# XPS Insulation Thickness in Basement



## Logan176 (Feb 18, 2016)

So, I know there are countless threads in this forum and others about basement insulation. I have spent many hours reading them as well as articles from BuildingScience.com. With that being said, I'm not 100% sure if I should go with 1" or 2" XPS foam boards. 

Background:
I have a Cape Cod style home built in 1951. I live in southern New York, climate zone 5. The foundation is made from concrete block. The house had water infiltration problems when I first moved in 8 years ago. Since then, I have installed an interior perimeter drainage system below the slab and a sump pump. 

I currently have a moisture issue in one corner of the basement after several days of heavy rain. I plan on digging down and sealing the exterior foundation wall with tar extending out about 5-6 feet from the corner. I will also regrade and extend the downspout 10 feet from the house.

Question:
What combination of insulation should I use for my basement? 

I started out by insulating my rim joists with 2” XPS as described in this article from Family Handyman. Based on the reading that I’ve done, I want to go with XPS rigid foam boards glued directly to the foundation. My building inspector said that I need to have at least R13 insulation in the walls, so the 2” XPS alone (R10) is not enough.

This second article from Family Handyman says to use ¾” XPS on the walls, frame, and later fill with fiberglass.

Then I came across two articles from Building Science Corporation. The first BSC article says using 2” XPS along with R13 fiberglass works very well at controlling moisture when used with spray foam on the rim joists and another 2” of XPS below the slab. But I don’t have anything below the slab.

If I’m reading the second BSC article correctly, it says that using 1” XPS with R13 fiberglass can also be effective at controlling moisture as long as the sheet rock is covered with vapor barrier paint and a dehumidifier is located in the basement. But… the article states that “some variations not recommended” and the article does not explain what that means.

So, will 1” XPS with fiberglass be thick enough for moisture control or will I need to go with the 2” XPS and fiberglass?


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## GottaFixIt (Dec 3, 2010)

Have you considered using Polyiso? 2" of polyiso is R-13 and includes a vapor barrier.


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## Logan176 (Feb 18, 2016)

I thought about it, but according to Green Building Advisor, polyiso shouldn't be used for interior below grade applications because it prevents the foundation from drying to the interior.


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## Bud9051 (Nov 11, 2015)

It used to be people would ask their neighbors for building advice and end up getting conflicting information. Now the internet has evolved to do the same, from varied sources to varied time frames. It can be difficult to pick and choose something that will work the best. And meet codes.

I do like BSC, but I don't always agree with their advice. In the links you posted all is basically good, except, I don't like the interior vapor barrier mentioned in your last link. An interior vapor barrier (less so with the paint) will allow the moisture level of everything to the outside of the VB to equalize with the surrounding soil. I wouldn't want my wall assemblies and especially my fiberglass insulation and drywall to accumulate moisture. I want them to dry to the inside where I can manage that moisture through conditioned air or a dehumidifier. Recognize we are talking about a VERY small amount of moisture that is easy to remove and only becomes an issue if we allow it to accumulate, behind the vapor barrier or inside our basement.

I like the 1" rigid with a 2x4 wall filled with Roxul (r-15). Although the r-15 is slightly more than the r-13 listed, this is mostly below grade and Roxul is better suited, being very dense. An important part of any basement wall is to ensure it is air sealed to keep inside humid air from circulating towards those colder surfaces. 

As for the Code official, if they are following the 2009 energy codes the R-13 applies to the cavity fill and R-10 is for a continuous layer. But local codes rule and you will need to ask without offending, but the hybrid approach exceeds the total the code is allowing.

More questions welcome as I'm sure I didn't address all.

Bud


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## GottaFixIt (Dec 3, 2010)

Logan176 said:


> I thought about it, but according to Green Building Advisor, polyiso shouldn't be used for interior below grade applications because it prevents the foundation from drying to the interior.


But alas, not everyone agrees that a foundation should dry to the interior.

GBA (greenbuildingadvisor) and BSC (buildingscience) are good resources but they often contradict each other and even themselves. In my research, I found that over time, new research is done and methods change - in this case, getting away from "drying to the interior" methods (do we really want to invite moisture into our basements?). I guess you'll have to make your own conclusion. Watch the dates of the articles you're reading.

If you have a vapor barrier on the exterior, you don't want one inside and XPS is probably the way to go. I'm guessing your don't. If you do, please disregard anything I say and do not install an additional vapor barrier. Also, this all assumes that you've mitigated any liquid water infiltration and we're only working against vapor. 

Anyway, here's where it really gets fun. Vapor moves from wet to dry. When one side is warm and the other cold, it's very likely that the warm side is wet and the cold side is dry (look up temperature and relative humidity). So, basically, we can design a wall assembly to dry to the interior, but that's likely the warm side (and also the wet side). Make sense to you?

