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Basement Insulation With XPS

Posted by ricksample (My Page) on
Mon, Feb 11, 13 at 11:42

Hi everyone,
I've been doing my basement remodel over the past month. I have XPS insulation completely sealed on the block wall. I also took out the fiberglass insulation in the rim joists and put 2" XPS insulation in and then sealed with foam. After this I tossed the fiberglass insulation back in. Now a month later I have all of my framing completed. However, I read that fiberglass should be kept out of the basement. So I took out all of my fiberglass insulation in the rim joists to leave the XPS exposed. I have I think around 70-80 rim joist spaces. After I pulled all of the fiberglass insulation out to be tossed away I noticed 2 of the XPS boards had a small bit of sweat towards the top. I cut into the XPS and everything seams fine... just a surface sweat. It evaporated an hour after the fiberglass was out. It may have been that it wasn't sealed correctly and air was coming in.

My question... I would really like to put Roxul R-15 Stone insulation in front of that XPS board in the rim joist. To add a little more insulation R value and for fire safety. Will this eventually cause all the XPS boards to sweat? Obviously with the Roxul insulation in front of the XPS... the XPS will be cooler on both sides. I just don't want to trap moisture between where the XPS meets the Roxul insulation. I'm not sure how warm or cool it would be behind the Roxul.


Follow-Up Postings:

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RE: Basement Insulation With XPS

That sweat comment has me scared since I did the same rim joist thing you did. In my case I put the xps in place, place a piece of drywall in front and used fireproof caulk to seal it up. Of course if its sweating I can't see it now. Further I then put the preexisting fibergalss back in place. I now have a suspended ceiling up as well.

May I ask where you heard NOT to leave the fg in place.


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RE: Basement Insulation With XPS

surface sweat. It evaporated an hour after the fiberglass was out. It may have been that it wasn't sealed correctly and air was coming in.

It's likely condensation from the cause you mentioned.

You could have left the fg in place; it's one of the alternatives suggested by Building Science Corp. Rock wool is also suitable. Since the XPS has to be covered by a thermal barrier, I don't see that rockwool increases fire safety.

The point of the insulation against the rims and the walls is to keep the concrete warm, so the warm interior air doesn't condense on the cold foundation.

Information below is from BSD-103: Understanding Basements, Building Science Corp.

Here is a link that might be useful: Building Science Corp. on Basements

This post was edited by worthy on Mon, Feb 11, 13 at 19:38


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RE: Basement Insulation With XPS

I've heard that just by reading online. Fiberglass all by itself is a very bad idea... mainly because it doesn't seal. Once fiberglass absorbs moisture, it'll stay damp. Some sites say the best bang for your buck is 2" XPS sealed then additional insulation in front. Just like what you did.

I took my FG out mainly because I also wanted a fireblock in front of the XPS board in the rim joists. Roxul is a great fireblock... I tried to set it on fire just to see how well it worked. Then after reading from multiple sites about no FB in the basement whatsoever, I decided to take it all out to replace it with fireblock insulation.

That's when I noticed a little condensation on the top of 2 of the XPS boards and the back of the fiberglass insulation was a little damp. But after I took the FG out, the condensation evaporated. That's why I'm not so sure if it's a good idea to have 2 forms of insulation back to back or just one form so it can breathe.

Everywhere I look online they say that XPS with additional insulation is ok. However, I wonder if anyone has actually taken the additional insulation out to see if there was any condensation on the XPS board.


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RE: Basement Insulation With XPS

Thanks for the reply worthy... that makes me feel a little better. I think I'll add a little more spray foam to each of the XPS boards just to make sure everything is seal up tight... then I'll add the Roxul insulation. The reason for the fire barrier is because I have Roxul sound insulation (with no R value) on the basement ceiling.This insulation will be butted up against the R15 Roxul in the joists. If a fire did break out in the basement, it'll take longer for it to go to the first floor.

I know it varies from location to location, but I couldn't help but notice on the picture you posted. It said wood frame wall to be insulated with fiberglass. It this a requirement for the XPS or would it be ok leaving the wall empty? Our codes require R13 in the walls... but without the insulation on the basement walls and with no heat... it stays 60 degrees during winter. Once I finish the walls and turn on the heat it should stay warmer. I didn't want to go overkill on the insulation. I put the R5 owens corning Foamular board on the walls with R10 in the joists plus R15 Roxul. I wasn't planning on adding additional insulation the the walls.

