Another basement insulation thread.....

[Carl Weng]

Thanks Dana- Would it be advisable to maybe add an inch of foam board onto the bottom section - this would increase the R ratio and then I could use the 9.5 inch R30 fiberglass to fill both the stud bays and the cavity between the wall and foam board?

 

[Dana]

3" of EPS is R12.

If the R30 batt is at the full 9.5" manufactured loft it's R30, for (R12 + R30=) R42 total.

R12/R42= ~28.5% , which is above the 27% minimum, and you're good, but not with a whole lot of margin.

An R30 compressed into a 9.25" deep 2x10 bay performs at R28. Compressed to 7.25" in a 2x8 bay it performs at about R25. Compressing it to 8.0" due to the additional inch of foam taking up some of that space would make it about R26.5.

R12 + R26.5 = R38.5

R12/R38.5= 31%...

...which is greater than the 27% minimum. So it looks like you'd have some margin at 3" foam, 8% fiber.

 

[Sponsor]

 

[Carl Weng]

Understood- So if I add in an extra 2 inch of foam to be safe- it would give me a total of 3.5 inches which is a Foam R of 17.5 plus the compressed R30 Fiber to R25 = Math: 17.5 / 42.5

With this compromise would i still need a vapor permeable sheet?

 

[Dana]

The more foam R you have, the more dew point margin there is.

R17.5/R42.5= 41%

That is well above the minimum 27%.

Below 27% a vapor retarder is required on the interior. But above 27% there still needs to be a drying path to keep ground moisture or any foundation leak water from potentially accumulating in that space.

The fiber insulation still needs an AIR barrier on all six sides of the cavity but you actively don't want a vapor barrier on the conditiond space side- it needs to be at least somewhat vapor permeable. If the cavity is completely full of fiber, the wallboard & standard latex paint will serve as the somewhat vapor-permeable air barrier that allows moisture that manages to find it's way in to have a drying path out.

 

[Carl Weng]

Got it- What do you suggest for air barrier on the top and bottom of the wall?

 

[Dana]

A sheet of foam board trimmed to fit then caulked/taped in place works.

 

[JFM224]

A sheet of foam board trimmed to fit then caulked/taped in place works.

Similar question.....I’m in Northern NJ (Bergen County). I believe we are Zone 5 and the minimum R value for a basement wall is 10. I’m looking to finish my basement, which is below grade accept for the top 2’. I’m debating between gluing 2” XPS (R10) to my block walls and then framing with furring strips OR going with a 1” EPS, framing with traditional 2x4s and adding R15 mineral wool batt. The second option would provide a higher R value, but would require more work and at a higher cost. The basement has French drains so water is not an issue unless the sump pump fails, which I plan to add a back up pump for.

Would appreciate any advice on whether the first option is sufficient or better to spend more time and money on option 2.

 

[Dana]

Similar question.....I’m in Northern NJ (Bergen County). I believe we are Zone 5 and the minimum R value for a basement wall is 10.

The IRC calls out R15 c.i. for zone 5 (yes Bergen County is zone 5), R10 c.i. for zone 4. If NJ is operating at IRC 2009 your local code might only call for R10, but I believe NJ is using IRC 2015, with some amendments, but still calling for R15.

I’m looking to finish my basement, which is below grade accept for the top 2’. I’m debating between gluing 2” XPS (R10) to my block walls and then framing with furring strips OR going with a 1” EPS, framing with traditional 2x4s and adding R15 mineral wool batt. The second option would provide a higher R value, but would require more work and at a higher cost. The basement has French drains so water is not an issue unless the sump pump fails, which I plan to add a back up pump for.

Would appreciate any advice on whether the first option is sufficient or better to spend more time and money on option 2.

Neither option is ideal.

R10 XPS is only warranteed to R9, and you need R15 type performance. The reason for the warrantee fine print is that it's performance drops over a few decades as it's climate-damaging HFC blowing agents (a mixture of HFC, predominately HFC134a, which is ~1400x CO2 @ 100 years) diffuse out.

