Doubling up exterior walls during house remodel

[fubarcamry]

We are remodeling our house. I've done some projects over the years, and we have a contractor friend giving us some advice on things that come up during this project, but I am having trouble getting an answer on this, and looking for some thoughts from you guys. Here's some background on the project.

Our house is relatively old (1950's maybe), exterior walls have aluminum siding. It was remodeled by the previous owner about 12 years ago with all new batt insulation in the walls, new wiring, drywall etc. The house is 20x30. The ceiling joists run parallel to the ridge, and there are supporting walls every 10 feet, breaking the 30' long house into thirds. There are no rafter ties, so the 30' exterior walls are beginning to spread apart. Due to the condition of rafters and roof sheathing, our desire to move supporting walls, increase eave overhang, straighten up the exterior walls that are spreading, and the small size of our house, we've decided to rip off the entire roof and replace with trusses. So everything will be straight(er) and solid when it's done. We have removed the ceiling in the whole house, many interior walls ripped out, and trusses are on order. It seems extreme not to move but we really like the location and plan to stay, and enjoy the project.

My question is in regards to my strategy for doubling up the walls...
The exterior walls do not have a doubled-up cap on the top, so I plan to add another 2x cap on top of the walls before putting on the new trusses. I am considering doubling up the existing exterior walls from the inside for bad-ass insulation, extra support and load bearing for the roof trusses, and deep window sills for our cat and house plants :). I know the extra load bearing is basically unnecessary, but my thoughts are that it would be a nice benefit if I did it for extra insulation anyway, and the walls are old so it wouldn't hurt to have it. I plan to use a 2x8, 2x10, or whatever I need for the extra top cap to bridge over the top of the current existing wall and the 2nd wall I'm going to add. Then I can build my 2nd wall under that and nail from below, so the walls would be tied together nicely.

My main question is, should I leave the existing drywall and build over it, or could that create moisture problems within the wall? It's an unusual situation, I think, so I am not finding much on it. I've done some reading on moisture barriers within walls. I guess I am leaning towards removing it. Our house is in Ohio, so we have humid summers. What do you guys think about the drywall, or any other thoughts on the plan?

[Papa bear]

Massive project.  I’ve done some crazy stuff on remodels before but nothing to that extent. 

If you are set on building a new wall inside, I would definitely remove the drywall. I’m not an engineer, but I think I’ve read that painted drywall does act as a low quality vapor barrier, which you would definitely want removed.

If this was my house though?  I would tear off the aluminum siding and add exterior insulation to the house.  You would not lose interior space, you can air and water seal from the outside on your sheathing, and the insulation removes the thermal bridging you would have from the stud walls and headers.  I would look at 2-4” of xps foam board, staggered seems. That would give you an additional r-10 to r-20 over the entirety of the house.  See the perfect wall here: https://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall

You can also look at matt risinger on his YouTube channel, the build show. He has a lot of projects like this.  While he is in the south, he does have walkthroughs of projects in cold climates like Ohio.


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[NaN]

@fubarcamry, this might be a dumb question. If you are tearing off all the roof completely to rebuild it with trusses and are considering adding exterior walls, why not tear down the walls? I mean, that's a pretty serious framing retrofit. At this point it sounds like you should just build a new house.

Have you had this plan approved by a structural engineer? It is possible the weight of the new trusses can't be supported by the old 2x4 walls. Adding framing inside the old exterior walls may not be a great idea if the foundation wall is only built up right underneath the old walls. You would have the new load bearing interior walls sitting on the thinner slab section.

[Papa bear]

@fubarcamry, this might be a dumb question. If you are tearing off all the roof completely to rebuild it with trusses and are considering adding exterior walls, why not tear down the walls? I mean, that's a pretty serious framing retrofit. At this point it sounds like you should just build a new house.

Have you had this plan approved by a structural engineer? It is possible the weight of the new trusses can't be supported by the old 2x4 walls. Adding framing inside the old exterior walls may not be a great idea if the foundation wall is only built up right underneath the old walls. You would have the new load bearing interior walls sitting on the thinner slab section.

I’m assuming he still has all mechanicals, windows, sheathing, fixtures, cabinets, etc in there if there’s still drywall.  That’s a much bigger undertaking to tear down and start over. 

Also, 2x4 walls are already pretty overkill to handle the compression loads of a roof.  One 2x4 vertical can handle 3000 pounds. So I wouldn’t worry about the walls at all.


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[Cadman]

I'd be more concerned about loss of interior space. Of course the electrical would need to moved to the inner walls to reach switches, recepts, etc. and at that point, it's cheaper to re-wire from scratch. Can you do the electrical yourself in your area, or would you need to hire it out? What about perimeter floor vents? Then there's still thermal bridging at the top and bottom plates.

I think if you run the numbers just on the materials, you'll find the ROI is likely beyond your lifetime. If I were in your shoes I'd put that money towards a more efficient HVAC system (geothermal) or incorporate solar since you've got a new roof going on.

[bacchi]

Massive project.  I’ve done some crazy stuff on remodels before but nothing to that extent. 

If you are set on building a new wall inside, I would definitely remove the drywall. I’m not an engineer, but I think I’ve read that painted drywall does act as a low quality vapor barrier, which you would definitely want removed.

If this was my house though?  I would tear off the aluminum siding and add exterior insulation to the house.  You would not lose interior space, you can air and water seal from the outside on your sheathing, and the insulation removes the thermal bridging you would have from the stud walls and headers.  I would look at 2-4” of xps foam board, staggered seems. That would give you an additional r-10 to r-20 over the entirety of the house.  See the perfect wall here: https://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall

You can also look at matt risinger on his YouTube channel, the build show. He has a lot of projects like this.  While he is in the south, he does have walkthroughs of projects in cold climates like Ohio.

+1 This is a much better idea than doubling up the walls.

Some builders do a double-stud build, like so: https://foursevenfive.com/blog/the-double-stud-wall-simplified-low-cost-high-performance/

But your service cavity is the current wall, which would be the "exterior" wall. You'll need to extend pipes and wires and vents if you add an interior wall. That's a shit-ton of work. If you just rip off the siding and add rigid foam, your newish drywall and fixtures can remain in place.

See https://www.greenbuildingadvisor.com/question/eps-xps-or-polyiso-for-exterior-rigid-foam for more info. Note the typo for polyiso in the question -- it's R13 and not R31.

[Frugal Lizard]

We have been systematically doubling up our exterior walls.  We have just left the plaster walls and built new 2x4 walls with roxul insulation.  We spray foam the old electrical and tuck tape around the windows so the air sealing is super at the old plaster wall layer and there is an air gap between the plaster and the new stud wall. 

When we did the first room in 2016 we added a vapour barrier.  Since then, no vapour barrier because plaster is pretty air tight.  I don't know specifically where we sourced this wall assembly but the DH did extensive amounts of research.  We also had an energy/air quality engineering company prepare us a plan to get to net zero.  They confirmed the wall section and advised against using a second vapour barrier.  Our house is well below air changes per hour of Ontario Building Code, but from what I have been told, that is not such an easy bar to hit. 

Our house is huge so we aren't worrying about the loss of floor space.  But we are super enjoying how much more comfortable the house is getting as each room gets done.

Good luck on your project.

[BudgetSlasher]

If this was my house though?  I would tear off the aluminum siding and add exterior insulation to the house.  You would not lose interior space, you can air and water seal from the outside on your sheathing, and the insulation removes the thermal bridging you would have from the stud walls and headers.  I would look at 2-4” of xps foam board, staggered seems. That would give you an additional r-10 to r-20 over the entirety of the house.  See the perfect wall here: https://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall

I love this concept in a new build (and may do it if I build our next house). But, doing it in an old house ... one would need to redo all the widow/door bucks. In a new construction you can build a PT extension around the opening to make the window flush with the final insulation.

