# Best Strategy for Existing 2x4 Wall?



## Nealtw (Jun 22, 2017)

Leo55 said:


> Hey guys, I'm in the middle of a big remodel, and am working with existing 2x4 exterior walls. I live in climate zone 5, in central Ohio, where we get a lot of precipitation. It gets both pretty hot and pretty cold.
> 
> The house exterior is vinyl siding, with house wrap, and OSB. On the interior, everything is stripped to the studs. The old drywall had a plastic sheet under it, and lots of the old insulation was moldy. The old insulation was fiberglass batts.
> 
> ...


 You need 2 things to get condensation in the wall. Warm moist air and a cold surface. Wet dirty or moldy insulation would point out air leaks 

If you just replace the fiberglass thickness with glass or rock wool you could add 1" foam board properly sealed would be the air barrier and add 5R and a thermal break on the studs.


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## Colbyt (Jan 27, 2014)

I'll hazard a guess that is electric BB heat or some form of no forced air.


I think I read builders made a mistake in the 70's and early 80's using plastic as a vapor barrier.


Personally, I would make really sure it is dry and that any mold on the studs is killed and then go back with plain old kraft faced insulation.


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## Leo55 (Feb 14, 2020)

Yes, the old insulation was dirty in places, so there was air infiltration. 

And yes, the old place had baseboard heating. Out of curiosity, what made you guess that? 

A ductless mini-split is what I'm thinking to go to. The house sits on a slab, and there is no ductwork. It's so small, I think a mini-split might be a good solution.

Regarding the walls, do you think 1" of rigid foam on the interior would be in danger of creating a plane for condensation inside the wall? I plan to air seal the daylights out of the wall both to the exterior and interior, to eliminate as best I can gross air infiltration. But the ambient air here in the summer will still be very humid, and the inside of the place will be air conditioned. I don't know if 1" is thick enough to create enough of a thermal break so that condensation would not be a risk. 

Thanks again for the thoughts. I appreciate it.


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## Porsche986S (Dec 10, 2017)

Here is what I would do but I'm no expert :biggrin2: Let's say your studs are spaced at 16 " on center . Get some 3/4 " or 1 " thick rigid foam insulation and cut the foam so there is aprox. a 1/4 " gap all the way around in the stud cavity . Use a few nails/screws to hold the foam in place . Do this for all stud cavities . After all the foam board is in place take cans of spray foam like Great Stuff or whatever brand is available in your area and fill that 1/4 " gap all the way around . This will eliminate air infiltration . You may have to get a little creative around switch and outlet boxes .

I would then attach 1x furring strips to the face of the studs and then install kraft faced fiberglass insulation . Then finish with the wall product of choice . Use this same air sealing methodology for anything that goes from inside to outside like ceiling can lights as an example . The more drafts you can eliminate the better your home will feel regardless of heating or cooling .


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## SPS-1 (Oct 21, 2008)

We should really be more specific here. The term "foam", has been used maybe half a dozen times so far in this thread. Maybe somebody can educate me on the proper wording, but the term "foam" means nothing to me.

There is EPS, XPS, Polyiso, and polyurethane that can be purchased as board. Some of these are similar, some are very different. XPS is a good vapor barrier, EPS - not so much.


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## Bud9051 (Nov 11, 2015)

The readily available charts to provide some guidance for how much rigid (?) is needed have moved behind membership requirements.
https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation

My free time has expired but you can take a look.

The down side is these charts are dealing with exterior rigid and i'm not sure how easily we can apply them to an inside application, but it is a start.

Air sealing is a key to any assembly then thermal bridging. But without a continuous layer of rigid there are still major gains to be made with better insulation (like mineral wool), and a neat installation. If new siding and or new windows are somewhere in the future than exterior rigid could be considered.

Bud


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## Leo55 (Feb 14, 2020)

Yes, please feel free to be more specific about the rigid foam. I am not knowledgeable enough yet to be able to be more specific myself. I have not gotten that far yet. I've been mulling over the more general idea of whether or not it is advisable in my climate to have 1" of rigid foam on the interior of a 2x4 wall. I'm not sure if the wall is too thin for that, and you might get condensation between the rigid foam and the insulation. 

I'm pretty sure in my area you don't need to have a vapor barrier anymore, but instead a vapor retarder, which I think can be just painted drywall. 

Also, I've seen it suggested in my area that if you replace your siding, they encourage you to add 1" rigid foam with an r value of 5 to the exterior beneath the new siding.

My plan is to attend very carefully to air sealing. The old insulation was very dirty. I saw a chart not long ago that showed the curve created when heat flow was plotted against r value, and how most of the benefit occurs in the first part of the curve. My goal might be to get the wall to an r value of 20, if that would be possible. Here is a link to the page with that chart for anyone who wants to see it. 

https://www.energyvanguard.com/blog/76941/The-Diminishing-Returns-of-Adding-More-Insulation


Thanks for the link to that Green Building Advisor article.


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## APA (Jul 13, 2018)

Leo55 said:


> Hey guys, I'm in the middle of a big remodel, and am working with existing 2x4 exterior walls. I live in climate zone 5, in central Ohio, where we get a lot of precipitation. It gets both pretty hot and pretty cold.
> 
> The house exterior is vinyl siding, with house wrap, and OSB. On the interior, everything is stripped to the studs. The old drywall had a plastic sheet under it, and lots of the old insulation was moldy. The old insulation was fiberglass batts.
> 
> ...



Was the framing moldy or did the insulation just smell musty? I almost guarantee it was because of that plastic vapor barrier under the drywall. How old is the house. If it is old and hasn't rotted by now, it isn't. I would just get the newer 3.5-inch, r-15 kraft faced and staple it in. However, since the house is so small, look into spray foam. there will be no condensation with that stuff.


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## APA (Jul 13, 2018)

I would say that 90% of houses in this area has some mustiness in the walls. It is a product of how they are constructed.


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## SPS-1 (Oct 21, 2008)

APA said:


> Was the framing moldy or did the insulation just smell musty? I almost guarantee it was because of that plastic vapor barrier under the drywall.


I don't understand the suggestion that a vapor barrier on the warm-in-winter side is a problem in a cold climate. The warm moist air is _inside_ the house. The cold air outside contains very little moisture. If warm moist air hits a cold surface, it will condense to liquid water, so I can understand the desirability of a vapor barrier behind the drywall. 

Things can be different in a warm climate, but OP indicated he is in Zone 5.


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## Nealtw (Jun 22, 2017)

SPS-1 said:


> I don't understand the suggestion that a vapor barrier on the warm-in-winter side is a problem in a cold climate. The warm moist air is _inside_ the house. The cold air outside contains very little moisture. If warm moist air hits a cold surface, it will condense to liquid water, so I can understand the desirability of a vapor barrier behind the drywall.
> 
> Things can be different in a warm climate, but OP indicated he is in Zone 5.


People have seen all kinds of problems with VB but they never consider that it was just not done properly. :vs_cool:


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## Leo55 (Feb 14, 2020)

Thank you guys for the thoughts. 

To answer your questions---the house was built in 1985. It's very small---about 800 sq ft. The insulation was moldy, not really the framing. It seems to have been from condensation on the plastic vapor barrier. 

I'm in zone 5----we get a lot of precipitation. In the summer it gets hot and muggy, and in the winter we have snaps down in single digits. 

People used to run a window air conditioner in there in the summer. I think warm muggy air infiltrated the wall, and moisture condensed on the vapor barrier, which was touching the cool drywall. 

I like the idea of adding 1" of some kind of foam to the wall, to create a thermal break and raise the wall's r value. I just want to be sure that I don't somehow create a situation for condensation to form.


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## Nealtw (Jun 22, 2017)

Leo55 said:


> Thank you guys for the thoughts.
> 
> To answer your questions---the house was built in 1985. It's very small---about 800 sq ft. The insulation was moldy, not really the framing. It seems to have been from condensation on the plastic vapor barrier.
> 
> ...


If you are running the AC a lot then the worry is getting air from the outside in the summer. Gaps in the insulation are the real problem when you have mold inside the poly.


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## APA (Jul 13, 2018)

Nealtw said:


> People have seen all kinds of problems with VB but they never consider that it was just not done properly. :vs_cool:



Because it is so easy to do improperly.


