Insulating and remodeling 1970’s basement
Hi Martin/Dana/GBA Community:
Thanks for all your prior help, particularly Dana. Based on GBA articles and feedback, last year I replaced an aging, dirty oil furnace and HW heater with clean, efficient heat pumps. My whole house is not only more comfortable in the winter but now also in the summer. All at a running cost less than oil was for just one floor.
I am now focused on remodeling my basement for my 88 year old father to live in. I’ve read all the articles I can find on GBA and would really appreciate a sanity check on my plan as well as get guidance where I am not so sure. The house is in southern NH and built in the 1970’s. It’s a walkout basement in back with half below grade up front. All basement walls are full height poured concrete except the front and corners which are half poured, half stud wall. The prior owner finished it with drywall+paneling over 2×4 stud wall with interior faced F-13 fiberglass. The studs are ¼-1/2” off foundation, no poly or insulation on foundation walls nor under slab. Basement is dry and has no water issues, tho smells musty and has mold on bottom of walls probably bc it has no moisture barrier/retard under slab or on walls, was carpeted and old drywall contacts slab. House above is 2×4 construction with inner faced R-13. There’s currently no exterior insulation and I have no current plan or budget to reside and add insulation.
My plan for the basement remodel is to:
1) Remove perimeter stud walls to Insulate+tape rim joist, foundation, half height stud wall in front and all door/window surrounds with 1” ISO. Then put perimeter stud wall back, replacing 40+ year old drywall with new. I hope to re-use existing R-13 faced fiberglass from existing wall and will add more fiber around rim, filling any holes with canned spray foam. Questions:
a) This results in near R-20 wall insulation and moisture retarding, within code for climate zone, correct?
b) Do I need to keep drywall ½” off floor if insulating floor with foam and OSB subfloor?
c) I’m remodeling a bath that has tub and surround against back full foundation wall. I need 2-3” more space in bath to meet code. Current wall uses 6+” from foundation. Can 3 layers of 1” ISO be used: 1 on foundation then 2 interior layers ‘cut and cobble’ between 2”x2”’s for attaching cement board?
2) Insulate floor with 1” EPS with foil facing up and subfloor on top before putting laminate down.
a) This results in near R-5 floor insulation and moisture retarding, within code for climate zone, correct?
b) Is caulk needed/worthwhile between EPS if I’m taping?
c) Can I float two layers 1/2’” OSB instead of one ¾”? I’d prefer to float subfloor and not to drill+screw into slab but with 7’6” clearance I am limited on height.
d) Do walls, particularly perimeter ones, need to rest on ESP+OSB subfloor? If so what type/density/psi will I need? If not, do I caulk and tape to wall sole plates?
e) For bathroom, what type/density/psi EPS or XPS do I need under tub and file floor?
f) For utility/mechanical room that has floor drain, can I leave slab bare? What are pro/con’s?
Thanks,
Anthony
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Replies
> smells musty probably bc it has no moisture barrier/retard under slab or on walls
Since I started dehumidifying and improved air sealing, my similar basement never smells musty. Consider that you could address this concern at far lower cost.
Regarding slab floors, I'd build for for substantial ability to dry upwards. Plan on water soaking the floor (lots of causes - broken pipe, overflowing toilet, etc).
Thanks Jon,
I appreciate your perspective and cost reducing approach. When buying the house, I had originally hoped and planned to just insulate+laminate the floors then remove the paneling and paint the walls.
Unfortunately it’s not that simple. The basement has 5 rooms plus a bath and after removing the paneling I discovered that over half the bottoms of the drywall has mold on it. The basement is also cold in the winter, especially when windy.
I am ok, actually feeling good about the idea of properly insulating and moisture retarding the basement. The incremental cost is not appearing as bad as I thought and appears to have a good long term pay back. And now that I’ve switched to heat pumps and electric, I am hoping to eventually go solar and be near net zero.
I'd avoid all faced insulation and use unfaced EPS. In many cases, the facing creates a moisture trap with a reasonable chance of growing mold and the odor leaking out. Also verify that your laminate flooring doesn't create a moisture trap.
If want a vapor barrier, paint on direct to concrete is best (it leaves no air gap for mold).
