# Hybrid insulation for solid-wood walls



## Windows on Wash (Aug 30, 2011)

Where is the home...?


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## manbitesfilm (Jan 24, 2012)

Ooops... I accidentally edited that out... Home is in Montreal.


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## Windows on Wash (Aug 30, 2011)

Do you care about loosing a little footprint of the space by putting rigid foam to the interior side of the studs?

That will net you the best performance and you will only loose about 2" of wall space on 4 sides.


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## manbitesfilm (Jan 24, 2012)

Hmm... You're saying to put the studs against the stacked solid wood (with the rock wool) THEN put the EPS (faced with a vapor barrier while I'm at it I presume...) then drywall?

I would think that a) my Roxul would be less effective given the drafts through the wood wall and b) I have to go through the hassle of cutting out all the rigid foam for electrical boxes, further reducing it's efficiency... Am I over-thinking this?


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## Windows on Wash (Aug 30, 2011)

Roxul is very good, as compared to fiberglass, about resisting wind wash of the R-Value.

Rigid foam the interior would certainly thermally uncouple the wall and make the overall assembly much more effective. Rigid foam, if specified correctly, combines R-Value and vapor retarder in one fell swoop.

If you can do that route, that would be best.


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## AGWhitehouse (Jul 1, 2011)

Haven't had a chance to look into the codes, but I do remember seeing that in climate zone 5 (Connecticut) a continuous layer of R-10 foam placed anywhere in the wall assembly sufficed the vapor retarder requirements. The basis being that with that climate and that R-value the dew point falls within the foam layer. Since the moisture cannot (very minimally) enter the foam, condensation never happens within the system. If this code section can be found for your climate zone, you could place a continuous layer of foam against the outside that would suffice as both an insulation layer and vapor retarder. Then stud and batt like any traditional wall system.


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## manbitesfilm (Jan 24, 2012)

Thanks for all your replies. Unfortunately the iPad app does not quote well, so I'll mash it up...

1st, I'm pretty certain that Montreal is in Zone 6, not 4, so a full vapor barrier under drywall is the code here. I pretty much have the choice between poly or vapor barriers laminated to rigid insulation. Most laminated barriers are also reflective so they require air-space, which is another 3/4 inch. I don't mind loosing SOME space, but I start drawing the lines somewhere...

The note on dew point falling inside R10 insulation is interesting, but again, zone 6, code may be higher, so more space gobbled up on top of the two inches already taken up by R10 (or more). I probably would not use 2" of rigid AND framed wall with batts. It starts getting thick...

I was initially ok with only using Roxul in standard 2x4 framing directly against the solid wall. Until I found additional 2" of space to strip away (in the form of a layer of wood planks and an air space). I figured I could take half of that to beef up the assembly. My intention was to put the rigid insulation (if any) directly against the solid wood wall AND mainly to use as an effective air-barrier WITH +R5 as a bonus. My main concern was that 3.5 perms is not enough for the wall assembly to dry out IF it gets wet. But then again, with a poly barrier under the drywall, there is no reason it should get wet, right? Moisture comes from the warm air inside...

So permeable rigid foam against the solid wood then 2x4 batts finished with poly should be ok? I guess a cheaper variation could be to use 1" strips to break the thermal bridge and 2x3 studs to maintain the same 4" cavity with batts only, if Roxul stands up better to wind washing?... But 2x3 lumber is rarely straight around here... 

The solid wood wall btw looks like it's made from 3x10 old-school lumber.


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## Gary in WA (Mar 11, 2009)

Whoops, sorry about that!

I'll answer later.......

Gary


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## AGWhitehouse (Jul 1, 2011)

manbitesfilm said:


> My main concern was that 3.5 perms is not enough for the wall assembly to dry out IF it gets wet. But then again, with a poly barrier under the drywall, there is no reason it should get wet, right? Moisture comes from the warm air inside...
> 
> So permeable rigid foam against the solid wood then 2x4 batts finished with poly should be ok? I guess a cheaper variation could be to use 1" strips to break the thermal bridge and 2x3 studs to maintain the same 4" cavity with batts only, if Roxul stands up better to wind washing?... But 2x3 lumber is rarely straight around here...
> 
> The solid wood wall btw looks like it's made from 3x10 old-school lumber.


A class I interior vapor retarder and a class III exterior vapor retarder should function fine. If you were using a class II (0.1-1.0 perms), then you would want to bump up the perm rating on the exterior. As a point of reference, Massachussetts code reads that the exterior vapor retarder should be no less than 10x the permeability of the interior. So with poly at a <0.1perms, you'd want the exterior to be no less than 1.0 perms within climate zone 5 (Mass.). You're looking at 35x the permeability, which considering you have a 6 climate zone, you should be o.k. But, of course, check with your local codes before you take that for gospel.

