# Insulating Rim Joist - Questions.



## cjaustin81 (Sep 4, 2014)

this is the energy code, it mentions rim joists as just simply saying "rim joists should be insulated and include the air barrier"

http://www.palatine.il.us/assets/1/building_permits/2012iecc_residential_BECU.pdf

What does that mean by an air barrier? Would the rigid foam be the air barrier AND insulation?


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## cjaustin81 (Sep 4, 2014)

Can I just put the foam XPS insulation up and caulk it?

Or do I need to cover it somehow, I was reading it was combustible but I've seen other people just put it up and caulk. 

I was following this guide > http://www.familyhandyman.com/basement/insulate-basement-rim-joists/step-by-step


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## Windows on Wash (Aug 30, 2011)

According to the codes, spray foam is able to be left exposed. Rigid foam, to my reading, cannot be left exposed. 

Seal up the XPS and stick the fiberglass right back over top.


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## cjaustin81 (Sep 4, 2014)

Windows on Wash said:


> According to the codes, spray foam is able to be left exposed. Rigid foam, to my reading, cannot be left exposed.
> 
> Seal up the XPS and stick the fiberglass right back over top.


This is what I was worried about. Do I just need to contact my local village to figure out if I need to put drywall over that insulation? I know some locales don't require it I think. 

I threw away all the fiberglass. Are you saying that if I were to have fiberglass over the rigid foam that would be up to code?

Basically I have a bunch of sheets I bought that are cut of the Owens Corning R10 2" rigid foam right now.


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## Windows on Wash (Aug 30, 2011)

Call your local code department and get them to weigh in on the matter. 

Easy to safe than sorry. 

Be sure to know that I don't recommend code minimum as a standard. I would not leave foam uncovered in my home.


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## cjaustin81 (Sep 4, 2014)

Windows on Wash said:


> Call your local code department and get them to weigh in on the matter.
> 
> Easy to safe than sorry.
> 
> Be sure to know that I don't recommend code minimum as a standard. I would not leave foam uncovered in my home.


If I do have to cover it, what's the cheapest/best way to do that? Is there a tape you can use?

To get them to weigh in on the code, do I just call the village?

I see a lot of writeups online (such as the one I posted above) that show the rigid foam exposed when they install it, that's why it's really confusing. Not sure what I should be using to insulate the rim joist.


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## High Gear (Nov 30, 2009)

http://www.buildingscience.com/documents/information-sheets/basement-insulation

If covered my XPS with Roxul mineral wool ( Lowes sells it ) for added protection.
Fiberglass melts and gives little protection for the xps but it is used.


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## cjaustin81 (Sep 4, 2014)

High Gear said:


> http://www.buildingscience.com/documents/information-sheets/basement-insulation
> 
> If covered my XPS with Roxul mineral wool ( Lowes sells it ) for added protection.
> Fiberglass melts and gives little protection for the xps but it is used.


Thanks, I see you're also in Chicagoland. I wonder if this meet code (International Fire Code, 2009 Edition)?

What size thickness did you use?


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## High Gear (Nov 30, 2009)

My understanding is the 2" is acceptable in our area but I doubled up so 4" total, then covered with Roxul.
Walls are 1"(R5) plus R13 glass.( would have used roxul here also but it wasn't easily available here then).
Don't know what the code says but take a torch to glass and then to mineral wool and see which one you pick.
A big difference in basement temps ...I'm comfy in short pants and a t shirt.
( Winnebago county BTW)


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## cjaustin81 (Sep 4, 2014)

High Gear said:


> My understanding is the 2" is acceptable in our area but I doubled up so 4" total, then covered with Roxul.
> Walls are 1"(R5) plus R13 glass.( would have used roxul here also but it wasn't easily available here then).
> ( Winnebago county BTW)


You think the code says 2" of just the rigid foam, caulked is up to code?


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## Windows on Wash (Aug 30, 2011)

Roxul has more body and will stay the shape you cut it more easily. It is also a better performer in these applications as well.


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## cjaustin81 (Sep 4, 2014)

Windows on Wash said:


> Roxul has more body and will stay the shape you cut it more easily. It is also a better performer in these applications as well.


Are you saying to use the Roxul to cover the ridge foam? Or to use it instead of the ridge foam?

I'm really confused as to what insulation I should use. I bought $180 worth of XPS Owens Corning in R10 and now I have a couple concerns:

1) Is using this up to code without covering it?
2) If I do have to cover it can I use Roxul?
3) Is the R10 value high enough (Illinois is a category #5) and it seems we're supposed to use R20 in the wood walls. 

I'm not sure if I should scrap the XPS and use something completely different (due to fire risks) or if I should use the XPS I bought and cover it (if needed for code).


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## Windows on Wash (Aug 30, 2011)

To cover.

XPS is great in band/ribbon board areas. 

Get a good quality sealant, cut it to fit (2"), seal it up, cover with Roxul. 

Done in done. 

Don't get hung up on the R-Value. Once you have an R-10 in there, the bigger issue is the air seal. Covering it with the Roxul is to satisfy ignition/thermal barrier requirements. The additional R-Value is just a bonus.


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## cjaustin81 (Sep 4, 2014)

Windows on Wash said:


> To cover.
> 
> XPS is great in band/ribbon board areas.
> 
> ...


Thanks for the reply. I just want to make sure the XPS 2" covered w Roxul (R15) 
will meet fire code? 

I read that I need R20 on wood walls in IL, so R10 XPS + R15 Roxul might work.

What's a good sealant to use? I bought some cheap acrylic caulk. Maybe use silicone instead? I wasn't sure what to use on the wood to seal.


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## Windows on Wash (Aug 30, 2011)

Anything that is compliant with the foam is fine.


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## cjaustin81 (Sep 4, 2014)

I just want to make sure the Roxul will satisfy any ignition barrier issues. Also, do I use the XPS + Roxul in EVERY place I used fiberglass before?

For example behind the heating ducts. Does the Foam touch the ducts where the fiberglass used to be? Or do I cut the rigid foam and give some space for the hearing ducts? 

I guess these are all part of the fire code questions.


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## cjaustin81 (Sep 4, 2014)

Windows on Wash said:


> Anything that is compliant with the foam is fine.


Do you know if Roxul is compliant w the foam? Thanks!


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## 123pugsy (Oct 6, 2012)

cjaustin81 said:


> Do you know if Roxul is compliant w the foam? Thanks!


The Roxul won't eat the foam or vise versa.

Use Roxul against the duct work. It won't melt.


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## cjaustin81 (Sep 4, 2014)

Another question is the area by the electrical box. There was insulation on the wood before I removed the fiberglass. Do I have to be careful w the XPS + Roxul near the electrical box?

Safe to replace the fb above the box with XPS + Roxul?


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## cjaustin81 (Sep 4, 2014)

123pugsy said:


> The Roxul won't eat the foam or vise versa.
> 
> Use Roxul against the duct work. It won't melt.


Is it ok to use the XPS behind the ductwork as long as the Roxul
is in front and touching the duct?

In spots like this should I use XPS + RoxuL?




























Or do I want to avoid using XPS entirely in those sections (sides of basement) where the ductwork is?


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## Windows on Wash (Aug 30, 2011)

If you can get to it, yes.


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## cjaustin81 (Sep 4, 2014)

Windows on Wash said:


> If you can get to it, yes.


If I can't get XPS + Roxul in there would u recommend I just use Roxul only
or fiberglass only?


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## 123pugsy (Oct 6, 2012)

cjaustin81 said:


> If I can't get XPS + Roxul in there would u recommend I just use Roxul only
> or fiberglass only?


The only choice left, so go for it.
Go with the Roxul. Great stuff.


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## cjaustin81 (Sep 4, 2014)

123pugsy said:


> The only choice left, so go for it.
> Go with the Roxul. Great stuff.


So go w XPS + Roxul in the areas I can fit it, and the tight spaces that 
won't allow both I should just use a Roxul by itself?

I'm going to b putting the XPS right up against the wood rim joist. Same w
the Rolux, I'm hoping I don't need an additional barrier material 
to be attached to the wood initially.