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## NotYerUncleBob2 (Dec 29, 2017)

The poly vapor barrier is fine...if you're in Canada or Alaska. Not so much for most heating/cooling climates in the US. 
For the best comfort your order of importance is to air seal, then insulate the attic, then insulate the walls. 
If you're looking to break the thermal bridge and have already gutted the interior, consider the bonfiglioli wall approach: https://www.finehomebuilding.com/2014/11/26/breaking-the-thermal-bridge


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## Nealtw (Jun 22, 2017)

@APA @ NotYerUncleBob2


Mold does not grow on fiberglass or poly, it grows on dust and dirt left there by passing air. So there was an air leak one or both sides of this wall.
Are you saying with out poly, there would have been no mold? 

What would you blame if there was no poly?


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## Leo55 (Feb 14, 2020)

From what I've read it sounds like a poly vapor barrier isn't necessary where I live. 

Neal, yes, there was air leakage, probably on both sides of the wall. The mold was concentrated on the surface of the insulation touching the poly vapor barrier. 

I'm not sure what it would have looked like if there had been no poly. Maybe mold on the backside of the drywall? Seems like you still might have gotten some condensation, maybe just not as much? You'd still have had the air leakage, and the contact between humid air and a cool surface----just a more porous surface, the backside of the drywall. 

Air sealing is high on my priority list.


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## Nealtw (Jun 22, 2017)

Leo55 said:


> From what I've read it sounds like a poly vapor barrier isn't necessary where I live.
> 
> Neal, yes, there was air leakage, probably on both sides of the wall. The mold was concentrated on the surface of the insulation touching the poly vapor barrier.
> 
> ...


 Yeah, air getting in the wall from either side can deposit dirt and moisture and the mold spores are everywhere. 

With 2x4 walls that don't leak and are well insulated, we still can see ghosting on the walls where the dust collects where the studs are. 



Dew point is the temperature where moisture in the air will condense on a cold surface. 

I would think we would like to keep that temperature about the center of the wall and then if we have that we don't want any moist dirty air there. 

So keeping air out of the wall is very important.
I have tried to imagine what that dew point line might look like in a wall that is done with out air leaks and remembering that the studs have a very low R value. Ghosting on the wall is when the dew point has made it all the way to the inner surface, not enough R value or poor heat circulation behind furniture, maybe. 



So I tried to figure what the dew line might look like in a wall with out air leaks and the same wall with outside light or inside switch box.
I don't think it matters how you block the air as long as you are blocking it. 

So this is what I imagine and I did not try to figure what would happen with air leaks.


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## Bud9051 (Nov 11, 2015)

Here is an article from Building Science Corp to add to the debate. For disclosure, I haven't read all of it but it has nice pictures .
https://www.buildingscience.com/doc...ng-cold-weather-condensation-using-insulation

Bud


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## Leo55 (Feb 14, 2020)

Neal, thanks for the graphics. I plan to air seal both to the exterior and to the interior. I don't see any reason not to do both. The electric boxes might be a little tricky. Getting a good seal against the drywall. I've seen some boxes with gaskets, but not locally. You have to order them. 

Bud, that looks like a very interesting article. I need to spend a minute with it.


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## Nealtw (Jun 22, 2017)

Leo55 said:


> Neal, thanks for the graphics. I plan to air seal both to the exterior and to the interior. I don't see any reason not to do both. The electric boxes might be a little tricky. Getting a good seal against the drywall. I've seen some boxes with gaskets, but not locally. You have to order them.
> 
> Bud, that looks like a very interesting article. I need to spend a minute with it.


Both, Home Depot and Lowe's have Nu Tek boxes up here. Which is strange because we need VB so we don't need special boxes.


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## APA (Jul 13, 2018)

This is the maddening part of the insulation debates. It winds down to nobody has ever insulated anything the right way. There is no way to do it "right" it is part luck and part voodoo. What works for one might not work for the guy down the street...


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## Leo55 (Feb 14, 2020)

The part that I find tricky is how specific it all is. There's so much information out there, but does it apply to your situation? My situation, too, is very suboptimal. It's a 2x4 wall with no exterior insulation under the siding. And I'm sure it's the sort of situation a kajillion houses in my area are in. If you were starting from scratch, it would be a no brainer to build a thicker wall and both seal and insulate the daylights out of it. But what about when you are working within some suboptimal parameters? It isn't entirely easy to know. 