*The cheapest variation I can think of* off hand would be to put up an air barrier (tyvek or equal) over the whole existing wood wall. Gasketed fasteners and tape all seams and edges. Strip 2" insulation and use as thermal breaking spacers behind the studs. 2" because it will lend to a bay depth allowing for a 6" batt. Then fill the bays with the 6" batt. ensuring snug but uncompressed fit. poly vapor barrier. sheetrock, tape, paint, trim, drink beer.

*The energy efficient variation I can think of* is to run that rigid foam continuous right behind the sheetrock like WOW said. The minimal amount of foam you'll have to penetrate for electrical boxes will be gained back multiple fold in improved thermal efficiency of the system. To ensure you minimize the losses, you can air seal the boxes and provide the foam switch/plug plate backers.


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## Windows on Wash (Aug 30, 2011)

manbitesfilm said:


> Thanks for all your replies. Unfortunately the iPad app does not quote well, so I'll mash it up...
> 
> 1st, I'm pretty certain that Montreal is in Zone 6, not 4, so a full vapor barrier under drywall is the code here. I pretty much have the choice between poly or vapor barriers laminated to rigid insulation. Most laminated barriers are also reflective so they require air-space, which is another 3/4 inch. I don't mind loosing SOME space, but I start drawing the lines somewhere...
> 
> ...


See above.


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## manbitesfilm (Jan 24, 2012)

Good stuff gentlemen, good stuff... I didn't know my consumption of beer had insulating properties on the house! 

Ok, so last two clarifications, if you can indulge me: 

1. Thermal uncoupling of the framed wall is best done on the warm side? You uncouple the warm sheetrock from the (progressively) cooler studs mounted against the cold outside wall, correct? This is more efficient than uncoupling on the cold side?

2. When you say 2" of rigid "should do it", you mean using JUST rigid? That's only R10, R14 maybe if it has a radiant barrier and an air-space pointing inward... And then I have to dick around with holes for electrical, take reference pictures of my furings and wires before closing up, etc... 

I guess rigid would be the trade off for speed of installation vs cost. (2" rigid is still relatively expensive per sq.ft...)


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## AGWhitehouse (Jul 1, 2011)

If you are doing rigid sheathing, the most efficient is at the interior side.

If you're doing rigid foam blocks at the studs, I don't believe you'll change the overall wall's effective R-value much by placing the blocks at the interior or exterior. I'm sure there is a scientific method that says it's more efficient in one location over the other, but I believe you're really only talking minor differences, so ease of installation becomes the real driving factor. I would think, in your case, it would be easier to place the rigid blocks at the exterior to allow for easier gwb, vapor retarder, and electrical box installations.


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## Gary in WA (Mar 11, 2009)

I’m back, barely, home, sick. Anyway, you do have 4575 HDD in Celsius not Fahrenheit, big difference. http://gocanada.about.com/od/canadatravelplanner/tp/temperatures_canada.htm
To change to mine from yours; 9/5 =1.8 x 18c =32.4 70/32.4 = 2.16 x 4575 = 9884 HDD in U.S. Zone 7 as brought out here, much easier-LOL: http://www.buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design
Fig. 5a is what you have now, Fig. 5b is ideal—similar to AGW said. But the foam is outside keeping the cavity warm = no condensation on the solid wood. If you add foam on the studs inside, the cavity will be colder = more condensation.
Notice the “Delta T” = temp. of condensation on sheathing, graph bar just below each wall make-up, and the temps.

Figuring the condensing temp.; read the beginning of article. Here is an easier way using a Pysch. Chart or click on the link to figure your existing; http://www.greenbuildingadvisor.com/blogs/dept/musings/are-dew-point-calculations-really-necessary
So you raise or lower the dew point by where and how much insulation is added. 

Notice the R-10, AGW said, works for your zone, at the end of article. Remember – that is without a vapor barrier, though you are required to use one.
Read these if possible; http://www.buildingscience.com/docu...america-special-research-project-high-r-walls

http://www.buildingscience.com/documents/reports/rr-0412-insulations-sheathings-and-vapor-retarders

Gary


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## Windows on Wash (Aug 30, 2011)

manbitesfilm said:


> Good stuff gentlemen, good stuff... I didn't know my consumption of beer had insulating properties on the house!
> 
> Ok, so last two clarifications, if you can indulge me:
> 
> ...


You are right. It will be more of a pain in the butt and more expensive but doing it the best way (at least on paper) usually is.

Going conventional can still net you 95% of the desired effectiveness if you get your air barrier and vapor retarder done right.


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## manbitesfilm (Jan 24, 2012)

WoW, I already read through a lot of Building Science website. Amazing stuff! But I had a hard time finding info on solid-wood walls. And a lot of their stuff is pro-sprayed foam, for obvious reasons, but a path I would still like to avoid... I'll check the links a little later.

Thank you both for your input and clarification. Much appreciated. I may actually mix techniques... Use all rigid for a stairwell and bathroom for example (where I'm tighter for room and there are no plugs) and hybrid for the rest.

Thanks again!


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