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## cjaustin81 (Sep 4, 2014)

Just got off the phone w the village building director and she
said since this is the basement (100% underground) that the R
requirements are 10/13. That I will meet the R requirement w just the 2"
XPS foam. She also told me NOT to use any vapor barriers.

As far as the fire codes, she wasn't sure if the XPS has to be covered 
for residential applications. She's going to check and call me back.

I always thought for the rim joist it was the wooden wall R rating (20 for area 5) 
but she said it's only R10 for a rim joist.


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## cjaustin81 (Sep 4, 2014)

Update:

They said I need to cover any exposed rigid foam to meet code.

Roxul will meet this requirement so I'll just add Roxul to every 
Place I add rigid foam XPS.


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## cjaustin81 (Sep 4, 2014)

For the sealant, what caulk is best?

I bought some cheap acrylic caulk, would silicone be better?


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## cjaustin81 (Sep 4, 2014)

This is the caulk I bought >

http://www.dap.com/product_details.aspx?BrandID=12&SubcatID=3

*DAP® ALEX PLUS® Acrylic Latex Caulk Plus Silicone*


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## 123pugsy (Oct 6, 2012)

That caulk looks fine.


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## cjaustin81 (Sep 4, 2014)

Just wondering if an elastomeric caulk is needed such as >

http://www.homedepot.com/p/HDX-10-oz-Premium-Elastomeric-Plus-Silicone-Caulk-1416-1-66/204852647

or 

http://www.homedepot.com/p/DAP-Dynaflex-230-10-1-oz-Premium-Indoor-Outdoor-Sealant-18275/100035980#

Just want to make sure I use the right stuff that won't crack.

I noticed under specifications that the two caulks above say "yes" under permanently flexible. 

Whereas the Alex Plus says "no" under permanently flexible. 

http://www.homedepot.com/p/DAP-Alex...lic-Latex-Caulk-Plus-Silicone-18103/100097524

Wouldn't I want something flexible? (Elastomeric)


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## cjaustin81 (Sep 4, 2014)

After researching the problem these are the products I plan to use:

1) Dynaflex 230 10.1 oz. Premium Indoor/Outdoor Sealant - The reason I chose this over Alex caulk is because this caulk is permanently flexible, meaning it can bend without cracking as the weather changes. On the package it also lists:

"It has excellent adhesion to a wide range of building materials including wood, MDF, PVC, polystyrene, and composite trimboard." 

So it specifically lists polystyrene (rigid foam).

http://www.homedepot.com/p/DAP-Dynaflex-230-10-1-oz-Premium-Indoor-Outdoor-Sealant-18275/100035980

2) Loctite® PL 300® VOC Foamboard Adhesive - I chose this because you need to be careful with adhesives used on polystyrene. This product is specifically marked as being compatible for use with polystyrene. 

"PL® 300 VOC is a latex-based adhesive especially designed for bonding polystyrene foam to itself or to a variety of construction materials, including, but not limited to: wood"

http://www.loctiteproducts.com/p/pl_ca_300_voc/overview/Loctite-PL-300-VOC-Foamboard-Adhesive.htm

3) R15 Rolux - To safely create a fire-barrier in front of the XPS foam. 

Does this seem legit? I'm glad I took the extra time to figure out the correct caulk/adhesive to use BEFORE I started the job.


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## High Gear (Nov 30, 2009)

Get the large size 28oz? vs 10 oz? caulk gun as you'll save a bunch buying the larger 
tubes.
This time of year ....... I'm thinking the PL300 needs to be above 40f to cure....wondering what the band temp might be after it's covered.
PL Premium may be the ticket for these cold temps lately at least behind the foam.
I've got more premium than 300 on my foam walls ...little pricier but lots stronger.
Single digits tonight ....plowed the driveway yesterday ..what the heck.


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## cjaustin81 (Sep 4, 2014)

My friend just reminded me of something that's going to dictate the direction of this project. This past summer I had MOLD in the basement and I had to do a mold remediation.

With that being said, my friend told me I shouldn't use ANY caulk for this job and I should also skip the glue (since both can grow mold).

What are your thoughts on this? He indicate that I should use Great Stuff since that can't inhibit mold, is this true?

