Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > Insulation

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Reply
 
Thread Tools Search this Thread Display Modes
Old 03-27-2013, 01:46 PM   #1
Newbie
 
Join Date: Feb 2013
Location: Berks, PA
Posts: 11
Rewards Points: 0
Share |
Default

Pairing Rigid foam insulation with existing wall cavity


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...


Last edited by Izaura; 03-27-2013 at 01:50 PM. Reason: links
Izaura is offline   Reply With Quote
Old 03-28-2013, 07:40 AM   #2
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: VA, MD, DC
Posts: 6,252
Rewards Points: 84
Default

Pairing Rigid foam insulation with existing wall cavity


Quote:
Originally Posted by Izaura View Post
- 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.

Windows on Wash is offline   Reply With Quote
Old 03-28-2013, 10:49 AM   #3
Newbie
 
Join Date: Feb 2013
Location: Berks, PA
Posts: 11
Rewards Points: 0
Default

Pairing Rigid foam insulation with existing wall cavity


Quote:
Originally Posted by Windows on Wash View Post
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.
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?
Izaura is offline   Reply With Quote
Old 03-28-2013, 01:49 PM   #4
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: VA, MD, DC
Posts: 6,252
Rewards Points: 84
Default

Pairing Rigid foam insulation with existing wall cavity


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.
Windows on Wash is offline   Reply With Quote
Old 03-28-2013, 03:26 PM   #5
Registered User
 
Join Date: Mar 2009
Posts: 9,968
Rewards Points: 0
Default

Pairing Rigid foam insulation with existing wall cavity


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/docum...024-vocabulary

Compare your wall make-up/perms with the choices given, pp.14, Fig. 3e-1; http://www.buildingscience.com/docum...nd-wall-design

Using 3 low temps average of 33*F from; http://www.weather.com/weather/wxcli...ly/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&q...t5jlOsa_kZRRgw


Gary
__________________
If any ads are present in my answer above, I do not condone/support/use the product or services listed, they are there against my permission.
Gary in WA is offline   Reply With Quote
Old 03-29-2013, 06:37 AM   #6
Newbie
 
Join Date: Feb 2013
Location: Berks, PA
Posts: 11
Rewards Points: 0
Default

Pairing Rigid foam insulation with existing wall cavity


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.
Izaura is offline   Reply With Quote
Old 03-29-2013, 07:14 AM   #7
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: VA, MD, DC
Posts: 6,252
Rewards Points: 84
Default

Pairing Rigid foam insulation with existing wall cavity


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.
Windows on Wash is offline   Reply With Quote
Old 03-29-2013, 07:02 PM   #8
Registered User
 
Join Date: Mar 2009
Posts: 9,968
Rewards Points: 0
Default

Pairing Rigid foam insulation with existing wall cavity


Quote:
Originally Posted by Izaura View Post
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.

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?------------
correct, at 65*F it would be 50%RH.
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.
---- Yes, I agree--- with all.
__________________
If any ads are present in my answer above, I do not condone/support/use the product or services listed, they are there against my permission.
Gary in WA is offline   Reply With Quote
Old 03-30-2013, 07:56 PM   #9
Member
 
Join Date: Jan 2012
Location: Pacific North west
Posts: 1,248
Rewards Points: 8
Default

Pairing Rigid foam insulation with existing wall cavity


Here is a good chart to know the permeability rates.http://www.energy.wsu.edu/Documents/...05-Jan2011.pdf
There you go. Good Info.

Nailbags is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Wall construction for insulation support WillK Building & Construction 3 12-09-2010 01:55 PM
Hybrid wall insulation consisting of 2" rigid foam + 2x4 fiberglass batts? TitaniumVT Building & Construction 15 09-14-2010 02:50 AM
Foam insulation wall cavity soundproofing... CGehlhausen Building & Construction 3 09-05-2009 03:20 PM
Wall - LOAD BEARING: How to Move? user49172 Building & Construction 1 03-11-2009 04:48 PM
rigid foam insulation in walls gossamer Building & Construction 12 02-03-2009 09:26 PM




Top of Page | View New Posts

Copyright © 2003-2014 Escalate Media. All Rights Reserved.