So far what seems to me to be a no brainer is to air seal as well as possible. Prevent gross air infiltration, as that is probably what produces most of the opportunity for condensation. It also seems like a no brainer to get rid of the poly vapor barrier. If you air seal you shouldn't need it----all it really does it provide a plane for condensation to form. What I'm less sure about is if adding 1" of some kind of foam to the interior would be a good thing or a bad thing. Would it be sufficiently thick to provide an actual thermal break? Or would it too be a place where condensation could easily form?


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## SPS-1 (Oct 21, 2008)

Leo55 said:


> What I'm less sure about is if adding 1" of some kind of foam to the interior would be a good thing or a bad thing.


Martin Holladay (Green Building Advisor) seems to like the idea.

https://www.greenbuildingadvisor.co...n-to-the-inside-of-a-wall-instead-of-exterior 

https://www.greenbuildingadvisor.com/article/walls-with-interior-rigid-foam

One weak point of foam boards is the joints. Any small gap means you only have a .001" think piece of plastic tape between the inside and outside. Consider caulking the joints, in addition to tape.

Note that any fire code will require covering the interior foam with protection such as drywall.

.


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## APA (Jul 13, 2018)

Leo55 said:


> The part that I find tricky is how specific it all is. There's so much information out there, but does it apply to your situation? My situation, too, is very suboptimal. It's a 2x4 wall with no exterior insulation under the siding. And I'm sure it's the sort of situation a kajillion houses in my area are in. If you were starting from scratch, it would be a no brainer to build a thicker wall and both seal and insulate the daylights out of it. But what about when you are working within some suboptimal parameters? It isn't entirely easy to know.
> 
> So far what seems to me to be a no brainer is to air seal as well as possible. Prevent gross air infiltration, as that is probably what produces most of the opportunity for condensation. It also seems like a no brainer to get rid of the poly vapor barrier. If you air seal you shouldn't need it----all it really does it provide a plane for condensation to form. What I'm less sure about is if adding 1" of some kind of foam to the interior would be a good thing or a bad thing. Would it be sufficiently thick to provide an actual thermal break? Or would it too be a place where condensation could easily form?



If it is a small room, just pay someone to come in and spray foam it and be done. And you will be done.


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## Bud9051 (Nov 11, 2015)

I don't see your climate region listed, may be I missed it.
But a 3.5" wall assembly air sealed with good insulation will be just fine and a big improvement over that you have.

Adding 1" of rigid to either inside or outside is nice, but an expensive step when you consider how much it would add to an improved wall. Remember, walls are only 20% of your total heat loss (wild estimate) and an extra 20% improvement in the wall insulation would result is a 2% overall improvement (20% x 10% = 2%).

The big improvement comes from air sealing as it improves both insulation value and comfort.

Bud


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## Leo55 (Feb 14, 2020)

Hey Bud,

My climate zone is 5. I live in central Ohio. We get lots of precipitation, and it gets both hot and cold. Heck, anymore during the winter you can get snaps where it warms up close to 50 degrees one week, then is down around 10 degrees the next. 

My goal is to improve this place, fully realizing I can't make it perfect. I also want to make sure that I don't inadvertently do anything that causes problems.


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## Bud9051 (Nov 11, 2015)

Your zone can be cold but you also use ac so condensation issues can come from both directions. In that case the more important issue is to be sure the walls can dry in one or both directions. Air sealing reduces the concerns about condensation but best to just avoid the plastic inside or out.

More reading: https://www.buildingscience.com/documents/insights/bsi-073-macbeth-does-vapor-barriers

And: https://www.greenbuildingadvisor.com/article/questions-and-answers-about-air-barriers

Greenbuildingadvisor has a free membership which will give you access for a period of time. I used my free time tears ago but it had a lot of great information. Just not enough need for me to pay $90 a year.

Bud


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## Nealtw (Jun 22, 2017)

The biggest worry we have is mold or rot or any fungus growing in the wall.
They need a certain amount of moisture to grow. Warm moist dirty air entering the wall and leaving moisture and dirt will provide what ever it needs.
By the time new house is insulated the wood is already lower than what is need for this growth. 

Just with that info, I think there is a good argument for sealing both sides.
The only argument against poly that i have read so far is that the wall can't dry when it does get wet. 

If you have a leak, a flood or any other reason for the wall to get wet, it need to be opened for drying, with or with out poly makes no difference. 

I do agree, you might be able to seal a wall with out poly. 



People do talk about mold growing on poly. Mold does not find food in poly so mold on poly is an indication of moisture and dirt being deposited there from a leak in the system. Air flow and cold making it to that surface. 

http://www.rlcengineering.com/wood-moisture-content/


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## House Designer (Oct 4, 2019)

For a cost effective and FUNCTIONALLY effective solution, I would get spray-foam applied to a thickness of 1". That will completely seal you from infiltration. Then install standard R-13 batts in the cavity. The batts will be compressed from 3-1/2" down to 2-1/2", so the insulation factor will suffer some.
The finished assembly will give you:
R-0.5 for the exterior sheathing
R-5 for the 1" of closed-cell spray-foam
R-10 for the compressed R-13 batt
R-0.5 for the drywall
TOTAL = R-16

I recommend this for my remodels in Cleveland with 2x4 walls.

You should NOT have any problem with mold. You will still want to use FACED insulation batts with 1" of spray-foam. Thicknesses of 2" or more of closed cell spray-foam act as a vapor barrier themselves, and don't require another vapor barrier.

If price is no object, you can spray-foam the entire cavity, and achieve R-18.5, but you will pay dearly. An 800 sq. ft. house could cost well over $10,000 to spray-foam the walls.

Source of Compressed R-Values:
https://dcpd6wotaa0mb.cloudfront.ne...Value-Chart-Tech-Bulletin.pdf?v=1428909128000


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## SPS-1 (Oct 21, 2008)

House Designer said:


> You will still want to use FACED insulation batts with 1" of spray-foam.


On which side of the assembly are you suggesting the face paper should go ?


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## House Designer (Oct 4, 2019)

SPS-1 said:


> On which side of the assembly are you suggesting the face paper should go ?


The face paper should go on the inside (heated side). The closed cell spray foam isn't a fully effective vapor barrier unless it is 2" thick or thicker. In which case it works just fine as a vapor barrier, even though it's on the outside (unheated) wall.


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## Bud9051 (Nov 11, 2015)

The building science community is no longer in love with vapor barriers in your climate region:
https://www.buildingscience.com/documents/insights/bsi-073-macbeth-does-vapor-barriers

They have also switched to referring to them as vapor diffusion retarders so they can distinguish between a 100% VB and housewrap.

I'm afraid your recommendations sounds like yesterday's advice, not terribly bad, but not what science is recommending today.

Bud


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## House Designer (Oct 4, 2019)

I stand corrected, a 1" spray-foam may also provide vapor diffusion properties, so an additional kraft paper vapor barrier may not be required, according to Bud9051's website link. I have seen other sites that contradict this, so it may pay to survey several sources and make a decision based on the majority.
The rest of my advice stands as a cost-effective option to alleviate the original poster's mold concerns.


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## ablodneyget (Jul 15, 2018)

My 2x4 walls are sheathed with Celotex and are therefore extremely vapor-open to the exterior. When I redid my kitchen I caulked all seams (basically the perimeter of each stud bay) and applied Certainteed Membrain over it. The material (nylon as I recall) is more vapor-open when the cavity humidity is higher. At least that's what they claim... So it should be better than straight poly which is a perfect vapor barrier.

Time will tell whether that was a good idea, but it was the best compromise I could come up with once I chose Rockwool.

Dense-packed cellulose acts as a moisture buffer and air seals pretty well based on my experience from this house. The original walls of my home were dense packed and performed very well for 50 years without vapor retarders. Fiberglass and rockwool do not buffer humidity, whereas cellulose shares moisture with the framing, enabling the wall assembly to ride out those fluctuations.

WUFI software from ORNL has a free trial for personal use. I used it to simulate my assembly in my climate. (I used another product in its database similar to MemBrain.)
https://web.ornl.gov/sci/buildings/tools/wufi/


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## Leo55 (Feb 14, 2020)

Hey guys, just wanted to say thank you for all of the feedback. This was very helpful. I appreciate the help a lot.


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