Further, we need to think about dew points. That is, if moisture migrates within our wall assembly, it becomes a problem only when it hits the dew point and becomes condensation. Where this happens depends on several factors, but when you introduce air movement (convection) into it, it all goes to hell. That's a big part of why conventional vapor barriers got such a bad rap. They enclosed a wall that was packed with fiberglass that also allowed convection (the moldy diaper).

A vapor barrier on the inside of an assembly with minimal convection (such as foil faced polyiso tight to the foundation) forces the wall to dry to the outside, but only to the extent that vapor from outside migrated into the wall assembly to begin with (since we're not allowing interior humidity into the assembly). It will only migrate inwards to the point of equilibrium or until it hits the dew point and changes to liquid. If your insulation is adequate, then that should not happen inside the wall cavity and there is no concern. When the air outside the block is drier than the air inside the wall assembly, it dries outwards. That is how a vapor barrier should work...


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## Bud9051 (Nov 11, 2015)

Nick, the experts don't change their minds, they usually just change the discussion.
Just when i was getting comfortable with the "no VB on the inside of the foundation" approach, they had to come up with an answer for foil faced polyiso as a one step solution for foundation walls, since the foil facing is certainly a good vapor barrier. The resulting re-thinking resulted in the conclusion that the soil and foundation do very well when moist and thus a good Vb on the inside would be just fine. However, I believe they were looking at a VB directly in contact with the concrete. I looked briefly but couldn't find the reference. Moving the VB all the way to the inside makes a big difference as there can be a lot of mold food located inbetween. We have seen the BSC pictures of fiberglass insulation inside of a plastic vapor barrier, all be it made much worse by air circulation, but the results were scary.

On the other hand, I have seen no scary pictures resulting from a small amount of drying to the inside. I have seen data about how much moisture passes through a 1/2" layer of drywall and would assume it to be even less through 1" of rigid and then the drywall. In an application where the moisture through the floor and from other sources must be managed anyway, what little would pass through the wall would make no difference.
Here is a link discussing the change back to a Vb being ok, "News story #2: Basement walls don’t need to dry to the interior"
Bud

http://www.greenbuildingadvisor.com...usses-basement-insulation-and-vapor-retarders


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## GottaFixIt (Dec 3, 2010)

Thanks, Bud. I was looking for that article to cite, but couldn't seem to find it.

Here's a relevant quote:


> The experts at the Building Science Corp. believe that a concrete basement wall should be able to dry to the interior, but I think that the importance of this detail is exaggerated. After all, the soil on the outside of your basement wall is usually damp. If you encourage a basement wall to dry to the interior, you are inviting a lot of moisture into your home.


Logan, Sorry if we derailed your thread, but I think it's important to make an informed decision. Look at how many people installed the moldy diaper at the direction of inspectors, no less.


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## Logan176 (Feb 18, 2016)

No need to apologize. My head is still spinning, but I don't feel my thread was derailed. I'm just trying to the best I can with all this. The last thing I want is for mold to get people sick and cost thousands of dollars to fix. I'm already dealing with issues from what I believe to be a faulty icynene spray foam application in my roof. But I'll leave that story for another thread.

I've seen too many nasty photos online with a plastic sheet vapor barrier over fiberglass to know that's NOT what I want to try. Some people have recommended the plastic sheet between the foundation and the fiberglass, but the contractor who gave me a quote to sheet rock and tape said he's had to rip out a bunch of basement walls where mold formed between the plastic and the foundation. Then again, he suggested using no plastic vapor barrier with regular paper-faced fiberglass with a 1/2" - 1" gap between the fiberglass and the foundation so air can move... but I thought air movement helps to create condensation... sigh.

Nick, I do not have a vapor barrier on the exterior. I have some run-off issues in the front two corners of my house. The right side is easier to fix because I can extend the downspout and regrade. The left side is more tricky because run-off is coming from the driveway. Here's my new thread on that issue. In the spring, I'm going to dig out the front two corners of my house and apply tar to the outside of the foundation... probably about 5 feet from the corner in each direction in the hope that this helps. These will be the only areas with a vapor barrier. 

Bud, in your first post you said you liked the idea of the 1" XPS glued to the wall and 2x4 studs filled with Roxul, but no vapor barrier paint. I assume you'd rather see latex paint on the 1/2" sheet rock?


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## GottaFixIt (Dec 3, 2010)

Logan176 said:


> I'm going to dig out the front two corners of my house and apply tar to the outside of the foundation... probably about 5 feet from the corner in each direction in the hope that this helps. These will be the only areas with a vapor barrier.


I wouldn't worry about that as far as a double vapor barrier goes. A vapor barrier has to completely surround the space to be effective. Because of the porosity of CBUs and the cavities in the block, it will equalize with the rest of the wall anyway.

Do you have an interior drain tile & sump pump?


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## Logan176 (Feb 18, 2016)

Yes. I installed interior drain tile below the slab right before Hurricane Irene hammered my area. I called in a bunch of favors with the fellas and had two jack hammers going for several hours before we started hauling out the concrete. Some basements in my neighborhood had to be pumped out by the fire department. I had to drop a second sump pump into my pit, but I only had two damp discolorations about 6" in diameter on my basement floor. I really don't want to do that project again but I'd say it worked well!


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## GottaFixIt (Dec 3, 2010)

Did you install drain tubes from the block to the drain tile?


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## Logan176 (Feb 18, 2016)

I drilled two holes at the bottom of each block and installed a dimpled membrane so any water would seep down to the tile. However, that created another problem. The gap that the membrane created let more radon into the basement. So I had to seal up the top of the membrane with spray foam from a can.


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## GottaFixIt (Dec 3, 2010)

Gotcha. I ask because I'm knee deep in a similar project. I opted for the irrigation tube method ripped from Family Handyman. It took care of my minor leak problems and I was also able to bring Radon from almost 35 (yes, 35) to less than .10. 

If the block drains into the tile, you shouldn't have to worry about liquid. It may be worth excavating just that corner. You'll probably fix the problem from outside, but the fact that it's happening means there's something wrong with the drain tile. Maybe the foam sealed up your holes?

You don't want liquid getting into any insulation. In addition to potential for mold, it loses most all of it's RValue.


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## High Gear (Nov 30, 2009)

I'm at Ill/sconsin border (zone5 also).
Poured dry walls ,conditioned basement .
1" XPS with stud walls filled with R13 (if paper faced poke holes through the paper
Walls need to breathe)).
I can control the humidity quite well with April Air and Ac but in the times of the year the HVAC is not needed a dehumidifier is used and needed to prevent condensation .
Mineral Wool (Roxul ) is best to cover rim XPS , IMO. Burning foam is toxic and glass gives virtually no protection. ( take a propane torch to some fiberglass in your driveway ..then try mineral wool).
All my downspouts are run in tiles far away from the house...you need to solve your moisture problems first .
I had block walls in a previous house ,..window well covers ,regrading ect all help.


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## ProGreen (Oct 2, 2014)

Nice read all, thanks! :smile:


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## Logan176 (Feb 18, 2016)

Nick DIY said:


> Gotcha. I ask because I'm knee deep in a similar project. I opted for the irrigation tube method ripped from Family Handyman. It took care of my minor leak problems and I was also able to bring Radon from almost 35 (yes, 35) to less than .10.
> 
> If the block drains into the tile, you shouldn't have to worry about liquid. It may be worth excavating just that corner. You'll probably fix the problem from outside, but the fact that it's happening means there's something wrong with the drain tile. Maybe the foam sealed up your holes?
> 
> You don't want liquid getting into any insulation. In addition to potential for mold, it loses most all of it's RValue.


A radon level of 35! Holy crap! My basement was only between 6-7 units before I had the radon system installed.

I left the foam out of that one corner because that slight trickle of water hasn't been solved yet. I will most likely leave it without foam since it is not impacting the operation of the radon system. I think the water is penetrating the foundation wall through cracks in the exterior. The foundation only has a 65 year old skim coat of mortar as an exterior seal. I'm sure years of downspouts without extenders and having tall hedges close to the house probably created a few holes. 

High Gear, after people on this forum recommended Roxul, I did some research and I'm sold on the product. The house I grew up in burned down before we moved out. Seeing someone hold a blow torch to a Roxul batt without it catching fire is pretty amazing. I wish I knew about this stuff when I gutted by entire house 7 years ago.


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## Logan176 (Feb 18, 2016)

Bud9051 said:


> As for the Code official, if they are following the 2009 energy codes the R-13 applies to the cavity fill and R-10 is for a continuous layer. But local codes rule and you will need to ask without offending, but the hybrid approach exceeds the total the code is allowing.
> 
> Bud


I have a good relationship with my building inspector. He knows I'm always asking questions because I'm trying to do the right thing instead of trying to beat the system. 

My building inspector just confirmed the R-10 continuous insulation is permitted. However, 2" of XPS costs about the same as 1" of XPS with R-15 batts of Roxul on top. The Roxul approach will give me an R-20 plus added flame and sound resistance vs the R-10 of the XPS alone. Looks like this just sealed the deal.


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## Logan176 (Feb 18, 2016)

Here's another question about the XPS. I have been using 2" boards to insulate the rim joists. I've been using caulk to seal up the edges and foam from a can to seal up the larger spaces. Caulking is a real pain in the ass. Can I just cut the XPS rectangles a little smaller and use foam cans to seal around all 4 sides of the XPS?


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## ProGreen (Oct 2, 2014)

Logan176 said:


> Here's another question about the XPS. I have been using 2" boards to insulate the rim joists. I've been using caulk to seal up the edges and foam from a can to seal up the larger spaces. Caulking is a real pain in the ass. Can I just cut the XPS rectangles a little smaller and use foam cans to seal around all 4 sides of the XPS?


Hey Logan, yes you can do that. Most important is to get a tight seal all around. I will be starting my crawl rim joists soon: why do you find the caulking hard to do? Thanks.


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## Logan176 (Feb 18, 2016)

The subfloors my from the floor above the basement is not plywood. It is made up of individual boards installed diagonally. Combine the unevenness of this subfloor design with nails that are popping and through and my own flaws cutting the xps, I wind up using a lot of caulk. I think it might just be easier to foam all the edges with the stuff in the can.


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## GottaFixIt (Dec 3, 2010)

Logan176 said:


> The subfloors my from the floor above the basement is not plywood. It is made up of individual boards installed diagonally. Combine the unevenness of this subfloor design with nails that are popping and through and my own flaws cutting the xps, I wind up using a lot of caulk. I think it might just be easier to foam all the edges with the stuff in the can.


How are you cutting the xps? I was using my tablesaw for 2" foam and they seemed to be coming out almost perfect.


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## Gary in WA (Mar 11, 2009)

IMHO, caulk the wood joints before canned foam at the rims to stop infiltration of summer moisture/air infiltrating - wetting the XPS as it will hold moisture much longer than after the wood dries out. With a double facing, you also stop winter condensation on the XPS inside face (if not thick enough) right through air permeable fg or Roxul---though this may dissipate due to the surrounding air; http://www.diychatroom.com/f103/insulate-rim-joists-canned-spray-foam-roxul-307313/ Maybe it's not that important, IMO, I'd spend the extra time.

BSC *(IN 2002) *also recommended foil-faced foam board on the wood rims as an interior fix; pp. 7, 8, and 9, here- pp.8- top of second column, yet notice Fig.15, either or- maybe it didn't get proof-read...; http://buildingscience.com/sites/default/files/migrate/pdf/BA-0202_Basement_Insulation.pdf

In a later (2007)actual field study, not a simulation or analysis, I found (topic of my searching for) the warm below-grade temps of the soil; few problems with most insulation types, other than solid plastic wrapped FG. or possibly poly sheet on inside at drywall during summer, especially with AC in a lot of U.S., not southern Canada- and that this was an unoccupied house and we don’t know the basement RH. ( eg. Kitchener- check their HDD compared to yours); 

Slashing a vapor barrier does nothing as it is area weighed (surface area)- much different from an air barrier; http://www.ecohome.net/guide/difference-between-air-barriers-vapour-barriers


Using 3 coldest months average temps for your location; https://www.currentresults.com/Weat...w-york-city-temperatures-by-month-average.php
At 68*F in basement w. R-5 + 15R = concrete wall above grade and 6" down at 43.25*F safe to room RH of 41%...................... with R-10 XPS and 15R cavity, 48.2*F and 49%Relative Humidity. Though the plastic drainage board may not be conducting full temp to inside, so both temps/RH would change at the inside rigid board from interior RH. ADA the drywall; http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/, use acoustic sealant at perimeters, duct tape/mastic at seams as the board does shrink/change with temps...


Gary


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## Logan176 (Feb 18, 2016)

Hey everyone. Things have been going well in the basement. After listening to everyone's advice, I decided to go with the 1" xps and R15 Roxul. Propping up each board for 12 hours while waiting for the glue to dry was a pain, but I figured out a system using 2x4s. 

I was surprised that the 3M red construction tape has NOT held up. It stuck well in the beginning, but lots of air bubbles appeared a couple weeks later. The tape has not peeled off, but I hear a wrinkling/popping sound when I run my fingers along the tape. I've since learned that Owens Corning makes a tape specifically for xps, so I might go over the red tape with the Owens Corning stuff if I can find where to buy it. 

I have a new question though. I have a part of my foundation that sticks out to add extra support for the main beam spanning my basement. I insulated and framed around it on the family room side of the basement, but do you think I need to do so in the utility room? I only ask because that would leave very little space between that bump out and my furnace.


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## Bud9051 (Nov 11, 2015)

How about just boxing it in with XPS and then gluing the drywall to that?

Bud


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