To insulate or not to insulate a basement bathroom??

[Dana]

Leave the Drylok on. Drylok reduces the capillary draw of ground moisture, but is not a vapor barrier.

Above the frost line is where you need the full-R the most from an energy use/comfort point of view. Every where that it can be safe for the plumbing you should have both the foam board + fiber. The only place you should relieve the fiber insulation is where is the narrow passages where would be a potential pipe freeze-up problem. Pipes above the frost line are at risk if there is insulation between the pipe and the interior of the basement.

The foam board can be glued with purpose made "foam board construction adhesive" or cap-screwed to the foundation wall, but tape all of the seams. Most common adhesives used in construction use solvents that will degrade foam board. It's available at box stores.

If you use EPS it can run all the way down to the slab. If the wall foam is (higher R/inch) polyisocyanurate it's best to use housewrap tape or foil tape to seal the cut bottom edge, and stop a few inches above the slab, filling in the remainder with EPS. If exposed polyiso rests on the slab (or flood water even from a minor puddle) the potential for slowly taking on ground moisture and losing performance is non-zero. Polyiso can wick and retain water, taking forever to dry. EPS doesn't have that issue.

 

[Birdlove]

Leave the Drylok on. Drylok reduces the capillary draw of ground moisture, but is not a vapor barrier.

Above the frost line is where you need the full-R the most from an energy use/comfort point of view. Every where that it can be safe for the plumbing you should have both the foam board + fiber. The only place you should relieve the fiber insulation is where is the narrow passages where would be a potential pipe freeze-up problem. Pipes above the frost line are at risk if there is insulation between the pipe and the interior of the basement.

The foam board can be glued with purpose made "foam board construction adhesive" or cap-screwed to the foundation wall, but tape all of the seams. Most common adhesives used in construction use solvents that will degrade foam board. It's available at box stores.

If you use EPS it can run all the way down to the slab. If the wall foam is (higher R/inch) polyisocyanurate it's best to use housewrap tape or foil tape to seal the cut bottom edge, and stop a few inches above the slab, filling in the remainder with EPS. If exposed polyiso rests on the slab (or flood water even from a minor puddle) the potential for slowly taking on ground moisture and losing performance is non-zero. Polyiso can wick and retain water, taking forever to dry. EPS doesn't have that issue.

I looked at the pipes again and although they can be moved away from the wall to add space for 1 inch thick foam board, it will be a a lot of work. I'm wondering if it's really necessary. There will be about 5-3/4 inches of space between the foundation and the interior stud wall (with water supply and drain pipes in the cavity). Does that amount of space change anything, or is the 1 inch rigid foam board still required?

 

[Sponsor]

 

[Dana]

Do NOT leave an empty cavity space between the studwall and foundation. Empty cavities are fire spread paths, and codes require fire-stops. The empty cavity also becomes a potential thermal bypass for your insulated wall, and fiber insulation needs an air barrier on the exterior side of the wall too, not just the finished wallboard side. Without air barriers, air convection into & out of the batt on the cold side introduces a big thermal performance hit that becomes even bigger as temperature differences increase.

If it's 5.75" of space from the foundation to the interior-most side of the drain pipe, if filling most of the space with fiber you'll need more than an inch of foam board for dew point control on that much fiber insulation if you're in US climate zone 5. An inch of EPS is fine for for zone 4 at any amount of fiber, but even on the warmer half of zone 5 a mere inch of EPS would have at least wintertime condensation events even with just R13 of fiber insulation, but on the colder parts of zone 5 it would be better to use an inch of polyiso (R6-ish).

So to get the recommendations right (=relatively low mold risk at relatively low construction cost), can you share your ZIP code?

 

[Birdlove]

Do NOT leave an empty cavity space between the studwall and foundation. Empty cavities are fire spread paths, and codes require fire-stops. The empty cavity also becomes a potential thermal bypass for your insulated wall, and fiber insulation needs an air barrier on the exterior side of the wall too, not just the finished wallboard side. Without air barriers, air convection into & out of the batt on the cold side introduces a big thermal performance hit that becomes even bigger as temperature differences increase.