Mold prevention is my #1 issue in my basement so any help in regards to that would be appreciated, thanks.


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## cjaustin81 (Sep 4, 2014)

High Gear said:


> Get the large size 28oz? vs 10 oz? caulk gun as you'll save a bunch buying the larger
> tubes.
> This time of year ....... I'm thinking the PL300 needs to be above 40f to cure....wondering what the band temp might be after it's covered.
> PL Premium may be the ticket for these cold temps lately at least behind the foam.
> ...


I'm concerned about mold growing on any caulk and/or the PL300 glue. 

Do you know if Great Stuff is a mold-free product?


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## High Gear (Nov 30, 2009)

Just wait for warmer temps for the install should be some yet this fall.
Run a dehumidifier in the basement during the summer months and don't over humidify during the winter .


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## cjaustin81 (Sep 4, 2014)

High Gear said:


> Just wait for warmer temps for the install should be some yet this fall.
> Run a dehumidifier in the basement during the summer months and don't over humidify during the winter .


I'm not worried about the install, I'm worried about when summer comes and the humidity is high. I'm now running a 70-pint dehumidifier. 

For my situation, I'm wondering if using Great Stuff would be better than a mold resistant caulk to seal the rigid foam and make it air tight?


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## cjaustin81 (Sep 4, 2014)

Stupid question, but do you have to install the XPS foam in a specific direction (ie. so the text isn't upside down)?

Or can I cut a mix of upside down and sideways pieces?

The reason I ask is I have 2ft x 4ft pieces and I can get 8 squares vs 6 if I don't have the text all facing normally.

This would be for code reasons.


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## HDS (Jun 21, 2014)

The direction shouldn't matter. Instead of glue you could use long screws or nails with caps to hold the xps foam in place..

Before putting the xps in place, caulk around the perimeter to air seal each bay. 

Also don't forget to seal the sill plate and concrete too.


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## cjaustin81 (Sep 4, 2014)

HDS said:


> The direction shouldn't matter. Instead of glue you could use long screws or nails with caps to hold the xps foam in place..
> 
> Before putting the xps in place, caulk around the perimeter to air seal each bay.
> 
> Also don't forget to seal the sill plate and concrete too.


Because I had mold in my basement this past summer I'm going to try and avoid any caulk. I'll use the Great Stuff foam instead. I think this makes sense. 

I was thinking if I use XPS and then seal it AFTER, that should be good enough. I'm worried if I seal it first I won't get a nice flush seating w the wood rim joist.


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## cjaustin81 (Sep 4, 2014)

HDS said:


> The direction shouldn't matter. Instead of glue you could use long screws or nails with caps to hold the xps foam in place..
> 
> Before putting the xps in place, caulk around the perimeter to air seal each bay.
> 
> Also don't forget to seal the sill plate and concrete too.


I'm not going to secure the XPS to the wood, I don't see any reason to do that if I can trim it and seal it with foam. 

That's a code requirement for commercial buildings not residential.


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## HDS (Jun 21, 2014)

Excess moisture is what causes mold. Have you determined what was the source of moisture in your basement?


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## cjaustin81 (Sep 4, 2014)

HDS said:


> Excess moisture is what causes mold. Have you determined what was the source of moisture in your basement?


I wasn't running the AC or a dehumidifier in the summer.


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## 123pugsy (Oct 6, 2012)

Instead of worrying what mold grows on, you should be concentrating on sealing up, with caulk, all air gaps so the moist humid air from outside doesn't get in in the first place.

If your conditions are such that mold will grow on caulk, it will grow elsewhere. So now you have no caulk with mold, but you will have mold in other places.

Fight the moisture.

How about your walls below the rim board? How are they finished?


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## cjaustin81 (Sep 4, 2014)

123pugsy said:


> Instead of worrying what mold grows on, you should be concentrating on sealing up, with caulk, all air gaps so the moist humid air from outside doesn't get in in the first place.
> 
> If your conditions are such that mold will grow on caulk, it will grow elsewhere. So now you have no caulk with mold, but you will have mold in other places.
> 
> ...


But a Great Stuff also seals air gaps and has a higher insulation value than caulk
and also better for mold. Unless I'm missing something where caulk would be 
somehow better in this application?