Also, if the XPS boards in the rim joists sweat a little during the year.... will they be able to dry out because of the dehumidifier or will it be trapped between the insulations?


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RE: Basement Insulation With XPS

I took my FG out mainly because I also wanted a fireblock in front of the XPS board in the rim joists. Roxul

If you didn't install any other fireblock, the Roxul is needed as a fireblock between the stud space and the joist space. (See diagram.)

fireblocking photo Picture1.png

[FG]a requirement for the XPS or would it be ok leaving the wall empty?

Not required. However, you may want to consider that the 2012 International Energy Conservation Code, which is being gradually adopted in virtually all US building codes, prescribes a minimum R15 for continuous insulated sheathing or R19 for cavity insulation. (See table at bottom of post)

You're combining R5 foam with R10 between the studs, which works out to a bit less. The rim joists are well insulated; and tossing the old FG, which may have been mould contaminated, was a good idea.

Any moisture that might condense on the XPS will not affect either insulation and as long as there is no vapour barrier will be no problem. I had some of the same concern with a house of mine; before I put in the fg, I saw condensation on the concrete screws and fender washers. However, after six years, there were absolutely no signs of mould or water damage. Now, there are non-metallic fasteners available that completely eliminate that minor concern.

This post was edited by worthy on Wed, Feb 13, 13 at 10:15


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RE: Basement Insulation With XPS

Thanks for all the great info! I'm trying to do a lot with fire blocking, probably a little overboard. Most of it is for sound reasons, but the fire blocking is an added bonus. I actually have Roxul pinned at the top of each stud wall. After I put the XPS up to the top of the block wall, I had room to staple about a 4" wide strip of continuous Roxul directly over the XPS. After the walls went up, the top of the wall really smashed the roxul up against the seal plate creating a very tight fire barrier. This should be ok, in theory it seems like it would do a better job since the strips of Roxul is continuous and goes under all the first floor joists. To sum it up I have Roxul on the ceiling that butts up against the Roxul in the Rim Joists. Then the Roxul in the rim joists butts up against the Roxul fire barrier strips. Pretty much the entire joist area including directly above the XPS on the block wall is filled with fire blocking material.

I may consider additional insulation in the actual walls. That's crazy how they want a min R15 in the basement. It could be 10 degrees outside and 60 degrees in my basement with no heat. If I could do it over again, I would do away with the XPS and just go 100% Roxul. It would be kind of stupid to do both now because that would give me an R value of 20 in the basement walls. It would be a waste of money IMO. Unless I split the insulation in half to give me an R13 which doesn't sound to bad.

That's good to know that the moisture wont be a problem. I know both insulations are water resistant... so I was afraid of it making a puddle on the plates. But there is no vapor barrier, so it should evaporate.


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RE: Basement Insulation With XPS

60 degrees in my basement with no heat.

There may be no ducts, but unless the upstairs is totally sealed and insulated, the heat is being supplied by your HVAC system.

If I could do it over again, I would do away with the XPS and just go 100% Roxul.

Roxul touts its Comfort Board IS for that purpose. However, Dr. John Straube of Building Science Corp. told me a better practice would be to use foam board then a fibrous insulation such as Roxul. Martin Holladay of Green Building Advisor echoes that advice.

This post was edited by worthy on Thu, Feb 14, 13 at 14:45


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RE: Basement Insulation With XPS

Thanks for all the help! I think I'm about to move on with finishing the insulation this weekend so I can start drywall the following.

I just have a couple last questions... I have R10 XPS in the joists and R5 XPS on the walls. This weekend I'll add Roxul R15 to the joists to give me a total value of R25 and I'll add R15 to the walls to give me a total value of R20. Does this seem a little extreme here in my zone 5/6 Ohio? My first floor has R19.

I also wanted to verify that both the XPS and Roxul is vapor permeable or something. My biggest fear is trapping water with to much insulation. I think the XPS has a rating of 1.5... but I couldn't find anything on the Roxul.

Last one... with all this insulation... will my basement still be the coolest part of my house in the summer? I just don't want to lock the cool air out and keep the warmer air in. With my A/C on I have a good 15 degree difference in floor temps during summer months.

Thanks for all your help... I've learned a lot!


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RE: Basement Insulation With XPS

I have a similar project, and thought I'd jump in on this thread.