If it's 5.75" of space from the foundation to the interior-most side of the drain pipe, if filling most of the space with fiber you'll need more than an inch of foam board for dew point control on that much fiber insulation if you're in US climate zone 5. An inch of EPS is fine for for zone 4 at any amount of fiber, but even on the warmer half of zone 5 a mere inch of EPS would have at least wintertime condensation events even with just R13 of fiber insulation, but on the colder parts of zone 5 it would be better to use an inch of polyiso (R6-ish).

So to get the recommendations right (=relatively low mold risk at relatively low construction cost), can you share your ZIP code?

Dana, Thanks for this info. I am in 43214 zip code.
Thanks

 

[Dana]

Greater Columbus is in the middle of Zone 5, not the warm edge, not the cold edge, so ideally you'd want at least 1.5" of EPS or 1" polyiso for dew point control on a 2x4/R13 studwall snugged right up to the foam. But that's only 5" thick, not 5.75", and even 2" foam would only bring it out to 5.5", so you're looking at 2.5" of foam, which brings the interior stud edges 6" out from the foundation, or 2" of foam and a 2x6 studwall, which is 7.5", probably taking up more space than you'd like.

At 2.5" thick foam (any type) is almost be enough to meet code minimum performance on it's own (without fiber insulation) but cheap "contractor roll" R11s or R13s in the 2x4 studwall portion would still be worthwhile. To take the sting out of that much foam, buying 2"-3" reclaimed roofing foam ($24/sheet from folks like this but sometimes even cheaper) can be comparable in cost to 1" polyiso from box stores. At 3" fiber faced roofing polyiso would meet code min performance on it's own, and might make the whole assembly easier. With a 2x4 studwall that would put the interior facing stud edges 6.5" from the foundation and it wouldn't need any cavity insulation to meet code minimum (though it would be fine to install batts as well.) With polyiso you need to keep the cut bottom edge off the slab to keep it wicking ground moisture or any minor flood moisture. An inch of EPS extending from the foundation wall to under the bottom plate of your studwall protects both the polyiso and stud plate from ground moisture issues.

There are other vendors of reclaimed foam- deals come and go, but if you run this search every week or so you'll find quite a few, maybe even someone closer to you with the thickness you want.

 

[Birdlove]

Greater Columbus is in the middle of Zone 5, not the warm edge, not the cold edge, so ideally you'd want at least 1.5" of EPS or 1" polyiso for dew point control on a 2x4/R13 studwall snugged right up to the foam. But that's only 5" thick, not 5.75", and even 2" foam would only bring it out to 5.5", so you're looking at 2.5" of foam, which brings the interior stud edges 6" out from the foundation, or 2" of foam and a 2x6 studwall, which is 7.5", probably taking up more space than you'd like.

At 2.5" thick foam (any type) is almost be enough to meet code minimum performance on it's own (without fiber insulation) but cheap "contractor roll" R11s or R13s in the 2x4 studwall portion would still be worthwhile. To take the sting out of that much foam, buying 2"-3" reclaimed roofing foam ($24/sheet from folks like this but sometimes even cheaper) can be comparable in cost to 1" polyiso from box stores. At 3" fiber faced roofing polyiso would meet code min performance on it's own, and might make the whole assembly easier. With a 2x4 studwall that would put the interior facing stud edges 6.5" from the foundation and it wouldn't need any cavity insulation to meet code minimum (though it would be fine to install batts as well.) With polyiso you need to keep the cut bottom edge off the slab to keep it wicking ground moisture or any minor flood moisture. An inch of EPS extending from the foundation wall to under the bottom plate of your studwall protects both the polyiso and stud plate from ground moisture issues.

There are other vendors of reclaimed foam- deals come and go, but if you run this search every week or so you'll find quite a few, maybe even someone closer to you with the thickness you want.

The gap between the foundation and the stud wall is 5-3/4 inches. The room side of that stud wall (2x4 plus 1/2 inch drywall) will be approximately 8-3/4 inches from the foundation. I'll try to attach a sketch of what it looks like.