It's an unfinished basement do below the rim joists are concrete 
foundation walls w nothing covering them.


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## DexterII (Jul 14, 2010)

Great Stuff, as opposed to caulk, around the rigid foam seems to be most prevalent in my neck of the woods (Michigan). And verdicts on leaving the foam board exposed seems to be more localized, so always best to ask, but I cannot imagine any jurisdictions frowning upon going the extra step and covering it.


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## 123pugsy (Oct 6, 2012)

cjaustin81 said:


> But a Great Stuff also seals air gaps and has a higher insulation value than caulk
> and also better for mold. Unless I'm missing something where caulk would be
> somehow better in this application?
> 
> ...


That's cool.
Just don't leave any gaps and you should be good to go.


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## cjaustin81 (Sep 4, 2014)

I have some nails sticking thru the floor where I will be putting the foam. Do you just fit it under the nail to hold it? 

I would imagine I keep the nail ends and don't trim them?


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## DexterII (Jul 14, 2010)

Every once in a while I have seen places where somebody must have been target practicing, so you may have a couple of groups of nails that will be a problem, and you can either cut them or trim the rigid back a bit and fill around them with foam. Otherwise, it shouldn't be that much of a job to work around the nails as they are. And by the way, in my opinion, the gun with pro size cans is worth it. Only thing is that the gun needs to be cleaned when you stop for the day, so best to cut all of your pieces first and go down the line foaming them.


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## cjaustin81 (Sep 4, 2014)

DexterII said:


> Every once in a while I have seen places where somebody must have been target practicing, so you may have a couple of groups of nails that will be a problem, and you can either cut them or trim the rigid back a bit and fill around them with foam. Otherwise, it shouldn't be that much of a job to work around the nails as they are. And by the way, in my opinion, the gun with pro size cans is worth it. Only thing is that the gun needs to be cleaned when you stop for the day, so best to cut all of your pieces first and go down the line foaming them.


Thanks, maybe I'll just bend them. If I use great stuff
I won't need a caulking gun. But that's a god tip.


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## DexterII (Jul 14, 2010)

Maybe it's just one of my personal hangups, but be careful bending them. I hate seeing bent nails around, except if the piece is headed straight to the dumpster. Your body is more apt to get caught on a bent nail than a straight one, and beating on the nail is going to loosen it to some extent. Probably not anything measurable, but, again, that's me. I would try cutting them first. But that's one of the good things about spray foam over caulk; if you happen to run into a few in a tight spot, just knock the corner off of the rigid, and fill it with spray foam. As for the gun, I was referring to a gun for the Great Stuff, not caulk. As I mentioned, you would then use the "pro" size cans. I find the gun to be easier to wield than a standard can mounted nozzle.


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## cjaustin81 (Sep 4, 2014)

DexterII said:


> Maybe it's just one of my personal hangups, but be careful bending them. I hate seeing bent nails around, except if the piece is headed straight to the dumpster. Your body is more apt to get caught on a bent nail than a straight one, and beating on the nail is going to loosen it to some extent. Probably not anything measurable, but, again, that's me. I would try cutting them first. But that's one of the good things about spray foam over caulk; if you happen to run into a few in a tight spot, just knock the corner off of the rigid, and fill it with spray foam. As for the gun, I was referring to a gun for the Great Stuff, not caulk. As I mentioned, you would then use the "pro" size cans. I find the gun to be easier to wield than a standard can mounted nozzle.


Ahh, my fault. I did see those great stuff pro guns. That's a great idea, I think
I'm going to see if I can buy that instead. Hopefully the big box store
carries PRO. 

I wonder if I should use the window and door version since it's a thinner stream.

As for the nails, would u just use a metal cutter/clipper to snap the ends off?


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## AandPDan (Mar 27, 2011)

I just sealed my rim joists a few months back. I used the 2" foam board as well.

Cut it a bit small, so as to leave a little gap around it and then use expanding foam, Great Stuff, to seal around it. It worked great.

The only problem I had was with the Great Stuff. The big cans wouldn't fit into the joist bay far enough that I could fill the top. I ended up using an additional piece of plastic tubing to extend the reach of the can - worked great!

