, and overlap the seams by a foot or so. With EPS that has a foil or plastic facer no vapor retarders are needed. If unfaced, install a 6-mil polyethylene sheet on top of the slab, under the foam.

If sticking with XPS, the vapor permeance of 2" XPS (any density) is low enough to be able to skip the vapor barrier.

Stagger the seams of the bottom plywood and foam by a foot, and stagger the seams of the second layer by a foot or more. Glue and screw the plywood layers to each other, but let it float on the foam. Leave a quarter to half-inch clearance of the wood to the foundation walls to allow for seasonal dimensional changes due to moisture. Before putting down the finish floor, check for any bouncy or squeaky sections, and screw any suspect areas down to the slab with TapCons if needed. Alternatively, run a grid of TapCons 24" o.c. to make sure no wave or "potato chipping" curls show up over time, though with a glued & screwed double layer that risk is low. (Potato chipping and lifted edges are more common when using a single 3/4" subfloor- it can happen even with 3/4" t & g plywood.)

At Canandaigua's mid-40s F subsoil temperatures it's better to NOT go with a sleepers & foam-fill approach. With insulation on top of the slab the bottom side of the sleepers will be WAY below the summertime indoor dew point temperatures, and the sleepers will take on moisture. If the sleepers are pressure treated they'll survive, but it risks rotting the subfloor where it rests on the sleeper. The temperature striping of the sleepers would also be noticeable in bare feet.

If going with a sleeper & foam fill approach, use pressure treated sleepers, and install a 6 mil polyethylene vapor barrier above the foam & sleepers, under the subflooring.

 

[gregm]

Thanks for the great information, Dana! Sounds like we really need to get moving on eliminating the current XPS insulation. Are there health/indoor air quality concerns while the XPS is in your home subfloor?

One more question
1. Do I have to be choosey about which finished flooring material (or its underlayment) I use on top of the plywood? Like, do I have to pick something that is not vapor impermeable?

 

[Dana]

Thanks for the great information, Dana! Sounds like we really need to get moving on eliminating the current XPS insulation. Are there health/indoor air quality concerns while the XPS is in your home subfloor?

HFCs are not particularly dangerous to humans, and the rate at which it diffuses out of the foam is so slow as to be indetectible in the room air (assuming the room isn't hermetically sealed with zero air exchanges over the course of years.)

One more question
1. Do I have to be choosey about which finished flooring material (or its underlayment) I use on top of the plywood? Like, do I have to pick something that is not vapor impermeable?

It's never a good idea to install a true vapor barrier with the wood between the vapor barrier and soil, so yes, it matters if you have a wood subfloor.

While vinyl is an inherently vapor barrier type material, the crackage on vinyl plank flooring has sufficient diffusion area that the subfloor can still dry into the room air, albeit slowly, whereas glued down vinyl tile type flooring would be much riskier.

Another example of this sort of issue is aluminized fabric radiant barrier, which is also a true vapor barrier at <<0.1 perms. But "perforated radiant barrier" made of the identical material, with a ~1/2" grid of tiny pin-prick perforations comprising less than 0.1% of the total area raises it's vapor permeance to about 5 perms, similar to coat of standard interior latex paint on wallboard. That restricts the rate of moisture diffusion by quite a bit, but does not completely block it. There are applications where using unperforated RB would create a moisture trap, but are just fine using perforated versions despite restricting the drying capacity.

And to be clear, this is about diffusion of water vapor, not air movement and air-transported moisture. Even with zero air flow the water vapor responds to the vapor pressure differences related to the humidity levels on each side of the barrier.

 

[gregm]

Thanks again for more great info and advice

A couple more details:
1. regarding the 1/4" gap around the perimeter of the foam/plywood on the floor: What do I use to fill that? A can of spray great stuff foam? Loctite Foamboard adhesive? Edit: I think the answer here would be the can foam.