At full depletion it's no better than EPS of similar density (R8.4). XPS is by far the least-green insulation material in common use in the US. Had the US signed on to the Kigali Amendment to the Montreal Protocol HFC blown foam would be contraband as of the beginning of last year, but that didn't happen- it's still pretty bad stuff:

The reason EPS has so much lower impact is that it (like polyisocyanurate) is blown with lower impact hydrocarbons, not HFCs, usually a variant of pentane, about 7x CO2 @ 100 years. With EPS most of the pentane leaves the foam while it's at the factory, and recaptured, not vented to the atmosphere. Foil faced 1lb per cubic foot density polyiso (not shown in the chart) has a somewhat lower impact than EPS, coming in somewhere between fiberglass and rock wool, whereas 2lb density roofing polyiso is comparable to the HFO-blown closed cell spray foam.

The other solution, 1" EPS + 2x4/R15 wall fails due to insufficient dew point control at the foam/fiber boundary. The IRC prescriptive for 2x4 walls in zone 5 calls out R5 minimum to mitigate agains wintertime moisture build up at the foam/fiber boundary at the above grade portion of the wall, and that presumes only R13 in the cavities, not R15. With R15 you'd need a minimum of R5 x R15/R13= R5.8 to have the same mean temperature at the foam/fiber boundary. That can be done with an inch of foil-faced polyiso (R6) rather than an inch of EPS, or 3/4" polyiso (R5) and using only R13 in the cavities. A 2x4/R13 + R5 c.i. wall has about the same (or very slightly better) performance level of R15 c.i., and would still meet code on a U-factor basis.

If using polyiso, keep the bottom edge of the polyiso above the historical high tide mark of any incidental flooding, since it can wick moisture whereas EPS does not. If the bottom foot or so of the wall has to be EPS rather than polyiso due to flooding it isn't going to make a big difference in thermal performance, and if it's below grade the average wintertime temperature of the soil is much higher than the above grade portion of the wall, presenting no dew point control/moisture accumulation issues.

Putting an inch of polyiso under the bottom plate of the 2x4 wall is recommended, since that provides a thermal and capillary break from the cooler and potentially damp slab, keeping the wood above the summertime indoor dew point. That keeps the bottom plate framing warmer & drier year-round, and standard untreated lumber can then be used.

If there is a reasonably local source, 3" of reclaimed roofing polyiso (R17-ish) either strapped to the wall with furring or trapped with an uninsulated studwall can be quite a bit cheaper than either of the methods discussed so far. Any reclaimed foam (even XPS) is greener than any virgin-stock insulation, since no new polymer, blowing agent, or fiber was used in it's manufacture Most used roofing EPS is 1.25lbs density "Type VIII" goods, at about R4.1/inch. Derate any EPS to R4.5/inch for design purposes, and any used roofing polyiso to R5/inch (though some pre-Montreal Protocol CFHC blown polyiso from the 1980s can be performing as high as R7/inch, even after 30+ years of service.)

 

[JFM224]

The IRC calls out R15 c.i. for zone 5 (yes Bergen County is zone 5), R10 c.i. for zone 4. If NJ is operating at IRC 2009 your local code might only call for R10, but I believe NJ is using IRC 2015, with some amendments, but still calling for R15.



Neither option is ideal.

R10 XPS is only warranteed to R9, and you need R15 type performance. The reason for the warrantee fine print is that it's performance drops over a few decades as it's climate-damaging HFC blowing agents (a mixture of HFC, predominately HFC134a, which is ~1400x CO2 @ 100 years) diffuse out.

At full depletion it's no better than EPS of similar density (R8.4). XPS is by far the least-green insulation material in common use in the US. Had the US signed on to the Kigali Amendment to the Montreal Protocol HFC blown foam would be contraband as of the beginning of last year, but that didn't happen- it's still pretty bad stuff:

The reason EPS has so much lower impact is that it (like polyisocyanurate) is blown with lower impact hydrocarbons, not HFCs, usually a variant of pentane, about 7x CO2 @ 100 years. With EPS most of the pentane leaves the foam while it's at the factory, and recaptured, not vented to the atmosphere. Foil faced 1lb per cubic foot density polyiso (not shown in the chart) has a somewhat lower impact than EPS, coming in somewhere between fiberglass and rock wool, whereas 2lb density roofing polyiso is comparable to the HFO-blown closed cell spray foam.

