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Old 01-24-2012, 08:24 AM   #1
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Hybrid insulation for solid-wood walls


Howdy knowledgeable folks,

I'm looking for some advice about best approaches to insulating old solid-wood walls. We're talking about a 100-year old two-story house in Montreal, QC, where pretty much everything will be completely stripped from the inside. We have cedar shingles on the outside that I don't want to remove though... Not sure if there is anything between the shingles and the wood wall outside, but probably nothing more than tar-paper or something of that nature. So we're talking insulation from the inside only.

I'm looking for the most reasonable price-performance I can get, without going overboard AND mainly as not to cause problems to the wood structure. I'm 95% sure I won't be using any sprayed foam (since it's expensive and permanent...). What I was thinking is a hybrid approach:

1" of expanded polystyrene (Isolofoam HD, 3.5 perm) up against the sold-wall to provide an air-barrier and to break the thermal bridging, then 2x4 framed wall with Roxul R14 insulation, finished with poly vapor barrier and drywall. Total 5" R19 seems pretty adequate, easy to self-install and the 1" EPS and rockwool are the cheapest per sq.ft I could figure out.

My question is with respect to vapor... Is the EPS gonna cause trouble up against the solid wood? Is 3.5 perm enough to not get condensation and rot or mold in our climate?

Otherwise, what air-barrier would you recommend up against the solid-wood wall (it's obviously far from airtight...) from the INSIDE? Something that is easy to put up and I guess more permeable than EPS?

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Old 01-24-2012, 09:02 AM   #2
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Hybrid insulation for solid-wood walls


Where is the home...?
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Old 01-24-2012, 09:26 AM   #3
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Hybrid insulation for solid-wood walls


Ooops... I accidentally edited that out... Home is in Montreal.
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Old 01-24-2012, 09:30 AM   #4
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Hybrid insulation for solid-wood walls


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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Old 01-24-2012, 09:54 AM   #5
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Hybrid insulation for solid-wood walls


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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Old 01-24-2012, 02:23 PM   #6
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Hybrid insulation for solid-wood walls


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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Old 01-24-2012, 03:24 PM   #7
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Hybrid insulation for solid-wood walls


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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Old 01-24-2012, 08:51 PM   #8
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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... :P

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

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Old 01-24-2012, 09:46 PM   #9
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Hybrid insulation for solid-wood walls


Whoops, sorry about that!

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

Gary
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Old 01-24-2012, 10:29 PM   #10
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Hybrid insulation for solid-wood walls


Quote:
Originally Posted by manbitesfilm View Post
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... :P

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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Old 01-25-2012, 06:46 AM   #11
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Hybrid insulation for solid-wood walls


Quote:
Originally Posted by manbitesfilm View Post
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...

Don't worry about the radiant barrier aspect of insulation. It would only have some application value if that was a cooling degree climate and the foam was facing the air space behind the sheathing or it had and air space towards the inside of the home and was facing that way. I would go the rigid foam route and make sure it is properly seamed, sealed, and attached. Air barrier details in the foam are critical at this point.

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...

2" of rigid should do the trick. That is not that much of square footage loss when you think about it. In a 20x20 room, you only donate about 12 sq/ft in total to a 2" foam wall sacrifice.

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...

3.5 permeability rate should be good enough to allow drying to outside and the goal would obviously be to keep the bulk moisture out of that space as a default. Make sure that air gap is intact and you could use a much higher perm air barrier (i.e. Tyvek...roughly 50) on that outside wall and fill the walls with the Roxul at that point.

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... :P

I prefer the rigid foam to the interior to uncouple the stud assembly. That is the weak link in the comparable walls when you figure comparable air tightness numbers.

The solid wood wall btw looks like it's made from 3x10 old-school lumber.
See above.
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Old 01-25-2012, 09:42 AM   #12
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Hybrid insulation for solid-wood walls


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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Old 01-25-2012, 10:50 AM   #13
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Hybrid insulation for solid-wood walls


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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Old 01-25-2012, 01:16 PM   #14
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Hybrid insulation for solid-wood walls


I’m back, barely, home, sick. Anyway, you do have 4575 HDD in Celsius not Fahrenheit, big difference. http://gocanada.about.com/od/canadat...res_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/docum...nd-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/...ally-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/docum...t-high-r-walls

http://www.buildingscience.com/docum...apor-retarders

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Old 01-25-2012, 04:57 PM   #15
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Quote:
Originally Posted by manbitesfilm View Post
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?

I am with AG on this one in that I don't think it ultimately matters by any magnitude. I would be willing to bet that it depending if you are more heating degree or cooling degree day climate. In your case, probably makes better sense on the warm 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...

Yes, it will be more difficult. The primary benefit of full rigid foam layer in this case is the higher than batt R-value and the combination of the vapor barrier/retarder and the insulation layer into one piece.

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