2. regarding insulating the 2x6 wall cavities: I've read you recommending Rockwool R23 batts over R19 Faced Fiberglass batts. Sounds like a great idea, but the Rockwool is unfaced, do I need to add something between the Rockwool and the drywall? A little more info on the wall layers from outside-in: Siding -> GripRite wall wrap -> 1/2" plywood -> 2x6 stud framing filled with unfaced Rockwool or faced Fiberglass -> ??? (maybe something here next to the Rockwool?) -> drywall

 

[Dana]

Thanks again for more great info and advice

A couple more details:
1. regarding the 1/4" gap around the perimeter of the foam/plywood on the floor: What do I use to fill that? A can of spray great stuff foam? Loctite Foamboard adhesive? Edit: I think the answer here would be the can foam.

Leave a gap- don't fill it. Any bulk water leakage/seepage needs a place to go other than over caulk or can-foam into the wood, and the wood needs space to be able to freely expand during the summers when the indoor humidity is generally higher.

Can I assume the bottom plates of the exterior studwalls are pressure treated or have some sort of capillary break between the slab & wood?

2. regarding insulating the 2x6 wall cavities: I've read you recommending Rockwool R23 batts over R19 Faced Fiberglass batts. Sounds like a great idea, but the Rockwool is unfaced, do I need to add something between the Rockwool and the drywall? A little more info on the wall layers from outside-in: Siding -> GripRite wall wrap -> 1/2" plywood -> 2x6 stud framing filled with unfaced Rockwool or faced Fiberglass -> ??? (maybe something here next to the Rockwool?) -> drywall

If it's 2x6 above-grade walls with siding it depends on the siding or how it's mounted. Canandaigua is in US climate zone 5:

In zone 5 if the siding is back ventilated/rainscreened standard latex paint on reasonably air tight drywall is a sufficient vapor retarder. Vinyl siding would qualify as inherently back-ventilated, but other types of siding would need at least a 1/4" air gap between the siding and structural sheathing to get away with that. If your wall stack-up doesn't meet that test, a sheet of 2-mil nylon (Certainteed MemBrain) detailed as an air barrier under the wallboard would be cheap insurance. In my area box stores are starting to carry it, but apparently not in your area. But it looks like it can be ordered from the big orange box store for pickup at any of the outlets near you.

BTW: Current IRC code minimum for Zone 5 walls is 2x6/R20, not R19. While it may seem like it's kinda-mostly the same performance it's not. R19 batts are measured at a loft of 6-6.25" for labeling purposes, but only perform at R18 when compressed to 5.5" in a 2x6 cavity. They are also not very air retardent, barely slowing down any air leaks, and can even lose performance to convection within the batt when its in negative double-digits outside. It's really a "fluffed" R13 batt, performing at only R13 when compressed to 3.5" in a 2x4 framing:

An R21 HD fiberglass or R20 fiberglass (commercial distributors will have them, not box stores) will work, as will R23 rock wool. From a price/performance difference it's hard to rationalize upgrading to R23 rock wool over R21HD fiberglass at box store pricing, though IMHO it's nicer stuff to work with. It's a bit more fragile than HD fiberglass to work with, but somewhat easier to sculpt with a batt knife, with less lingering fiber shards hanging in the air than fiberglass.

If planning to install a 2x6 studwall up against the foundation wall in a crawlspace or basement, don't. Check back if you need more details on what to do there (or search this site for threads on basement insulation.)

 

[gregm]

Can I assume the bottom plates of the exterior studwalls are pressure treated or have some sort of capillary break between the slab & wood?

Yes, the bottom plates are pressure treated wood, and there's a layer of something under them (I'll have to ask my contractor what it is).

 

[gregm]

In zone 5 if the siding is back ventilated/rainscreened standard latex paint on reasonably air tight drywall is a sufficient vapor retarder. Vinyl siding would qualify as inherently back-ventilated, but other types of siding would need at least a 1/4" air gap between the siding and structural sheathing to get away with that. If your wall stack-up doesn't meet that test, a sheet of 2-mil nylon (Certainteed MemBrain) detailed as an air barrier under the wallboard would be cheap insurance. In my area box stores are starting to carry it, but apparently not in your area. But it looks like it can be ordered from the big orange box store for pickup at any of the outlets near you.

My siding is LP 76 Series Smooth Lap, some engineered wood product, so that it matches the rest of the house. So, I guess I'll go with Rockwool and the Certainteed MemBrain. Would the membrain just get stapled to the studs just behind the drywall? Edit: I found install instructions, so yes, it is stapled to the studs

 

[Dana]

The MemBrain gets stapled to the studs, and taped over the staples. Also overlap adjoining sheets by several inches, then staple and tape only where the seams are supported by framing. Detailing electrical boxes as air barrier is also important, made easier if using purpose-made air tight boxes.

If the LP siding wasn't already up, ripping 2" wide strips of 3/8" plywood ring-shank nailed over the housewrap at the studs makes a reasonable through-nailing surface for the siding fasteners while establishing a decent rainscreen gap. (In western British Columbia where rainscreens are required by code you can buy pre-made plywood strips like that at box stores.)

 

[gregm]

Detailing electrical boxes as air barrier is also important, made easier if using purpose-made air tight boxes.

Well, we've already installed electrical boxes that are not purpose-built air-tight . I assume that making them airtight requires covering them in great stuff foam? Or some sort of wrap?

And speaking of sealing stuff in the 2x6 wall cavities, I assume I need to foam the electrical wire penetrations, and maybe even caulk or foam the top and bottom plates?

 

[gregm]

If the LP siding wasn't already up, ripping 2" wide strips of 3/8" plywood ring-shank nailed over the housewrap at the studs makes a reasonable through-nailing surface for the siding fasteners while establishing a decent rainscreen gap.

This sounds like a really good idea, too. Would the plywood furring strips need to be pressure treated wood? Is 1/2" plywood enough? We have extra of that. After a little searching on the subject of rainscreen gaps, looks like I'll also need bug screen pieces on the top and bottom, too.

 

[Dana]

Well, we've already installed electrical boxes that are not purpose-built air-tight . I assume that making them airtight requires covering them in great stuff foam? Or some sort of wrap?

These li'l boxes are pretty slick, if you don't need a gazillion of them:

If you're good with a pair of scissors you could fab something similar and just as good using milk/juice cartons. The key is to cut an X in the MemBrain that gets folded over and taped to the inside of the cardboard box, then filling the carton with low-expansion latex foam. (Not polyurethane can-foam.) The square flat sides makes it easier to trim batts to a near-perfect fit too.

Would the plywood furring strips need to be pressure treated wood? Is 1/2" plywood enough? We have extra of that. After a little searching on the subject of rainscreen gaps, looks like I'll also need bug screen pieces on the top and bottom, too.

Half inch plywood is fine- more than enough, and no, it doesn't need to be pressure treated. It's also common to use 1x4 furring.

Stapling in a 3"-5" wide strip of fiber mesh cut from a ridge vent roll works fine as bug screen at the top & bottom of each furring bay:

Some prefer to use fiberglass window-screen mesh folded over & rolled, which is a bit more work:

Still others do "belt and suspenders" approach, folding window screening over a strip of ridge vent.

 

[gregm]

The MemBrain gets stapled to the studs, and taped over the staples.

Is it OK to put the memBrain directly between XPS foam board and drywall? Against the header I put 2" foam board to fill in the gap to the 2x6 wall. So I'm just wondering if I start the membrain below the header or put it over the foam board.

 

[Dana]

Is it OK to put the memBrain directly between XPS foam board and drywall? Against the header I put 2" foam board to fill in the gap to the 2x6 wall. So I'm just wondering if I start the membrain below the header or put it over the foam board.

The MemBrain is serving as both an air barrier and vapor barrier. So whether you fattened up some 2x4 framing to 5.5" with 2" XPS on either the interior or exterior side, the MemBrain would still be next to the wallboard in one broad continous sheet, with overlapping sheets stapled and taped where it's supported by framing.