The other solution, 1" EPS + 2x4/R15 wall fails due to insufficient dew point control at the foam/fiber boundary. The IRC prescriptive for 2x4 walls in zone 5 calls out R5 minimum to mitigate agains wintertime moisture build up at the foam/fiber boundary at the above grade portion of the wall, and that presumes only R13 in the cavities, not R15. With R15 you'd need a minimum of R5 x R15/R13= R5.8 to have the same mean temperature at the foam/fiber boundary. That can be done with an inch of foil-faced polyiso (R6) rather than an inch of EPS, or 3/4" polyiso (R5) and using only R13 in the cavities. A 2x4/R13 + R5 c.i. wall has about the same (or very slightly better) performance level of R15 c.i., and would still meet code on a U-factor basis.

If using polyiso, keep the bottom edge of the polyiso above the historical high tide mark of any incidental flooding, since it can wick moisture whereas EPS does not. If the bottom foot or so of the wall has to be EPS rather than polyiso due to flooding it isn't going to make a big difference in thermal performance, and if it's below grade the average wintertime temperature of the soil is much higher than the above grade portion of the wall, presenting no dew point control/moisture accumulation issues.

Putting an inch of polyiso under the bottom plate of the 2x4 wall is recommended, since that provides a thermal and capillary break from the cooler and potentially damp slab, keeping the wood above the summertime indoor dew point. That keeps the bottom plate framing warmer & drier year-round, and standard untreated lumber can then be used.

If there is a reasonably local source, 3" of reclaimed roofing polyiso (R17-ish) either strapped to the wall with furring or trapped with an uninsulated studwall can be quite a bit cheaper than either of the methods discussed so far. Any reclaimed foam (even XPS) is greener than any virgin-stock insulation, since no new polymer, blowing agent, or fiber was used in it's manufacture Most used roofing EPS is 1.25lbs density "Type VIII" goods, at about R4.1/inch. Derate any EPS to R4.5/inch for design purposes, and any used roofing polyiso to R5/inch (though some pre-Montreal Protocol CFHC blown polyiso from the 1980s can be performing as high as R7/inch, even after 30+ years of service.)

Thanks for the input. I’m not able to find anything reclaimed but can get new 2.5” polyiso, which the vendor is claiming to be R15. The boards are square edge and paper faced though. Is that sufficient or can I double up to get to 3 or 3.5”? Also can the polyiso be glued directly to the block and should the block be sealed with a waterproofing paint prior?

We will likely encounter some flooding at some point if the sump pump fails and therefore will go with EPS on the bottom 1’. That said should EPS be used for the bottom plate since it won’t wick up any moisture?

 

[Dana]

Thanks for the input. I’m not able to find anything reclaimed but can get new 2.5” polyiso, which the vendor is claiming to be R15. The boards are square edge and paper faced though. Is that sufficient or can I double up to get to 3 or 3.5”? Also can the polyiso be glued directly to the block and should the block be sealed with a waterproofing paint prior?

If the product is labeled R15 it's good enough. Most 2lb density roofing polyiso would only be labeled R14 at that thickness, but paper faced 1lb density goods could easily hit the R15 mark.

Gluing paper faced foam to the wall with blobs of foam-board construction adhesive works, but to put the code-required thermal barrier against ignition (like half-inch wallboard) would still require either a studwall or 1x4 furring through screwed to the foundation. (Don't cheap out with 1x3s- they have lots of twists and are more prone to splitting if taking the furring approach.) With 2.5" foam and 1x4s you a 4" TapCon or other masonry screw works. A pancake-head (not pan-head) would be ideal, but other head types will still work using fender washers to keep from splitting the furring. Use housewrap tape to seal the seams between sheets, and a bead of foam board construction adhesive to seal the top of the foam to the foundation to prevent convection. Leave the bottom edge unsealed so that any bulk water coming through the foundation can drain.

If using a studwall tight to the foam board the adhesive is good enough on it's own.

If there are any signs of efflorescence on the CMU wall it's worth cleaning it up and sealing it with a silane/siloxane type masonry sealer. Any visible cracks or holes can be sealed with a polyurethane caulk formulated for concrete walls, but avoid the soupy "self leveling" stuff used for sealing slabs.

We will likely encounter some flooding at some point if the sump pump fails and therefore will go with EPS on the bottom 1’. That said should EPS be used for the bottom plate since it won’t wick up any moisture?

