Sill Plate Insulation Codes

[HSBW]

Does anyone have experience in determining the options available for sealing and insulating one's sill plate/rim joists? My current preferred option would be XPS foam with a spray foam around the edges to get a good seal. I'm not sure if a fire barrier would be required to go over it though. Having to install gypsum board over the insulation would be a major annoyance. Any other suggested methods? Currently there is some old fiberglass in place from previous owners. Given the dark spots I see in many places, I don't think a proper air sealing job was done before placing the insulation and I'd like to fix that. Thanks!

[ncornilsen]

Is your crawlspace ventilated?

If so, air sealing is a waste of time. you WANT airflow in your crawlspace if it's not conditioned.

[bacchi]

Yes, a thermal or fire resistant barrier is required, depending on the usage of the space.

You can use rigid foam that has a thermal barrier or, if the crawl space isn't used for storage, even OSB or plywood. It doesn't have to be drywall.

[Sibley]

If you're talking about balloon framing, yes you need to air seal the sill plate. Less so for the sake of the crawl, more for the sake of lowering your heat bill. I've always seen some sort of spray foam used, possibly with a harder material to help fill larger areas (foam board, etc).

I have no idea what the codes might be for newer structures, old houses basically get grandfathered into a ton of stuff. I also don't have anywhere near an air tight crawl space, so allowing for air flow isn't a concern for me.

[sonofsven]

Does anyone have experience in determining the options available for sealing and insulating one's sill plate/rim joists? My current preferred option would be XPS foam with a spray foam around the edges to get a good seal. I'm not sure if a fire barrier would be required to go over it though. Having to install gypsum board over the insulation would be a major annoyance. Any other suggested methods? Currently there is some old fiberglass in place from previous owners. Given the dark spots I see in many places, I don't think a proper air sealing job was done before placing the insulation and I'd like to fix that. Thanks!

Air sealing that detail is a fairly recent practice, code minimum for floor insulation depends on your climate zone, but is probably immaterial to your question.
Practice in my new construction experience for air sealing is:  bottom of plate to foundation, top of plate to rim joist, top of rim joist to bottom of sub floor, and any penetrations through the rim and through the sub floor.
I'm insulating 2x12 framed floors, typically, with unfaced fiberglass r-38. Climate zone 4

Also: the installers have very good control and precision from their foam installation guns, much better than the little cans you buy at the hardware store. I've had better diy luck using caulk for air sealing than spray foam.

[BudgetSlasher]

Cutting rigid foam to fit and using one part foam (great stuff) is probably a decent cheap method.

I've found the 2 part spray foam (froth-pak) to be far superior to the one part. It costs a little more and it takes a little getting used to (plus some PPE).

One way to make 2-part a little cheaper is what some call flash-n-batt. Basically a thin coat of 2 part to block air movement and a batt insulation (rockwool or fiberglass) to bulk up the R-value.

As for a barrier yes you will likely need a barrier with foam. What type seems to depend on the amount of access/use the space will see. I am not at all an expert there so I will just point you to google thermal vs ignition barrier in relation to foam insulation and/or wait for someone else with more experience to chime in.

[HSBW]

Is your crawlspace ventilated?

If so, air sealing is a waste of time. you WANT airflow in your crawlspace if it's not conditioned.

The initial use case would be for the sill plate area of an unfinished half of my basement. I would eventually like to do something similar with my crawl space in conjunction with encapsulating and bringing it into the building thermal envelope.

Yes, a thermal or fire resistant barrier is required, depending on the usage of the space.

You can use rigid foam that has a thermal barrier or, if the crawl space isn't used for storage, even OSB or plywood. It doesn't have to be drywall.

So that would mean something like Dow Thermax PIR faced board wouldn't need an additional thermal barrier?

If you're talking about balloon framing, yes you need to air seal the sill plate. Less so for the sake of the crawl, more for the sake of lowering your heat bill. I've always seen some sort of spray foam used, possibly with a harder material to help fill larger areas (foam board, etc).

I have no idea what the codes might be for newer structures, old houses basically get grandfathered into a ton of stuff. I also don't have anywhere near an air tight crawl space, so allowing for air flow isn't a concern for me.

Yes, looking to lower air infiltration for comfort and lowered heating requirements.

