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Old 02-18-2016, 10:17 AM   #1
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XPS Insulation Thickness in Basement


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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Old 02-18-2016, 10:25 AM   #2
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Have you considered using Polyiso? 2" of polyiso is R-13 and includes a vapor barrier.

Last edited by Nick DIY; 02-18-2016 at 10:28 AM. Reason: should say, often includes a vapor barrier
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Old 02-18-2016, 10:54 AM   #3
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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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Old 02-18-2016, 11:12 AM   #4
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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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Old 02-18-2016, 01:47 PM   #5
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Quote:
Originally Posted by Logan176 View Post
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...

Last edited by Nick DIY; 02-18-2016 at 01:49 PM.
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Old 02-18-2016, 03:01 PM   #6
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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/...apor-retarders
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Old 02-18-2016, 03:16 PM   #7
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Thanks, Bud. I was looking for that article to cite, but couldn't seem to find it.

Here's a relevant quote:
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.

Last edited by Nick DIY; 02-18-2016 at 03:21 PM.
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Old 02-19-2016, 08:56 AM   #8
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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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Old 02-19-2016, 10:18 AM   #9
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Quote:
Originally Posted by Logan176 View Post
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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Old 02-19-2016, 10:29 AM   #10
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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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Old 02-19-2016, 11:33 AM   #11
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Did you install drain tubes from the block to the drain tile?
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Old 02-19-2016, 11:52 AM   #12
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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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Old 02-19-2016, 12:00 PM   #13
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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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Old 02-19-2016, 06:53 PM   #14
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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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Old 02-19-2016, 07:22 PM   #15
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Nice read all, thanks!
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