 

[Dana]

If the studwall is not next to the foundation wall, don't insulate between the studs. Slip in some 1" foam insulation (thicker, if it fits) between the pipes and foundation. Above/below/around the pipes increase the thickness of the foam to 3". You can use a combination of foam board adhesive and/or large cap screws to secure the foam to the foundation, then use foam board adhesive to mount wallboard to the foam as a code-required thermal barrier against ignition for the foam. Alternatively you could strap the foam to the foundation with 1x4 furring and 5" masonry screws 16" o.c., mounting the wallboard to the furring. But it may not need that layer of wallboard:

Code inspectors might let you get away without wallboard directly on the foam in the 5-3/4" cavity which code treats as a "concealed space" if there is no wiring or ignition sources in there, treating the the finish wall's wallboard as the thermal barrier. At a minimum there needs to be a fireblock at the top of the concealed space. For ease of assembly maybe you want to use a 2 x 10 covered with a sheet of half-inch plywood or OSB for the top plate for that purpose. (See the code specs for alterative fireblocking materials .) A 2 x 10 nominally 9-1/4" wide, which is happens to be the depth from the foundation wall to the interior facing stud edges. You'll still want to insulate the top of the foundation , the foundation sill and band joist.

 

[Birdlove]

If the studwall is not next to the foundation wall, don't insulate between the studs. Slip in some 1" foam insulation (thicker, if it fits) between the pipes and foundation. Above/below/around the pipes increase the thickness of the foam to 3". You can use a combination of foam board adhesive and/or large cap screws to secure the foam to the foundation, then use foam board adhesive to mount wallboard to the foam as a code-required thermal barrier against ignition for the foam. Alternatively you could strap the foam to the foundation with 1x4 furring and 5" masonry screws 16" o.c., mounting the wallboard to the furring. But it may not need that layer of wallboard:

Code inspectors might let you get away without wallboard directly on the foam in the 5-3/4" cavity which code treats as a "concealed space" if there is no wiring or ignition sources in there, treating the the finish wall's wallboard as the thermal barrier. At a minimum there needs to be a fireblock at the top of the concealed space. For ease of assembly maybe you want to use a 2 x 10 covered with a sheet of half-inch plywood or OSB for the top plate for that purpose. (See the code specs for alterative fireblocking materials .) A 2 x 10 nominally 9-1/4" wide, which is happens to be the depth from the foundation wall to the interior facing stud edges. You'll still want to insulate the top of the foundation , the foundation sill and band joist.

Thanks.
I would rather not deal with the additional layer of wall board if I don't need to. I can picture a 2 x 10 top plate, but what is the half-inch plywood for?

If I have to 'cut & paste' pieces of foam board to work around the pipes, I will need to glue foam board to foam board. I will need 3 layers of 1 inch foam board, total. Do I need to tape seams at each layer?
Is this the proper foam board to use: https://www.menards.com/main/buildi...1553-c-5779.htm?tid=395335551418927476&ipos=1

So with 3 inches of foam against the foundation wall, there would be no need for additional 'fiber' insulation between the studs? Is that because the dew point would be somewhere within the 3 inches of foam board, which is then not a concern because the moisture can't reach it?

Sorry for the million questions! I really do want to understand the science behind insulation and moisture control. Can you recommend a website that may have info that would help me understand? I am a visual learner, so pics & illustrations are always good for me!

Thanks again,
Steve

 

[Dana]

Thanks.
I would rather not deal with the additional layer of wall board if I don't need to. I can picture a 2 x 10 top plate, but what is the half-inch plywood for?

If I have to 'cut & paste' pieces of foam board to work around the pipes, I will need to glue foam board to foam board. I will need 3 layers of 1 inch foam board, total. Do I need to tape seams at each layer?
Is this the proper foam board to use: https://www.menards.com/main/buildi...1553-c-5779.htm?tid=395335551418927476&ipos=1

So with 3 inches of foam against the foundation wall, there would be no need for additional 'fiber' insulation between the studs? Is that because the dew point would be somewhere within the 3 inches of foam board, which is then not a concern because the moisture can't reach it?

Sorry for the million questions! I really do want to understand the science behind insulation and moisture control. Can you recommend a website that may have info that would help me understand? I am a visual learner, so pics & illustrations are always good for me!