Tape the EPS to the polyiso with housewrap tape, and yes, put EPS under the bottom plate as a capillary break (= won't wick moisture), and a thermal break to keep the summertime temperature of the wood above the dew point of the room air. In your area the slab temperatures will usually be <65F (or even <60F) even in summer, but the outdoor dew point averages in July/August are in the mid to high 60s. Ventilation air into the basement in summer will be losing moisture to the cool slab or any wood resting on the slab via adsorption, even if it never reaches the point of outright liquid condensation. But with an inch or more of EPS under the bottom plate the wood will be several degrees warmer than the slab, and will not be taking on moisture.

With 2.5" wall foam, install the bottom plate/slab foam to extend under the wall for with a 1/2" -1" gap to the foundation wall as a drain-space for any bulk water that happens to come down the foundation.

There is probably a ledge at the top of the foundation before the foundation sill plate(?), which can be sealed and insulated with 1.5" foam board that extends fully out over the top of the wall foam. The band joist can also be insulated with cut'n'cobbled foam board cut 1/2" narrower than the width & height to allow sealing in place with can-foam. Using 1.5" foam for the cut'n' cobble and adding trimmed batts to fatten out the R is fine.

^^Note the foam cut into the top of the foundation ledge.^^

^^It's better to not have fiber insulation touching the sill plate as shown here.^^ A second layer of whatever foam was used on top of the ledge would be better at keeping interior-air moisture from getting into the cold sill plate in winter.

 

[JFM224]

If the product is labeled R15 it's good enough. Most 2lb density roofing polyiso would only be labeled R14 at that thickness, but paper faced 1lb density goods could easily hit the R15 mark.

Gluing paper faced foam to the wall with blobs of foam-board construction adhesive works, but to put the code-required thermal barrier against ignition (like half-inch wallboard) would still require either a studwall or 1x4 furring through screwed to the foundation. (Don't cheap out with 1x3s- they have lots of twists and are more prone to splitting if taking the furring approach.) With 2.5" foam and 1x4s you a 4" TapCon or other masonry screw works. A pancake-head (not pan-head) would be ideal, but other head types will still work using fender washers to keep from splitting the furring. Use housewrap tape to seal the seams between sheets, and a bead of foam board construction adhesive to seal the top of the foam to the foundation to prevent convection. Leave the bottom edge unsealed so that any bulk water coming through the foundation can drain.

If using a studwall tight to the foam board the adhesive is good enough on it's own.

If there are any signs of efflorescence on the CMU wall it's worth cleaning it up and sealing it with a silane/siloxane type masonry sealer. Any visible cracks or holes can be sealed with a polyurethane caulk formulated for concrete walls, but avoid the soupy "self leveling" stuff used for sealing slabs.



Tape the EPS to the polyiso with housewrap tape, and yes, put EPS under the bottom plate as a capillary break (= won't wick moisture), and a thermal break to keep the summertime temperature of the wood above the dew point of the room air. In your area the slab temperatures will usually be <65F (or even <60F) even in summer, but the outdoor dew point averages in July/August are in the mid to high 60s. Ventilation air into the basement in summer will be losing moisture to the cool slab or any wood resting on the slab via adsorption, even if it never reaches the point of outright liquid condensation. But with an inch or more of EPS under the bottom plate the wood will be several degrees warmer than the slab, and will not be taking on moisture.

With 2.5" wall foam, install the bottom plate/slab foam to extend under the wall for with a 1/2" -1" gap to the foundation wall as a drain-space for any bulk water that happens to come down the foundation.

There is probably a ledge at the top of the foundation before the foundation sill plate(?), which can be sealed and insulated with 1.5" foam board that extends fully out over the top of the wall foam. The band joist can also be insulated with cut'n'cobbled foam board cut 1/2" narrower than the width & height to allow sealing in place with can-foam. Using 1.5" foam for the cut'n' cobble and adding trimmed batts to fatten out the R is fine.

^^Note the foam cut into the top of the foundation ledge.^^

^^It's better to not have fiber insulation touching the sill plate as shown here.^^ A second layer of whatever foam was used on top of the ledge would be better at keeping interior-air moisture from getting into the cold sill plate in winter.

 

[JFM224]

Thanks again for the insight.

