# Do I need housewrap if I am using 2" rigid foam with taped seams?



## Nealtw (Jun 22, 2017)

DBD2022 said:


> I just thought of this as I was doing my estimate. Part of the reason for having rigid foam is to move your dew point out of your wall cavities. If you put housewrap against the sheathing, doesn't that kind of interfere with the drying process as moisture builds up on the plywood? It feels to me like the moisture can't readily escape. I'm also wondering if the foam and tape by themselves are a sufficient weather barrier to replace tyvek, and I'm not really finding a reason why they aren't. Tyvek isn't very expensive, so this isn't a cost cutting thing. I'm genuinely curious if housewrap defeats the purpose of the foam. I could be totally wrong, as I'm not a weatherization specialist, but I understand there is a lot of debate about best practices over the past few years. Does anyone have any personal experience with this issue? Is anyone aware of any studies that deal with this?
> 
> Another related question: Is 2" rigid exterior foam sufficient in climate zone 6? This is in addition to r21 fiberglass wall cavity. Apparently, having thinner rigid foam is potentially dangerous, because the dew point is moved onto the sheathing, which leads to moisture buildup. I want to know how thin is too thin for colder climates.
> 
> Thanks!


You will build out the frames from windows and doors that will not be covered with foam , so you need house wrap in those areas. 
Dew point some where in the wall means nothing unless you intend to allow moist air into the wall that will reach that area, and if air can get there would not that move the dew point. 
So if you are planning on stopping air from entering from either side, dew point is a non issue. 
Walls that are kept dry do not have to dry out. If moisture can not get out, how did it get there?


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## SPS-1 (Oct 21, 2008)

DBD2022 said:


> If you put housewrap against the sheathing, doesn't that kind of interfere with the drying process as moisture builds up on the plywood? It feels to me like the moisture can't readily escape.


No.
You are equating housewrap to vapor barrier. They are two totally different materials. Housewrap is specifically engineered to be vapor permeable. 

_DuPont™ Tyvek® HomeWrap® is the original house wrap, incorporating unique material science that helps keep air and water out, while letting water vapor escape._


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## DBD2022 (8 mo ago)

Nealtw said:


> You will build out the frames from windows and doors that will not be covered with foam , so you need house wrap in those areas.
> Dew point some where in the wall means nothing unless you intend to allow moist air into the wall that will reach that area, and if air can get there would not that move the dew point.
> So if you are planning on stopping air from entering from either side, dew point is a non issue.
> Walls that are kept dry do not have to dry out. If moisture can not get out, how did it get there?


Can't I just use flashing tape for the areas around the windows and doors? I say that, and then realize how expensive it is. On the other hand, we do it all the time for remodels. It also looks like the model window I have selected comes with some kind of flashing tape.

Regarding air infiltration spots, nail holes, staple holes, and controlled vents (soffit, gable) come to mind. Do we even do vented gables and soffits anymore? My plan calls for them, but now I'm trying to figure out if they are necessary, or even counter productive. As you say, if the space is well sealed, it seems unnecessary, and could even reduce the performance of the attic insulation. Do you have a take on that?


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## DBD2022 (8 mo ago)

SPS-1 said:


> No.
> You are equating housewrap to vapor barrier. They are two totally different materials. Housewrap is specifically engineered to be vapor permeable.
> 
> _DuPont™ Tyvek® HomeWrap® is the original house wrap, incorporating unique material science that helps keep air and water out, while letting water vapor escape._


Ah, thank you. I remember learning that, but I must have forgotten. 

Since that is the case, do you think it is necessary with my rigid foam scenario? I'm seeing it included in a lot of sample plans, but to me, it seems like it isn't doing much besides a backup in case the foam fails. Rigid foam with taped seams is not vapor permeable, apparently.


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## Nealtw (Jun 22, 2017)

DBD2022 said:


> Can't I just use flashing tape for the areas around the windows and doors? I say that, and then realize how expensive it is. On the other hand, we do it all the time for remodels. It also looks like the model window I have selected comes with some kind of flashing tape.
> 
> Regarding air infiltration spots, nail holes, staple holes, and controlled vents (soffit, gable) come to mind. Do we even do vented gables and soffits anymore? My plan calls for them, but now I'm trying to figure out if they are necessary, or even counter productive. As you say, if the space is well sealed, it seems unnecessary, and could even reduce the performance of the attic insulation. Do you have a take on that?