Thanks again,
Steve

Dew point is a temperature, not a location- the temperature at which the relative humidity is 100%, and further cooling results in fog or dew. In this instance the relevant dew point is that of the indoor air. If the surface of the foam, or any fiber insulation is below the dew point of the indoor air moisture will accumulate on it. If it's above the dew point temperature it stays dry.

With latex painted wallboard as an interior side vapor retarder only the average winter temp at the front surface of the foam- it can dip below the dew point temp for hours or even many days at a time without accumulating enough moisture to cause a problem, due to the vapor retardency of the paint slowing down the moisture transfer. In a foam/fiber insulation stackup the ratio of the R-values, average (over several weeks) indoor & outdoor temperatures, and the dew point of the indoor air determine the temperature at that first foam surface that would accumulate condensing water.

Unless you are actively adding humidity to the air (almost always a bad idea), the dew point of the indoor air in winter would normally run between 35-40F in houses that have at least a modest ventilation (or infiltration) rate. At an indoor temperature of 68F that corresponds to an indoor relative humidity of 30-36%, which is the human-healthy and comfortable range. In very tight houses mechanical ventilation would be needed to keep it that low.

The reason you don't need fiber insulation when you have 3" of foam is solely about code compliance. IRC 2015 calls out a minimum of R15 continuous insulation for US climate zone 5 or higher, R10 if zone 4. At 3" polyisocyanurate (any density) would be more than R15. At 3" Type VIII or Type II EPS would be about R12.5. With the performance of additional trapped air films inside the empty wall the performance would hit (barely) R15 performance, or close enough to reach compliance on a total U-factor basis. It's OK to add fiber insulation if you like, as long as at least the ratio of foam-R to total-R is sufficient for wintertime dew point control at the above-grade section.

With foam-only insulation the face of the foam is essentially at the same temperature as the room air. As long as you don't have 100% relative humidity indoors with fog forming in the basement, it won't be condensing on the foam.

A single 1.5" thick milled 2x10 top plate does not meet the code requirements for fireblocking- it has to be a true 2" thick (or more), ergo you either need to cut in some plywood, or double up on the 2x10s.

 

[Birdlove]

Dew point is a temperature, not a location- the temperature at which the relative humidity is 100%, and further cooling results in fog or dew. In this instance the relevant dew point is that of the indoor air. If the surface of the foam, or any fiber insulation is below the dew point of the indoor air moisture will accumulate on it. If it's above the dew point temperature it stays dry.

With latex painted wallboard as an interior side vapor retarder only the average winter temp at the front surface of the foam- it can dip below the dew point temp for hours or even many days at a time without accumulating enough moisture to cause a problem, due to the vapor retardency of the paint slowing down the moisture transfer. In a foam/fiber insulation stackup the ratio of the R-values, average (over several weeks) indoor & outdoor temperatures, and the dew point of the indoor air determine the temperature at that first foam surface that would accumulate condensing water.

Unless you are actively adding humidity to the air (almost always a bad idea), the dew point of the indoor air in winter would normally run between 35-40F in houses that have at least a modest ventilation (or infiltration) rate. At an indoor temperature of 68F that corresponds to an indoor relative humidity of 30-36%, which is the human-healthy and comfortable range. In very tight houses mechanical ventilation would be needed to keep it that low.

The reason you don't need fiber insulation when you have 3" of foam is solely about code compliance. IRC 2015 calls out a minimum of R15 continuous insulation for US climate zone 5 or higher, R10 if zone 4. At 3" polyisocyanurate (any density) would be more than R15. At 3" Type VIII or Type II EPS would be about R12.5. With the performance of additional trapped air films inside the empty wall the performance would hit (barely) R15 performance, or close enough to reach compliance on a total U-factor basis. It's OK to add fiber insulation if you like, as long as at least the ratio of foam-R to total-R is sufficient for wintertime dew point control at the above-grade section.

With foam-only insulation the face of the foam is essentially at the same temperature as the room air. As long as you don't have 100% relative humidity indoors with fog forming in the basement, it won't be condensing on the foam.

A single 1.5" thick milled 2x10 top plate does not meet the code requirements for fireblocking- it has to be a true 2" thick (or more), ergo you either need to cut in some plywood, or double up on the 2x10s.