Is this the ledge your referring to? Is the idea to basically to tuck in some foam board insulation within that space to avoid a cool spot? Anything special I would have to do around the windows? My plan was to frame out using wood and then paint, would I need to add foam board there as well?

Also as you can see my ceiling is already finished and therefore debating whether to cut near the wall for framing. If I go with furring strips I think I can avoid cutting, otherwise assuming I will need to cut to locate joists and blocking. Thoughts?

Lastly the only polyiso I could find is typically used for roofing (Firestone ISO 95GL). Again it’s paper based, but wanted to understand if it was suitable for basement walls.

 

[Dana]

Translating the metric to US units in the spec sheet, it's 2lbs density and the 2.5" thick sheets test at R13.85, (R5.54/inch) even though the 1" sheets test at R6.30. From a total performance point of view that's a "who cares?", but it can't be legitimately labeled R15. If installing an empty wallboard clad studwall in front of it the R value of the additional air films & gypsum board as well as the R-value of the CMU block will bring up the performance enough to meet/beat code on a U-factor basis.

Glass reinforced mat facers (which looks like dark gray paper) and the lower permeance 2lbs polyiso are just fine for this application.

It looks like they sculpted a mortar bed the top of the ledge to seal the tops of the CMU cores. This is going to make air sealing the foundation sill and insulating that curve-over ledge a bit awkward do to without using a bit of spray foam. As much as I hate to use it, this may call for sealing and insulating the top of the foundation to the foundation sill and band joist with a 2-part spray foam kit (which uses HFC blowing agents.) A strong inch of coverage is enough for dew point control on R15 high density or rock wool batts. It also looks like you won't be able to air seal and insulate the band joist without doing a bit of ceiling demolition & repair.

 

[JFM224]

Translating the metric to US units in the spec sheet, it's 2lbs density and the 2.5" thick sheets test at R13.85, (R5.54/inch) even though the 1" sheets test at R6.30. From a total performance point of view that's a "who cares?", but it can't be legitimately labeled R15. If installing an empty wallboard clad studwall in front of it the R value of the additional air films & gypsum board as well as the R-value of the CMU block will bring up the performance enough to meet/beat code on a U-factor basis.

Glass reinforced mat facers (which looks like dark gray paper) and the lower permeance 2lbs polyiso are just fine for this application.

It looks like they sculpted a mortar bed the top of the ledge to seal the tops of the CMU cores. This is going to make air sealing the foundation sill and insulating that curve-over ledge a bit awkward do to without using a bit of spray foam. As much as I hate to use it, this may call for sealing and insulating the top of the foundation to the foundation sill and band joist with a 2-part spray foam kit (which uses HFC blowing agents.) A strong inch of coverage is enough for dew point control on R15 high density or rock wool batts. It also looks like you won't be able to air seal and insulate the band joist without doing a bit of ceiling demolition & repair.

Thanks, I took a closer look and there is some batt insulation above the sheet rock to seal the band joist. Rather than ripping that ceiling up I’m leaning towards leaving it as is and adding some spray foam to cover the exposed gap. I’ll likely then go with the polyiso foam board / EPS glued directly to the wall and frame tight with 2x4s.

 

[Dana]

Thanks, I took a closer look and there is some batt insulation above the sheet rock to seal the band joist. Rather than ripping that ceiling up I’m leaning towards leaving it as is and adding some spray foam to cover the exposed gap. I’ll likely then go with the polyiso foam board / EPS glued directly to the wall and frame tight with 2x4s.

If there is a convenient location it's worth checking to see if the band joist behind the batt insulation is caulked to both the foundation sill and the subfloor above. Those seams may not look very big, but the total size of the hole usually adds up to more than all window & door crackage combined.

It's not necessarily worth demolishing the ceiling to rectify that, but if it's pretty leaky (which can be determined with a blower door and infra-red camera) there are other ways to deal with it short of tearing it all apart.

[edited to add]

As it happens air sealing and insulating band joists is the topic of today's GBA-Prime blog (there are free trial subscriptions to get around the paywall), which is probably worth a read.

Air leakage under the foundation sill can be dealt with by spray foam, but that could be less than half the total leakage path in that area if the sheathing & band joist seams & penetrations aren't fully up to snuff: