# Hybrid wall insulation consisting of 2" rigid foam + 2x4 fiberglass batts?



## TitaniumVT (Nov 27, 2009)

I was flipping through some of my back issues of Fine Homebuilding, and reading different articles about spray foam, convection currents in wall cavities (worse with fiberglass insulation; non-existant with spray foam), and one about using 2" rigid foam instead of OSB sheathing to improve insulation performance. Which got me to thinking...

I want to build a guest house on my property next spring, and am debating what type of insulation to use. Spray foam performs the best, but is expensive, and becomes a permanent part of the structure. The former is not a big deal, but the latter concerns me because of the long history of finding out years later that certain building materials are defective and a health hazard (e.g., asbestos, urea formaldeyhde foam, mold in open-cell foam, etc). With a spray foamed house, the structure is basically a loss if you have to remove the foam later for health reasons.

All the insulation types seem to have downsides:

- Spray foam performs well, but is messy, needs to be professionally installed, and is next to impossible to remove easily once it's in the wall cavity. 

- Using 2" rigid foam instead of OSB on the outside of the framing is suboptimal because of noise transmission and the fact that it doesn't strengthen the framing. OSB or plywood sheathing not only provide a surface for exterior cladding, but add to the rigidity of the shell and cut noise transmission between the interior/exterior.

- Just using fiberglass batts gives you flexibility for abatement later, but is a poor insulant and the worst solution for convection currents that siphon away heat



*My question:* Could a hybrid solution be used that balances out the pros and cons of each? Here's what I have in mind:
Conventional 2x6 framing and OSB sheathing on the outside (adds rigidity, cuts noise transmission, provides a solid cladding surface)
Use 2" rigid foam (vapor permeable) on the inside wall cavities, right against the OSB sheathing (performs better than fiberglass batts, and cuts down on convection currents and heat loss)
Fill the rest of the cavity with 2 x 4 fiberglass batts (adds to the R-value and provides flexibility if I ever need to get into the wall cavities again)
Is there any reason this couldn't work? I never hear of anyone promoting this approach, and wonder why it's not commonly done. Thanks for any insight!


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## Tom Struble (Dec 29, 2008)

you still have thermal conductivity of the studs


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## High Gear (Nov 30, 2009)

S.i.p. ??


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## TitaniumVT (Nov 27, 2009)

tomstruble said:


> you still have thermal conductivity of the studs


That's a very fair point, Tom. Conductivity through the studs wouldn't be a problem with the "rigid foam as outside sheathing" approach, but it would still be a problem with spray foam or traditional fiberglass bats. I don't want to go with SIPs mostly because I'd like to build a conventionally framed structure.

Beyond thermal conductivity of the studs, can anyone think of any issues with the approach I highlighted above? Why shouldn't I do it this way?


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## TitaniumVT (Nov 27, 2009)

One other thought...if I were to try to address Tom's point, what about a variation of the original idea:

- Put 1" or 2" of rigid foam on the _outside_ of the framing (addresses thermal conductivity through the studs)

- cover the foam with OSB sheathing (adds a solid cladding surface, increases the strength of the framing, and cuts noise)

- fill the inside bays with 2x6 fiberglass batts

It would be more of a pain to nail/screw the OSB sheathing into the studs, but it should effectively address the conductivity and convection issues. Plus, the added benefit is that the R value of the wall would be higher, since now I'd be using 2x6 fiberglass batts _and_ 1" to 2" of rigid foam.


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## Tom Struble (Dec 29, 2008)

you can keep adding but there comes a point of the returns not equaling the investment,there are actually other ''more important'' areas that also need to be dealt with,from roof insulation to windows and doors


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## High Gear (Nov 30, 2009)

BIB ( blown in blanket ( fiberglass)) will get you a true R23 on a 2x6 or a R15 on a 2x4 wall and is no longer uncommon around here for walls.

Energy truss, triple panes ect. , make your guests to comfortable and they 

may never leave. ( yuk yuk )


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## TitaniumVT (Nov 27, 2009)

Thanks for all the good suggestions guys. Will blown in fiberglass settle over time? A common problem with blown in cellulose is that it settles over the years, leaving a large air pocket at the top of the bays, which siphons out heat.

Looking into things a little further this afternoon, I discovered "structural insulated sheathing". It's a relatively new class of products in which a thin structural layer (OSB-like) is bonded to foam and a water impermeable face. DOW's Styrofoam SIS is an example. http://building.dow.com/na/en/products/insulation/sis.htm

Wonder if this would meet my needs? It combines the structural properties of OSB sheathing, with the thermal break of rigid foam, and it had a waterproof outer shell which eliminates the need for housewrap. The only thing I'm not sure of are the noise insulating properties of this type of sheathing.

Thanks all for your ideas!


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## Red Squirrel (Jun 29, 2009)

If I was building a house, and provided it would not violate any kind of code (can't see why) I'd make the outside walls double 2x4 studded walls, with a seperation between the 2x4's. Basically, two walls very close to each other, maybe equivalent to like a 2x12 in thickness. Fill that up with batts of insulation, maybe some foam too for the first wall section. Though I don't know if this would be like having two vapor barriers since there is also plastic that would go over the wall before the drywall. 

Also the roof type has a certain effect on insulation. For example my house has a roof where the overhang is lower then the ceiling. The slope of the roof creates a "dead zone" where there is little to no insulation at the very end of the ceiling. I would love to have one of those heat cams to check that out, bet it is a major heat loss, and probably an issue for any house with a roof like mine. 

I would personally avoid something that is very new and not yet widely used. It might be the best thing to come out in construction history, or it might turn out to be a flop, and you'll only know in 10 years from now, kinda like Chinese drywall.


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## steveel (Sep 10, 2010)

This is a guest house in VT, right? My first thought was "strawbale construction" because I know some folks that did that up your way. 

What I mainly wanted to ask is how much of the year will it be used? Will the temp be turned way down a lot? If so, I'm thinking it would be easy to spend more on super-efficiency than you will ever get back on dollar savings or comfort. Not sure where the point of dimishing returns may be. Certainly, sealing against stack effect is high on the list, as is good weatherstripping for doors/windows. 

Have you considered the cladding you'll use? What do the specs say about substrate for fasteners?

SteveEl


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## Gary in WA (Mar 11, 2009)

I understand why you are concentrating on the walls – where 35% of heat loss occurs: http://www.explainthatstuff.com/heatinsulation.html

SPF is the Cadillac, although it depends on the mode of application with spray foam, some lose +30% of R-value over time: http://www.kwikbuildpanels.com/pages/article_facts.html

That SIS looks good, is it OSB backed? I didn’t wait for the video, but read two infos there. Will it crush under the 3”o.c. nailing at the edges? Because of only 20#psi compression and the nailing will be within 3/8” of the edge at the joints to break on 1-1/2” sleeper. Be sure to use ply on the wall for shear rather than inferior metal braces or 1x4 which have 1/5 the shear value and less. It would be easier to sheath the corners with ply, every “X” number of running wall or the house size by configuration, as required, with ½” foam over those and 1” foam elsewhere: http://bct.eco.umass.edu/publications/by-title/insulating-on-the-outside/ http://www.awc.org/pdf/WFCM_90-B-Guide.pdf
A house wrap is not required when using foam sheathing as long as the seams are taped: http://docs.google.com/viewer?a=v&q...lEbqKY&sig=AHIEtbQYwz6vG-jPF6_Uz5KEzgJVBCTo-g
The flat house wraps, like Tyvek, shouldn’t be used with wood or fiber cement lap siding without a drainage air space because the in-blown (or sun dried-one-side) water sits on the wrap, degrading it: http://docs.google.com/viewer?a=v&q...WO8aAe&sig=AHIEtbTHa3XRIv3GvYAMWrZBbBZoIUXgxw
Much safer to use a drainable house wrap or asphalt felt: http://www.dickseibert.com/martin.pdf

I dislike fiberglass for the loops, fire qualities, health, etc.: http://michaelbluejay.com/electricity/insulation.html It's an excellent filter for the incoming attic air leaking from below and the gaps at wall sheathing and plates, though.


Gary


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## steveel (Sep 10, 2010)

GBR in WA said:


> I understand why you are concentrating on the walls – where 35% of heat loss occurs: http://www.explainthatstuff.com/heatinsulation.html
> 
> (((snip)))
> 
> I dislike fiberglass for the loops, fire qualities, health, etc.: http://michaelbluejay.com/electricity/insulation.html It's an excellent filter for the incoming attic air leaking from below and the gaps at wall sheathing and plates, though.



I assume you're joking about filtering that attic air. This OP is in Vermont. IMO, they should seal attic air leaks against stack effect before worrying about thermal bridging on the walls. Of course if they can do both great, but I bet there is a huge financial difference on the return-on-investment between the two. (Just my guess)

Also I wonder if that 35% number is based just on radiative heat loss? If so it doesn't take stack effect into account at all and could be misleading.

SteveEl


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## Gary in WA (Mar 11, 2009)

Fiberglass is a great filter, look in any wall or attic, yes, that was a late night joke. 

"This OP is in Vermont. IMO, they should seal attic air leaks against stack effect before worrying about thermal bridging on the walls. Of course if they can do both great, but I bet there is a huge financial difference on the return-on-investment between the two. (Just my guess)" ---- This should be directed to the OP, as you already said it once (before my post so I didn't repeat it as you are again), it has nothing to do with my answer.I understand the stack effect but the OP has pin-pointed his questions to walls. http://www.wag-aic.org/1999/WAG_99_baker.pdf

"Also I wonder if that 35% number is based just on radiative heat loss? If so it doesn't take stack effect into account at all and could be misleading." --- Why don't you research that and get back to me..........

TVT, I would use dense-pack cellulose and an exterior foam board---- no convective loops, no settling, with a drainage plane- no thermal or capillary transfer from the siding or the studs. The type of siding would dictate whether or not a rain screen would be recommended. 
There is a cost/value of insulation: http://www.anchorinsulation.com/pdfs/the economic thickness of thermal insulation.pdf

Gary


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## steveel (Sep 10, 2010)

GBR in WA said:


> I understand the stack effect but the OP has pin-pointed his questions to walls.


Pardon me for trying to be helpful in mentioning the bigger picture... maybe the OP understands it and has thought about it, but then again maybe not. And of course there are many newbies and lurkers just reading. 



GBR in WA said:


> Why don't you research that and get back to me..........


I did. My most recent research effort was to read the link you posted. In that article the way they arrived at 35% was not defined yet the author was advocating more insulation, more insulation, more insulation... but without mentioning other important things to do. In my experience, when one is advocating a type of product and doesn't define the numbers they use to support their claims, a healthy skepticism regarding their number is justified. 

My earlier other non-scientific research was to watch my actual gas consumption (CFM) before/after sealing stack effect in two different homes. Both homes had gas hot water (but not cooking). Both times resulted in around 50% reduction in gas consumption. Not bad for 30-50 of caulk and blow foam. 

So on the basis of those two things, I think that 35% number in that article is likely radiative loss from an otherwise properly sealed house, which is a great prelude to advocating folks can't possibly have too much insulation. I sure don't want 20 foot thick walls. So there IS a point of diminishing returns.

Pardon me for helping lurkers and the OP if they don't already know keep the bigger picture in mind.

SteveEl

PS I have a very clean drier duct now, Gary. Thanks again.


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## Gary in WA (Mar 11, 2009)

Steve, I was pointing out you brought up attic air sealing. IF the OP was interested, they usually tell you. Ask him....

Show me another site with a different percentage, I may use that one. In 12 searches, that number comes up. It may be a government published scientific finding, could you look? I'll keep looking too. Research is great to prove a point other than experiences. Though they help understanding. Example: attics have to be vented no matter where you live: http://www.fpl.fs.fed.us/documnts/pdf1999/tenwo99a.pdf

Venting attics is the best way to cool asphalt shingles: http://www.fpl.fs.fed.us/documnts/pdf2002/rose02a.pdf

Gary


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## forresth (Feb 19, 2010)

Red Squirrel said:


> If I was building a house, and provided it would not violate any kind of code (can't see why) I'd make the outside walls double 2x4 studded walls, with a seperation between the 2x4's. Basically, two walls very close to each other, maybe equivalent to like a 2x12 in thickness. Fill that up with batts of insulation, maybe some foam too for the first wall section. Though I don't know if this would be like having two vapor barriers since there is also plastic that would go over the wall before the drywall.
> 
> Also the roof type has a certain effect on insulation. For example my house has a roof where the overhang is lower then the ceiling. The slope of the roof creates a "dead zone" where there is little to no insulation at the very end of the ceiling. I would love to have one of those heat cams to check that out, bet it is a major heat loss, and probably an issue for any house with a roof like mine.
> 
> I would personally avoid something that is very new and not yet widely used. It might be the best thing to come out in construction history, or it might turn out to be a flop, and you'll only know in 10 years from now, kinda like Chinese drywall.


I believe the double wall thing is called an envelope house, but I may be wrong. 
you could do a 2 compartment wall (I'd stagger the studs if you do that to cut back on the stud transmission), and stick the questionable spray foam on the ouside half, and whatever on the inside.
you could always do the ICF thing, and add some foam sheets or whatever to the outside.

you could do an earth home.

there really are a million different ways to get a super insulated home. most of them involve thicker walls.


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