Air sealing that detail is a fairly recent practice, code minimum for floor insulation depends on your climate zone, but is probably immaterial to your question.
Practice in my new construction experience for air sealing is:  bottom of plate to foundation, top of plate to rim joist, top of rim joist to bottom of sub floor, and any penetrations through the rim and through the sub floor.
I'm insulating 2x12 framed floors, typically, with unfaced fiberglass r-38. Climate zone 4

Also: the installers have very good control and precision from their foam installation guns, much better than the little cans you buy at the hardware store. I've had better diy luck using caulk for air sealing than spray foam.

So in this case you would air seal the mentioned locations with caulk and then an unfaced fiberglass (or potentially rock wool) batt?

Cutting rigid foam to fit and using one part foam (great stuff) is probably a decent cheap method.

I've found the 2 part spray foam (froth-pak) to be far superior to the one part. It costs a little more and it takes a little getting used to (plus some PPE).

One way to make 2-part a little cheaper is what some call flash-n-batt. Basically a thin coat of 2 part to block air movement and a batt insulation (rockwool or fiberglass) to bulk up the R-value.

As for a barrier yes you will likely need a barrier with foam. What type seems to depend on the amount of access/use the space will see. I am not at all an expert there so I will just point you to google thermal vs ignition barrier in relation to foam insulation and/or wait for someone else with more experience to chime in.

In the flash and batt method, would a batt of rock wool suffice as a thermal barrier?

[Sibley]

Just be careful - if you tighten up older buildings too much, you can cause problems with moisture. The classic insulation for old houses without vapor barriers is to air seal around doors/windows, top and bottoms of the exterior walls (basement/attic), and then insulate the attic. It's much harder to do the walls. I'm not sure about crawls, so do your research. If the source is only talking about new construction, its not something you can just go with. There are adjustments needed when factoring in old buildings.

[bacchi]

Dow Thermax and rock wool are both thermal barriers.

There is fire resistant caulk and foam. I've only used the foam and it's messy as you'd expect. It's definitely not for finish work.

[HSBW]

Just be careful - if you tighten up older buildings too much, you can cause problems with moisture. The classic insulation for old houses without vapor barriers is to air seal around doors/windows, top and bottoms of the exterior walls (basement/attic), and then insulate the attic. It's much harder to do the walls. I'm not sure about crawls, so do your research. If the source is only talking about new construction, its not something you can just go with. There are adjustments needed when factoring in old buildings.

For the moisture problems you mention, is the concern with moisture within the wall assembly or more general moisture in the house interior? At the moment my best guess is that the house is very leaky (1950s construction with a 1970s addition) though I’ve never had a blower door test performed to confirm that suspicion. The crawl section is only below the addition. Maybe a quarter of the overall building footprint. The sill plate area I have easier access to is in the remaining footprint of the house.

[HSBW]

Dow Thermax and rock wool are both thermal barriers.

There is fire resistant caulk and foam. I've only used the foam and it's messy as you'd expect. It's definitely not for finish work.

Good to know. Thanks. Sounds like caulking the area and then placing rock wool would be a viable strategy.

[BudgetSlasher]

Just be careful - if you tighten up older buildings too much, you can cause problems with moisture. The classic insulation for old houses without vapor barriers is to air seal around doors/windows, top and bottoms of the exterior walls (basement/attic), and then insulate the attic. It's much harder to do the walls. I'm not sure about crawls, so do your research. If the source is only talking about new construction, its not something you can just go with. There are adjustments needed when factoring in old buildings.

For the moisture problems you mention, is the concern with moisture within the wall assembly or more general moisture in the house interior? At the moment my best guess is that the house is very leaky (1950s construction with a 1970s addition) though I’ve never had a blower door test performed to confirm that suspicion. The crawl section is only below the addition. Maybe a quarter of the overall building footprint. The sill plate area I have easier access to is in the remaining footprint of the house.

I won't speak for Sibley, but there are plenty of examples of tightening up the air leakage on an older house, even one that has been standing for 100+ years, and it quickly develops all kinds of moisture related issues (mold/rot/so-on). It seems to be 2 fold, first putting what can amount to a moisture barrier on the inside with foam prevents the structure from drying to the inside and second the air that was previously flowing through the assembly was carrying away moisture that had gotten in.