My basement is using SuperiorWall Xi, which has built in styrofoam R-12.5 insulation into the basement wall. There is also a built in cavity in between the embedded studs to add additional insulation. Is it ok to just add fiberglass insulation without a vapor barrier, and close up with sheetrock?

Also, in the rim joist, I have 1" of closed cell foam covering the entire rim joist. Should I add XPS boards and seal, or just add unfaced fiberglass there as well?

Thanks!

Here is a link that might be useful: SuperiorWall


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RE: Basement Insulation With XPS

Does this seem a little extreme here in my zone 5/6 Ohio? My first floor has R19.

It's over Code. But Building Science Corp.'s Dr. Lstiburek's rule of thumb is that for any home north of the Mason Dixon Line, follow the 10, 20, 40, 60 rule. That's R Value for, respectively, basement slab, basement walls, above grade walls and attic. If you're planning to stay for awhile and the labour is yours, there may be an economic payback. The space will definitely be more comfortable.

My biggest fear is trapping water with too much insulation.

At Roxul sales displays, they like to run a water fountain through a chunk of their product. The water runs right through and is not absorbed.

will my basement still be the coolest part of my house in the summer?

Well, mine have been. But that's probably because of the leakage of HVAC-cooled air into the basement, where the cooling equipment is also located.

Is it ok to just add fiberglass insulation without a vapor barrier, and close up with sheetrock?

Unless you're in an extreme cold climate such as northern Alaska, leave the poly vapour barrier out. All you need is painted drywall as a vapour retarder.

in the rim joist, I have 1" of closed cell foam covering the entire rim joist. Should I add XPS boards and seal, or just add unfaced fiberglass there as well?

Either one works. See illustration below from Building Science Corp. Info. Sheet 511, p. 2.


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RE: Basement Insulation With XPS

I'm just beginning to work on finishing my basement as well. I have a 1.5 yr old home. My rim joists are all coated with spray foam. Do I need to add anything else?


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RE: Basement Insulation With XPS

If necessary, I would increase the R Value to at least meet the International Energy Conservation Code 2012 minimums for walls in your climate zone. (I posted the Table earlier in this thread.) Any insulation will do.


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RE: Basement Insulation With XPS

Thanks Worthy!


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RE: Basement Insulation With XPS

Thanks, worthy.

One more question -- I have a cantilevered bay window, and I noticed that the foam has been sprayed on the rim joist, the subfloor of the 1st floor, but not the wood facing the ground. There is obviously a notable amount of air leakage going on. I'm going to use Roxul in the basement ceiling, mostly for sound, and I was wondering if I should just fill the cantilevered cavity with roxul, or use an XPS board and seal it.

If I use XPS, then does it need to be next to the cantilevered rim board, or can I just put it on top of the foundation wall, leaving 2' of unreachable space?

Thanks! Roxul sounds simplest of course, but I want to make sure to do it right.


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RE: Basement Insulation With XPS

The bottom of the bay needs air sealing, best supplied by spray foam; XPS could work as long as all the joins are caulked/spray foamed. Roxul is not an air block.


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RE: Basement Insulation With XPS

Thanks. I thought I need to cover XPS with a fireblock, such as 1/2" sheetrock, but that's obviously tough in this case. Or do I only need to do that for walls and ceiling, and that's enough since XPS will be between sheetrock below and subfloor above?


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RE: Basement Insulation With XPS

since XPS will be between sheetrock below and subfloor above?

I was assuming "the bottom of the bay" was that part on the exterior of the home and that it would be covered by siding of some sort. The sheetrock and most subfloors are acceptable thermal barriers.

(Incidentally, fireblocking, firestopping, thermal and ignition barriers are not interchangeable terms, even though some of the same materials are acceptable in each category.)


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RE: Basement Insulation With XPS

Thanks, worthy. Based on other posts I saw, I figured I would need to cover up the XPS that's on the rim joist with a thermal barrier, even though it will be between sheetrock ceiling for the basement below, and subfloor for floor above.

Seems like a lot of pain to cut up many pieces of sheetrock. Foam insulation for what's about 40sf isn't very economical, since most companies want a minimum charge. Any DIY kits you can recommend?


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RE: Basement Insulation With XPS

I've never used a diy kit.

Here's the good and the bad: http://www.greenbuildingadvisor.com/blogs/dept/musings/spray-foam-jobs-lingering-odor-problem

Professional application isn't always
foolproof either.


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