Insulating Around Basement Windows

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ngen33r

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I am starting to insulate the basement and frame the walls. I am in Zone 5 so I am using R-5 XPS on the walls and I will do R15 paper backed fiberglass inbetween the studs. I am gluing the seams with PL300 vs taping. I may tape the seams after but not sure if I will need to. I have glass block windows that I need to frame around. What is the standard procedure to insulate around the windows. The bottom of the windows is about 14in below grade. The CMU block is 12in thick and the windows are cemented right to the block. I don't want to block them more than I have to as that would make future replacement a nightmare. Do I insulate and frame around them or do I need to insulate the sides as well?

I wanted to use reclaimed polyiso, but the local company that sells it never returns my calls. I am not going to beg for their business and the cost difference I can reclaim some with reward points.

I had all of the moisture issues corrected and I put a coat of drylok on the block for an added moisture barrier.
 

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Reach4

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How about a frame that holds a sheet of clear acrylic (e.g., Plexiglass) in place. Represented by the red line.
img_6.jpg
 

ngen33r

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Would that be only for the winter months? I open the windows frequently during the summer months. I would also think that the plexi would condensate when it is cold. and damage the drywall.
 

Cacher_Chick

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I used 2" xps on the walls sealed to 3/4 xps on the cmu window opening and drywall adhered the drywall directly to the xps in the opening.

Most of the problem in a basement is air leakage in the sill and between the cmu and wood plate, which leaks through fiberglass insulation like it is not even there.
 

Dana

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Tape the seams. PL300 bonds to the foam well, but isn't flexible enough to accommodate the shrinkage of the foam over time. In 25 years a 48.0" wide sheet of XPS will be between 47.5 - 47.75" wide which is a big improvement over the XPS manufactured in the 1970s, but sealing with adhesive just won't cut it. Use the PL300 to hang the foam on the wall, and snug the studs up firmly against the foam. (Some XPS is manufactured with ship-lapped edges, which won't have quite the temperature striping of well aged square edged XPS since there is always at least half-the thickness, not a gap.)

To hit R5 XPS is relying on the effects of the HFC blowing agents, most of which will escape the foam over time. The "lifetime warranty" is only for 90% of the labeled-R, or R4.5. But at full depletion it will reach R4.2, the same as EPS of similar density. It's greener & cheaper to just go with EPS, or better yet polyisocyanurate, both of which is blown with variations of pentane, which are far more environmentally benign. The chemistry of polyisocyanurate polymer has a somewhat lower overall environmental impact than polystryene, the polymer in EPS and XPS.

For the above grade section R4.5 is not quite sufficient for dew point control on R15 in the studwall in Cleveland's zone 5A climate, but if you go as air tight as possible on the wallboard the kraft facer will offer substantial protection against excessive wintertime moisture accumulation. The wintertime temperature of the below-grade section of each stud bay will be warmer, the fiberglass will wick and redistribute a modest moisture burden accumulating in the upper part of the wall to the warmer lower part, where it can dry toward the interior. It would be better with R6 though. The IRC prescriptive minimum for above grade R13 walls would be R5 in your climate zone, but you have R15. The simple math would be (R15/R13) x R5= R5.77.

Ideally you'd be able to insulate the sides of the window opening too, but it will impinge on the glass a bit, or at least the mortar line. About the best you can do thickness-wise is to glue 0.5" thick fire-rated foil faced polyiso (eg Dow Thermax), with 1/4" hardboard (eg Masonite) glued to the polyis0. In this instance you'll want to put a continuous bead of PL300 between the foam and foundation at the exterior edge side, and tape over the other seams with a high quality foil tape such as Nashua 324A (available at most box stores.) The hardboard has a very smooth finish that can be painted with standard interior latex. If using a non-fire rated polyiso it needs half-inch wallboard as an ignition barrier on the foam, which would be 1.0" thick stackup. A half inch of polyiso is only R3, but that's a gia-normous improvement over R0.
 

Reach4

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Would that be only for the winter months? I open the windows frequently during the summer months. I would also think that the plexi would condensate when it is cold. and damage the drywall.
The plexi would be closer to the window than the drywall. You want a moisture barrier on the wall side of any drywall. The right foam is also a moisture barrier.

You would make it removable.
 

ngen33r

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I will switch over to eps also since my ceiling is only 82in can I use the 14in scraps between the studs and foam them in place? That would give me R10 above grade 4 foot down the wall. I know I will have some thermal leakage through the studs but in my mind it sounds like it will be a good compromise to using 1in material.
 

ngen33r

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Problem solved. HD sells PolyIso for the same price as XPS. They just don't clearly mark it. I exchanged the XPS for 1in PolyIso R6.5 and grabbed some foil tape. Back to work now.
 

ngen33r

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Just venting because the cmu walls aren't square and its making the framing that is supposed to hold the poly iso against the wall have gaps of 1/4in - 1/2in in some places. Not sure if that is acceptable. Do I need to foam the gaps? I also installed the foil side facing in so the foil tape is on foil. I tried to have all the seams with less than 1/8in gaps. I really do not want any mold problems.

When it comes time to do the rim joist area, can I just foam the cavity to the poly iso or do I need a topper that is taped to the top edge and then foam?
 

Cacher_Chick

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Just venting because the cmu walls aren't square and its making the framing that is supposed to hold the poly iso against the wall have gaps of 1/4in - 1/2in in some places. Not sure if that is acceptable. Do I need to foam the gaps? I also installed the foil side facing in so the foil tape is on foil. I tried to have all the seams with less than 1/8in gaps. I really do not want any mold problems.

When it comes time to do the rim joist area, can I just foam the cavity to the poly iso or do I need a topper that is taped to the top edge and then foam?

By experience is that basement walls are often not plumb, so the wall framing needs to be offset away from the foundation. I install 2" ridgid foam using the adhesive and 1x2 furring strips tapconed into the cmu and then frame a straight and plumb wall to the inside of that.
 

Dana

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A gap of 1/4" between the stud edge and foil isn't a show-stopper and quick & easy to fill with caulk and a powered caulking gun. For gaps in the 0.5" range it may take can-foam.

A gap of greater than 1/4" means installing thicker batts to guarantee a compression fit an a full fill of the cavities. R15 fiberglass or rock wool is nice & dense, but doesn't easily fluff to 4"+ for a guaranteed full fill, and loses performance to convection inside the cavity. A mid density unfaced R20 performs at about R14 when compressed to 3.5" where there is no gap, and would be about R15 when compressed to 4.0". A low density R19 performs at R13 when compressed to 3.5", and runs a bit under to R15 at 4.0". Either would be an easier and more reliable full-fill fit than R15s if there are sections with half-inch gap.
 

ngen33r

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What about the top of the blocks and rim joist area? Is a foam kit enough if I overlap the top of the PolyIso or should I use a PolyIso topper and then foam to fill?
 
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