A table saw makes nice neat cuts. A drywall saw can be used to cut out for any obstructions.

Just the air sealing is going to make a tremendous improvement in your comfort and reduce the humidity in the basement. You really should run a dehumidifier to keep humidity down.


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## cjaustin81 (Sep 4, 2014)

Thanks for the info! Did you use the regular great stuff or the one for
windows and doors? Also, did I attach the foam using glue or screws?


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## cjaustin81 (Sep 4, 2014)

What kinda table saw blade are u using to cut the XPS? I saw one
Online, but it was about $90.


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## cjaustin81 (Sep 4, 2014)

I have a code question when using mineral wool (Roxul) as a fireblocking material in front of the XPS. 

My town is using the International Building Code, 2009 Edition.

For using mineral wool as a fireblock is says the following:

_"7. Batts or blankets of mineral wool, mineral fiber or other approved materials installed in such a manner as to be securely retained in place."

http://publicecodes.cyberregs.com/icod/ibc/2009f2cc/icod_ibc_2009f2cc_7_par279.htm

_I was just going to cut the mineral wool and friction fit it in front of the XPS on the rim joists. According to the code it's saying I have to securely retain the mineral wool in place.

How would you secure the mineral wool in place? Or is friction fitting it "good enough" to be called securely retained?


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## AandPDan (Mar 27, 2011)

cjaustin81 said:


> Thanks for the info! Did you use the regular great stuff or the one for windows and doors? Also, did I attach the foam using glue or screws?


I used regular Great Stuff. On my rim joist there were nails from the siding poking through so I was able to push the foam onto that to hold it.



cjaustin81 said:


> What kinda table saw blade are u using to cut the XPS? I saw one Online, but it was about $90.


Regular blade, nothing fancy. You can put in backwards if you want but it cut fine the regular way and there wasn't much dust.


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## AandPDan (Mar 27, 2011)

cjaustin81 said:


> How would you secure the mineral wool in place? Or is friction fitting it "good enough" to be called securely retained?


It never hurts to ask your local AHJ.


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## cjaustin81 (Sep 4, 2014)

AandPDan said:


> It never hurts to ask your local AHJ.


I told them I was going to friction fit it and they seemed ok with that but after reading about how it needs to be secured I wonder about there reply. 

Maybe I can use some pins if it comes down to it.


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## DexterII (Jul 14, 2010)

Since they seemingly don't require anything in particular, look at "lightening rods" (wires that you put under the insulation, and the ends poke into the joists). Fast and easy. Available in the insulation section, and no nailing required.

And, tool purchases can be fun, especially something like a table saw, but, in case this is the only project that you have in mind for it, it's not necessary to use a table saw. A circular saw will work, as will a hand saw, as will a utility knife, etc...


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## cjaustin81 (Sep 4, 2014)

DexterII said:


> Since they seemingly don't require anything in particular, look at "lightening rods" (wires that you put under the insulation, and the ends poke into the joists). Fast and easy. Available in the insulation section, and no nailing required.
> 
> And, tool purchases can be fun, especially something like a table saw, but, in case this is the only project that you have in mind for it, it's not necessary to use a table saw. A circular saw will work, as will a hand saw, as will a utility knife, etc...


Thanks for the tips. I bought a 4" putty scraper and used a belt grinder to sharpen it so it's very sharp. I'm going to use that to cut the XPS since I'm low on funds. I'd love to buy a table saw when the time is right though. 

I'll check out that lightening rod too. Ideally I would use something that doesn't require a lot of screws and/or holes in the wood.


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## Sean Lynch (Oct 9, 2021)

cjaustin81 said:


> Thanks for the tips. I bought a 4" putty scraper and used a belt grinder to sharpen it so it's very sharp. I'm going to use that to cut the XPS since I'm low on funds. I'd love to buy a table saw when the time is right though.
> 
> I'll check out that lightening rod too. Ideally I would use something that doesn't require a lot of screws and/or holes in the wood.


So did you put the roxul over the XPS foam board? If so, have you noticed and forst or moisture build on the foam board now?


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