Yes you still have soffet vents and any time you have a chance you increase the size of that. 
Often the soffets are lower than the top of the wall so you do need to deal with them. 
Windows and doors are always the best place to find water leaks and re and re them with a proper pan is almost a must do. 
If you were to put wrap over the foam you do it all with tape. I don't know if it is required by code. 
Most all siding leaks so making sure water has a way out is important


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## Old Thomas (Nov 28, 2019)

I would use housewrap along with the foam, behind the foam. Over the next few years the foam will shrink and will pull on the tape, and some of the tape will fail. 8 foot foam boards outgas and shrink about 1/4” in length over a few years.


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## SPS-1 (Oct 21, 2008)

If you are planning a build, you may wish to check out Green Building Advisor. You can dig up a lot of very useful information even without paying for their subscription. But if you are building a house or addition, the cost of the subscription is negligible in the overall cost of your project.

For example, by doing a search on GBA, I find your question of required foam thickness is addressed in an article.







This particular article is behind their pay-wall, but you can see in the preamble they mention looking at the IRC table. You could then Google "IRC minimum foam thickness table"

But actually, your question is flawed from the beginning. You did not specify what type of foam you are planning on using. There are big differences in R Value and vapor permeability.


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## SLSTech (Jan 19, 2021)

Let me give you 2 links: the first covers do I need housewrap if I use foam? If I do where should I put it, etc... More importantly is towards the bottom where if you do want to use foam as a WRB - best practices Common Sense Building: To Tyvek or not to Tyvek, that is the question (thehtrc.com) 
The second is a look at optimal insulation values & a chart on the codes & "should" have insulation values for condensation control: Common Sense Building: How much insulation should I install? (thehtrc.com) FYI - you should have R25 & if you don't let moisture in a cavity you don't have to worry about getting it out


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## XSleeper (Sep 23, 2007)

Most WRB seam tapes aren't going to stick to foam forever. Anyone who thinks they will is fooling himself. A lot depends on the exact product you are using, though. Dow Weathermate is made to work with Dow Styrofoam products, for example. But a generic WRB... no way. The answer mainly depends on where the windows are flashed. If they have fins and the fin is going to be flush with the foam, then yes I would put a WRB on top of the foam. All dripcaps above window trims would then get tucked under the WRB.


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## DBD2022 (8 mo ago)

SPS-1 said:


> If you are planning a build, you may wish to check out Green Building Advisor. You can dig up a lot of very useful information even without paying for their subscription. But if you are building a house or addition, the cost of the subscription is negligible in the overall cost of your project.
> 
> For example, by doing a search on GBA, I find your question of required foam thickness is addressed in an article.
> View attachment 695273
> ...


Thank you, I will check that out. My current plan actually exceeds the IRC code, but I was told by someone on here that even if your plan meets code, you could still have problems. There was another GBA article that talked about it. 









Combining Exterior Rigid Foam With Fluffy Insulation - GreenBuildingAdvisor


It’s becoming increasingly common for builders to install one or more layers of rigid foam on the exterior side of wall sheathing and roof sheathing. Typically, these walls and roofs also include some type of air-permeable insulation (fiberglass, cellulose, or mineral wool) between the studs or...




www.greenbuildingadvisor.com





"These wall assemblies and roof assemblies perform extremely well, as long as the rigid foam is thick enough to keep the sheathing above the dew point during the winter. (Exterior foam reduces the ability of sheathing to dry to the exterior. Thin rigid foam is more dangerous than thick rigid foam because it isn’t thick enough to prevent moisture accumulation in the sheathing during the winter; however, it’s just thick enough to lower the rate of outward drying."

Regarding the foam I have in my plan, it is Owens Corning Foamular NGX 2" R10 with staggered and taped seams. It says on the Home Depot website that it has low vapor permeability and is highly water resistant. In other words, I would have to check with my local building department to see if they would approve it as both a weather membrane and vapor barrier. But, as someone just stated above, housewrap is a good buffer against failure of the tape and shrinkage of the foam, which are likely to happen over time. Basically, I just want to know if the 2" foam is sufficient for climate zone 6.


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