Thanks for the explanation. I think that helps. I will have to read through it a few more times to get it!

I was able to move all the pipes to get enough space for 1 inch foam against the foundation, then I cut & paste the rest to get 3 inches total thickness.
I have found EPS at various price ranges here. One has Poly-facer on both sides, but the other seems to be just EPS with no laminate. Does it matter which one I use?

 

[Dana]

The stuff without the laminate allows at least some drying of the foundation wall toward the interior. The concrete is happy as clams in wet sand even when fully saturated, but a damp foundation wall can compromise a wood foundation sill if there isn't a good capillary break (such as sheet EPDM) between the concrete and wood. If you have a foot of exterior exposure everywhere on the foundation wall and good grading keeping the surface water flowing away from the foundation it usually doesn't need to dry toward the interior to protect the foundation sill.

The stuff with the laminate is also usually only R3.9/inch. It is low enough density that it is a bit fragile without the facers. EPS sold without facers is usually higher density, but also R4.1 / inch to R4.2 / inch.

The facers takes the vapor permeance of an inch of 1lb density "Type-I" EPS from about 4.5-5 perms (unfaced) to less than 0.1 perms, which is a true vapor barrier. Unfaced 1.25lbs (Type VIII) or 1.5lbs (Type-II) EPS runs between 2.5-4 perms @ 1", and would be between 0.8-1.3 perms @ 3", which would be nearly ideal. If the unfaced goods are Type-I it would be between 1.5-2 perms @ 3", which is also fine, but it has to be handled with care to keep from knocking off the corners.

For the sections where the foam is only going to be 1" thick you might want to use 1" foil faced polyisocyanurate, which runs about R6-R7. The foil facer facing an air gap, not in direct contact with the plumbing would improve that by another R1 or so, which is a worthwhile upgrade from R4-ish @ 1" EPS.

For cutting foam board cleanly use a 3-5" steel wallboard taping knife that has been sharpened on the sides, and a straight edge. That's saves you from being covered in EPS beads or polyiso dust/shred, which is what happens if you use saws. See this vidi.

 

[Birdlove]

The stuff without the laminate allows at least some drying of the foundation wall toward the interior. The concrete is happy as clams in wet sand even when fully saturated, but a damp foundation wall can compromise a wood foundation sill if there isn't a good capillary break (such as sheet EPDM) between the concrete and wood. If you have a foot of exterior exposure everywhere on the foundation wall and good grading keeping the surface water flowing away from the foundation it usually doesn't need to dry toward the interior to protect the foundation sill.

The stuff with the laminate is also usually only R3.9/inch. It is low enough density that it is a bit fragile without the facers. EPS sold without facers is usually higher density, but also R4.1 / inch to R4.2 / inch.

The facers takes the vapor permeance of an inch of 1lb density "Type-I" EPS from about 4.5-5 perms (unfaced) to less than 0.1 perms, which is a true vapor barrier. Unfaced 1.25lbs (Type VIII) or 1.5lbs (Type-II) EPS runs between 2.5-4 perms @ 1", and would be between 0.8-1.3 perms @ 3", which would be nearly ideal. If the unfaced goods are Type-I it would be between 1.5-2 perms @ 3", which is also fine, but it has to be handled with care to keep from knocking off the corners.

For the sections where the foam is only going to be 1" thick you might want to use 1" foil faced polyisocyanurate, which runs about R6-R7. The foil facer facing an air gap, not in direct contact with the plumbing would improve that by another R1 or so, which is a worthwhile upgrade from R4-ish @ 1" EPS.

For cutting foam board cleanly use a 3-5" steel wallboard taping knife that has been sharpened on the sides, and a straight edge. That's saves you from being covered in EPS beads or polyiso dust/shred, which is what happens if you use saws. See this vidi.

Thanks for the tips and the video.
I sketched what I believe you're saying about the fire stop. Please let me know if I got it wrong.
When I piece some of the foam board in around the pipes, should I use a spray foam (Great Stuff) insulation to fill in around the pipes? How about GS between foam board edges? Tape?
Thanks!