If I redid my basement floor, I'd look into wall-to-wall rugs with thick, insulating, air impermeable pad underneath. Easy to remove and dry. Basement dehumidifiers are always wise in humid climates (no matter how you insulate).
https://aceee.org/files/proceedings/2006/data/papers/SS06_Panel1_Paper27.pdf
Thanks Jon,
What you're suggesting is what created the moldy walls and musty smell. The prior owner used paint, with fiberglass batts in wall and carpet on the floor. Which puzzles me bc this approach seems to be counter to what Martin, Dana and others advise in GBA articles and comments. My understanding from them is to moisture retard, not barrier, using foam board insulation on walls and floor that is at least 25% of total wall R, thereby preventing interior condensation that leads to mold and musty smell.
The kraft facers on batts aren't true vapor barriers, and will let some of the moisture out when the humidity in the cavities are high, but with no rigid insulation or vapor retarder between the foundation and studwall the humidity on the cold side of the assembly stays pretty humid from ground moisture 3/4 of the year, and even more mold risk in the summer, when the infiltration/ventilation air's dew point is above the temperature of the lower part of the foundation (which is the case for 8-10 weeks of the summer in most of southern NH.)
As long as there is reasonable above grade exposure to the exterior on the foundation (a foot or more), or a good capillary break between the foundation and foundation sill (probably not, if built in the '70s) it's pretty safe to use foil faced polyiso directly on the foundation wall. Let the concrete dry toward the exterior, and the studwall dry toward the interior. The foil facers block the ground moisture drive into the susceptible studwall, but if the concrete can't dry toward the exterior it may raise the moisture content of the foundation sill.
If using unfaced EPS the foundation will still be able to dry toward the interior through kraft faced batts, but it'll take at least 1.5" to have sufficient dew point control on the above-grade section to keep the batts & stud edges from taking on winter moisture, assuming you're in the zone 5 tier of counties bordering MA. If further north 2" would be safer.
I'm not totally clear on the geometry you're looking for around the bathtub. Code is looking for the performance equivalent of R15 continuous insulation, which is only 2.5" of foil faced polyiso, 2.75" of fiber-faced roofing polyiso, but takes 3.75" if EPS. Using 1x4 furring through-screwed to the foundation with masonry screws instead of 2x2s for the framing adds another R3+ of thermal break on the framing, if EPS, R4+ if polyiso.
If insulating the floor with foam it has to be EPS (or XPS), and it should extend under the wall foam leaving a half-inch gap from the floor-foam edge to the foundation as a drainage space. Tape-seal the floor foam to the wall foam before installing the subfloor and studwall. With as little as 3/4" of EPS the risk of mold in the floor goes down considerably, but up to 2" of EPS is still financially rational on a lifecycle basis if you can make it work with all the doors, stairs, total headroom, etc.
Thanks Dana, very helpful. I’m in Zone 5A next to MA. The two Fujitsu heat pumps you recommended last year worked out fantastic. It took a while tho to find a knowledgeable installer. Richard Nolan from Townsend Energy was fantastic – timely, courteous, knowledgeable and significantly less than others who kept pushing for a single inverter with multiple heads instead of one inverter and head for each floor. Thanks again for your guidance, I LOVE these heat pumps!
For the foundation and front knee wall, I’m planning on using non-foil faced 1” Dow Super Tuff R-6.5 polyiso from Home Depot (cheapest I can find when including shipping). All the foundation is exposed to the exterior more than 1 ft except for the front which ranges between 6”-12”. Will the 1” polyiso without foil work for the front? For the bath wall I was thinking thicker 2x2 studs were needed for tiling. 1x4 strapping and better continuous 2.5” insulation is great, thanks.
For the foundation slab I’m planning on using foil faced 1” (I have limited height) Poly Shield EPS from Lowes (again cheapest I can find). I will shorten the outer stud walls and put on top of subfloor as described, thanks. Will I need to screw the wall bottom plates to slab or subfloor? All the walls are non-load bearing. And do interior walls need to be on top of the subfloor too or can they remain on the slab?
For the subfloor, can two layers of 7/16” or 1/2’” OSB be floated instead of one ¾” screwed? I’d prefer to not drill+screw into slab. When installing laminate, should the underlayment be taped or just overlapped? Under the bathroom and laundry, is higher density EPS/XPS needed than the 13 psi Poly Shield? And lastly, for the mechanical room with a floor drain, can the slab be left bare or should the drain be raised, EPS+subfloor put down with cement board and tile on top?
Thanks again for all your help, Anthony.
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