I've seen some video, though not in person, of people using various methods, like dimpled drainage mat on the back side of the exterior walls before foam insulation, to ensure the airflow that has been drying a house for decades in allowed to continue.

[Sibley]

Just be careful - if you tighten up older buildings too much, you can cause problems with moisture. The classic insulation for old houses without vapor barriers is to air seal around doors/windows, top and bottoms of the exterior walls (basement/attic), and then insulate the attic. It's much harder to do the walls. I'm not sure about crawls, so do your research. If the source is only talking about new construction, its not something you can just go with. There are adjustments needed when factoring in old buildings.

For the moisture problems you mention, is the concern with moisture within the wall assembly or more general moisture in the house interior? At the moment my best guess is that the house is very leaky (1950s construction with a 1970s addition) though I’ve never had a blower door test performed to confirm that suspicion. The crawl section is only below the addition. Maybe a quarter of the overall building footprint. The sill plate area I have easier access to is in the remaining footprint of the house.

I won't speak for Sibley, but there are plenty of examples of tightening up the air leakage on an older house, even one that has been standing for 100+ years, and it quickly develops all kinds of moisture related issues (mold/rot/so-on). It seems to be 2 fold, first putting what can amount to a moisture barrier on the inside with foam prevents the structure from drying to the inside and second the air that was previously flowing through the assembly was carrying away moisture that had gotten in.

I've seen some video, though not in person, of people using various methods, like dimpled drainage mat on the back side of the exterior walls before foam insulation, to ensure the airflow that has been drying a house for decades in allowed to continue.

You're absolutely correct. It's also quite difficult to turn an unfinished, unconditioned attic into a conditioned living space - because that attic NEEDED air flow to not rot out, so if you're going to remove the airflow you also have to solve the moisture problem.

Basically, old houses are simply built differently. Part of it is the materials, part is the methods and techniques used. It's not that its better or worse, because frankly arguments can be made both ways. Its different, and sometimes those differences are quite subtle.

[index]

Just be careful - if you tighten up older buildings too much, you can cause problems with moisture. The classic insulation for old houses without vapor barriers is to air seal around doors/windows, top and bottoms of the exterior walls (basement/attic), and then insulate the attic. It's much harder to do the walls. I'm not sure about crawls, so do your research. If the source is only talking about new construction, its not something you can just go with. There are adjustments needed when factoring in old buildings.

For the moisture problems you mention, is the concern with moisture within the wall assembly or more general moisture in the house interior? At the moment my best guess is that the house is very leaky (1950s construction with a 1970s addition) though I’ve never had a blower door test performed to confirm that suspicion. The crawl section is only below the addition. Maybe a quarter of the overall building footprint. The sill plate area I have easier access to is in the remaining footprint of the house.

I won't speak for Sibley, but there are plenty of examples of tightening up the air leakage on an older house, even one that has been standing for 100+ years, and it quickly develops all kinds of moisture related issues (mold/rot/so-on). It seems to be 2 fold, first putting what can amount to a moisture barrier on the inside with foam prevents the structure from drying to the inside and second the air that was previously flowing through the assembly was carrying away moisture that had gotten in.

I've seen some video, though not in person, of people using various methods, like dimpled drainage mat on the back side of the exterior walls before foam insulation, to ensure the airflow that has been drying a house for decades in allowed to continue.

You're absolutely correct. It's also quite difficult to turn an unfinished, unconditioned attic into a conditioned living space - because that attic NEEDED air flow to not rot out, so if you're going to remove the airflow you also have to solve the moisture problem.

Basically, old houses are simply built differently. Part of it is the materials, part is the methods and techniques used. It's not that its better or worse, because frankly arguments can be made both ways. Its different, and sometimes those differences are quite subtle.

Sealing rim joists and caulking airlinks is going to be fine in an old house. Most of the moisture problems I have seen or read about are related to blowing cellulose into old house walls. Without a vapor barrier, the cellulose takes on moisture and rots the lathe and sheathing. If you seal a vented crawl space or attic, you will likely have to add some form of dehumidification either through supplying conditioned air or installing a dehumidifier. Wireless humidistats are $10 and will be able to tell you if your crawl or attic is having humidity problems.