I am buying an existing 2005 built home in Grand Blanc, MI (48439) Currently an unfinished poured concrete walls. No insulation under slab. We are finishing the basement. Rec area and bathroom. I plan on removing the fiberglass around the band joist and putting in 2 part spray foam. For walls, I can get 2.5" factory seconds polyiso at $19 / sheet. I know EPS and xps is recommended, but is polyiso ok? Plan for floor was 2" XPS and osb subfloor tapcon'ed into concrete. XPS and osb would extend to wall and polyiso and stud wall would sit on top. Thanks in advance.

Sorry for formatting.

Polyiso is fine.
Just make sure to leave a half inch gap from any cut edges of the board.
Otherwise you have to worry about the insulation wicking up moisture from the concrete.

I prefer the method of letting the moisture flow through the wall so that you know about a leak sooner and can fix it before you have a wall full of mold. And it does dryout quicker if there is a small seasonal leak.

Posted By ian_upton on 08 May 2019 10:42 PM
I am buying an existing 2005 built home in Grand Blanc, MI (48439) Currently an unfinished poured concrete walls. No insulation under slab. We are finishing the basement. Rec area and bathroom. I plan on removing the fiberglass around the band joist and putting in 2 part spray foam. For walls, I can get 2.5" factory seconds polyiso at $19 / sheet. I know EPS and xps is recommended, but is polyiso ok? Plan for floor was 2" XPS and osb subfloor tapcon'ed into concrete. XPS and osb would extend to wall and polyiso and stud wall would sit on top. Thanks in advance.

Fiber faced roofing foam or foil faced polyiso is fine on walls- just don't let the bottom edge rest on a potentially damp (even if not visibly damp) slab or it can wick up moisture over time.

Installing 1-2" of polystyrene (EPS or XPS) under both the polyiso and the bottom plate of any studwall works well enough as a capillary and thermal break for the wood. Leave some space between the polystyrene and the foundation wall as channel for any bulk water incursions.

If the basement has a history of flooding, stop the polyiso above the high tide mark and continue down with polystyrene, or leave it blank.

At 2.5" most 2lb roofing polyiso is good for about R14, just shy of the IRC code minimum R15. Building a studwall tight to the foam and insulating it with R11-R15 fiberglass or rock wool would be moisture-safe from a condensation point of view, and give it substantial margin over code minimums. Kraft faced or unfaced is fine, but not foil faced, which would create a potential moisture trap.

An alternative to a studwall would be 1x4 furring through screwed to the foundation with 4" masonry screws 16-24" on center, hanging the wallboard on the furring. If you want that to meet/beat code min, install cut'n'cobbled 3/4" EPS (unfaced) between the furring and it'll meet code on a U-factor basis. With the furring approach it's a bit of a PITA to route & install wiring and electrical boxes, but cutting wiring channels with a router and backfilling with can-foam (trimmed flush after it cures) works. The furring approach eats up less floor area, which may or may no matter.

For the floor use EPS not XPS. XPS is the least-green insulating material in common use today due to it's extremely high global warming potential HFC blown agents (predominantly HFC134a, ~1400x CO2 @ 100 years.) Most of the HFCs get out in the first decade or two, and as the HFC levels drop, so does the performance. XPS is only warranteed to R9 @ 2", but at full depletion it would be R8.4, just like EPS of comparable density. EPS (and most polyiso) is blown with a variant of pentane, with a GWP ofabout 7x CO2, and most of the pentane leaves the foam and is recaptured at the factory, often burned for process heat (creating CO2, at about 1x CO2 :-) ).

With any floor foam, stagger the edges/seams of the OSB with those of the foam to avoid rockering compression of the foam. Even cheap Type I EPS has sufficient compressive strength to handle residential loads- just don't over-torque the TapCons. Be sure to vaccuum up the concrete dust out of the hole before inserting the screws or some of them will bottom out.

Thank You.

 

Basement does not have a history of flooding.

 

I like the idea of EPS for the bottom foot. (I have 9 foot basement walls so I can do 8' of Polyiso up top and EPS on bottom.

 

I plan on studding out. Dana already answered question. I can add unfaced fiberglass or mineral wool to increase R value. What about Cellulose or do I want to hold the "batt" insulation away from the foam slightly?

 

 

So a good plant would be to do the EPS on floor covered with OSB. Leaving 1/2" or so around the perimeter for any bulk water potential. EPS on wall down to the top of OSB and Poly ISO on top.

 

Fiber faced polyiso seems more cost effective, but what typoe of tape would I need to seal? Wondering is foil faced is better overall because of substrate for tape to stick.

 

 

Thanks in advance.

If the basement has no history of flooding, but DOES have a history of damp, also possibly think about putting dimple mat down on the floor before the OSB/EPS.
Or you could use Dricore (Basically an OSB product with a dimple mat integral on one side.

This gives the floor a breathing space in case you have low (but not no) amounts of moisture.

Thank you. I was wondering about dimple mat.

In basements with known wall moisture and bulk water inursion issues dimple mat between the foam & foundation works.

With a fiber insulated studwall the fiber insulation needs to be fully in contact with the wall foam. Cellulose is fine, but with even minor flooding it would wick moisture all the way to the top and need to be replaced. If rock wool or fiberglass only the wallboard and insulation up to about a foot above the high tide mark would need to be replaced.

Most housewrap tapes stick to paper or fiberglass facers on roofing polyiso just fine.