# Pairing Rigid foam insulation with existing wall cavity



## Izaura (Feb 11, 2013)

Short story: Shotgun exterior remodel of the house - windows and siding have to go, much sooner that we'd have liked but so it goes.

Silver lining: This would be a great time to add rigid foam insulation to the exterior... 

Conundrum: Trying to avoid a "vapor barrier sandwich" (argh!) while keeping existing cavity insulation.

Climate: Zone 5 (Berks Co. is just north of the zone 4 boundary, if it matters) -- 3-month cold weather average seems to be 30F here.
Wood frame 2x4
Sheathing: 1/2" Asphalt-impregnated fiberboard + plywood at corners 
Existing cavity insulation: Kraft-faced FB batts (age unknown)
Interior wall: 1/2" Gypsum, regular latex paint
Desired siding: vinyl -- open to insulated insulated / rigid core options at the moment. Tyvek is anticipated.

My current understanding:
- For Zone 5, over 2 x 4 walls, we'd want a minimum R-5 of exterior foam insulation (Naima)
- I could "get away" with just Class III latex paint as a vapor barrier given our siding choice, but I'm stuck with Class II (BS). 
- Strictly speaking, I wouldn't have a classic cold weather assembly (see p.14) to the extent that R-5 + rigid foam would be semi-impermeable..?
- Calculating hypothetical sheathing temperatures and dew points has thus far failed to help me make a decision, mainly because I don’t have enough data about our living habits and house performance to determine what interior design conditions are sustainable for us.

When shooting for approximately R-5 on the outside, there seems to be an awkward tradeoff between minimizing the risk of condensation (via thicker/denser foam) and maximizing the wall's ability to dry to the exterior. A minimum 1" of unfaced XPS would hover around 1 perm, give or take, faced polyiso is definitely under 1 perm... 
- Where should I seek to position myself in this tradeoff? 
- If I am interpreting this correctly (Sidebar 2), it seems as though with Class II on the interior, I do not need to obsess about preventing condensation at all costs as long as my exterior assembly is at least semi-impermeable (> 0.1 perm). That seems to leave all unfaced foam options open, no?
- Can the wall still dry to the interior with kraft-faced FB within, if need be? 

Thanks in advance... Having enough knowledge to know these things matter, but not enough to weigh tradeoffs where there do not seem to be absolutes, is a pretty uncomfortable place to be...


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## Windows on Wash (Aug 30, 2011)

Izaura said:


> - Where should I seek to position myself in this tradeoff?
> - If I am interpreting this correctly (Sidebar 2), it seems as though with Class II on the interior, I do not need to obsess about preventing condensation at all costs as long as my exterior assembly is at least semi-impermeable (> 0.1 perm). That seems to leave all unfaced foam options open, no?
> - Can the wall still dry to the interior with kraft-faced FB within, if need be?
> 
> Thanks in advance... Having enough knowledge to know these things matter, but not enough to weigh tradeoffs where there do not seem to be absolutes, is a pretty uncomfortable place to be...


You pretty much have it nailed down....trust your research. I promise you know more than most of the contractors of the world.

XPS (minimum 1") to the exterior and Gary has the psychrometric data to make sure you are covered on the dew point consideration.

Are you pulling down drywall? If so, ditch Kraft faced batts and go high density and unfaced.

Make sure all the all the gaps and cracks are spray foamed from the back side with the siding off as well as from the inside. 

If the drywall is coming down, making the interior wall air tight will dramatically reduce any chances for condensation given the fact that you will be keeping and exponentially larger amount of water out of the wall.

Trust your science. :thumbup:


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## Izaura (Feb 11, 2013)

Windows on Wash said:


> You pretty much have it nailed down....trust your research. I promise you know more than most of the contractors of the world.
> 
> Trust your science. :thumbup:


 Thank you for saying that. The more I was researching the topic, the more I became concerned I had not considered all variables, or that I might be grossly misinterpreting what I'm reading. This is especially true since none of the canonical wall assemblies detailed matched what I was proposing to do (e.g., batts should be unfaced). In other words, I am doubting the "my" in "my science" -- this is not my field by a longshot.

The drywall or, most of it anyway, can and will stay, and so will the FB for the time being. 
That said, we're tearing down paneling that was up in a good chunk of the house and just discovered that the drywall behind it was never taped and mudded. Any special considerations?


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## Windows on Wash (Aug 30, 2011)

No biggie.

Are you pulling off the fiberboard? 

That would be ideal if you aren't dropping the drywall. It would allow you to seal the back of the drywall against any leaks with a combination of foam and/or sealant.

If the blackboard isn't structural (not usually consider such and doesn't appear to in this case with the let in bracing for the plywood corners), I would rip it off and just pull out all of the old insulation, seal at the gaps and cracks, wrap all the outlets, install new batt insulation (FG or Roxul), install foam exterior, seal all the seam, new foam backed siding.


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## Gary in WA (Mar 11, 2009)

What will the interior finish on the drywall be, after you tape the seams? 

This on kraft faced batts, Ginger vs. Mary-Ann; http://www.buildingscience.com/documents/insights/bsi-024-vocabulary

Compare your wall make-up/perms with the choices given, *pp.14, Fig. 3e-1*; http://www.buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design

Using 3 low temps average of 33*F from; http://www.weather.com/weather/wxclimatology/monthly/graph/19604

At 70*F inside, dew-point is 38%RH at foamboard temp of 43*F. That is with medium density R-13, if R-9 as older f.g.= 46* foam temp, 42%RH. R-15 HD or Roxul= 42*F= 36%RH.

Use foam-backed siding if you have children, it resists dents better; https://docs.google.com/viewer?a=v&...bY44FW&sig=AHIEtbT1X6_1p3ibUPYAt5jlOsa_kZRRgw


Gary


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## Izaura (Feb 11, 2013)

Gary, thanks for the Ginger vs. Mary-Ann link. I had no idea the permeance of my FG would vary as a function of humidity, which is quite comforting.

Interior finish... Was going to go for "regular" (perm > 1, not BIN or the like) latex primer & paint? 

Looks like the interior face of my foam should be warm enough more or less all the time as long as we keep RH < 35%. 
... To the extent that we tend to keep the house cooler than 70F (65F is typical), our wall assembly is slightly more forgiving, correct?
That said, I really do need to get a hygrometer and see where our RH lies when we're not actively trying to keep it in check.


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## Windows on Wash (Aug 30, 2011)

The wall setup you are proposing will work great.

I would worry about the hygrometer if you keep the humidity in the winter months manageable.


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## Gary in WA (Mar 11, 2009)

Izaura said:


> Gary, thanks for the Ginger vs. Mary-Ann link. I had no idea the permeance of my FG would vary as a function of humidity, which is quite comforting.--- You are welcome.
> 
> Interior finish... Was going to go for "regular" (perm > 1, not BIN or the like) latex primer & paint? ----------- yep, no oil-based paints, hanging plate glass mirrors, etc.
> 
> ...


---- Yes, I agree--- with all.


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## Nailbags (Feb 1, 2012)

Here is a good chart to know the permeability rates.http://www.energy.wsu.edu/Documents/BFG%20Chapter%205-Jan2011.pdf
There you go. Good Info.


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