I would also be worried about diminishing the amount of overhang of the eves (shorter eves could mean more rot in the splash zone).

[Papa bear]

If this was my house though?  I would tear off the aluminum siding and add exterior insulation to the house.  You would not lose interior space, you can air and water seal from the outside on your sheathing, and the insulation removes the thermal bridging you would have from the stud walls and headers.  I would look at 2-4” of xps foam board, staggered seems. That would give you an additional r-10 to r-20 over the entirety of the house.  See the perfect wall here: https://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall

I love this concept in a new build (and may do it if I build our next house). But, doing it in an old house ... one would need to redo all the widow/door bucks. In a new construction you can build a PT extension around the opening to make the window flush with the final insulation.

I would also be worried about diminishing the amount of overhang of the eves (shorter eves could mean more rot in the splash zone).

Agree. You would have some finishing fun, but could just do jamb extensions and flash them appropriately.  That would leave a bigger sil outside rather than inside.  Or OP could take out the windows and reinstall.

If the 2-4” is a problem, even just 1” of xps foam at r-5 would bring the house up to current code for new construction in Ohio (either r-20 in wall cavity, or r-13 + r-5 continuous exterior layer). Air and water sealing would be the biggest gain in this. 


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[Dicey]

Posting so I can find this later.

[Jon Bon]

Holy crap, sounds like are trying to reinvent the wheel here?! Or is this just going to be the worlds most expensive retrofit?

I guess I am just not grasping the overall objective here. Energy efficieny? Increase structural strength? I think there are much easier ways to accomplish both of these things. You house is 70 years old, I don't know the condition of your roof, but it feels like some foam board and rafter ties would be 100x cheaper and just as effective?

[zolotiyeruki]

Deep window sills are nice (I have some in our newly-finished basement). However, if the primary goal is improving energy efficiency, and you're already willing to rip out the drywall, then I'd suggest a different alternative:  rip out the drywall and existing insulation, and replace with spray foam.  In a house that old, air infiltration is going to be a huge contributor to energy costs.  Building a second wall inside the first, and then making sure every gap and seam gets properly sealed, is going to be a huge amount of effort/time/expense.  Spray foam is expensive, but it may be cheaper/easier/faster/more effective than the exterior insulation approach or the double wall approach.

A cheaper alternative to spray foam is wet-spray cellulose.  The installation process is fairly similar, but instead of spray foam, the installers spray in cellulose that has an adhesive added.  It's not quite as air tight as spray foam, but it's still great, and it has all the other nice benefits--great insulation, conforms around everything in the wall, installs quickly and seals pretty well, and it's less expensive and more environmentally friendly than spray foam. (video here: https://www.youtube.com/watch?v=PO_LGJ-176E)

One last option:  dense-pack cellulose.  In this one, the installers cut a hole near the top or bottom of each stud cavity, and pack each stud bay tightly with cellulose.  Since it's not wet-sprayed, it doesn't air seal quite as well as the sprayed in cellulose, but it still does a pretty good job of retarding air flow, fills in nooks and crannies, gets very good insulation levels, and is cheaper.  If you're ripping off the roof anyway, you could drill holes in the top plates (rather than your drywall) in order to blow in the insulation. (video here: https://www.youtube.com/watch?v=ce-mM7R11Lo)

If it were me, I'd pick one of the last two options.

[lthenderson]

Like others, structure isn't an issue so the only reason to do this would be to increase efficiency. I would do a calculation of how much you would spend in time and materials to do everything needed to build the second wall through the finish stages and then assume a percentage efficiency gain of say 25% and figure the payback. 25% might be way overly optimistic since you will still have the biggest sources of heat loss unchanged, (i.e. windows, doors, etc.) My guess is it might be decades if not more to recoup your investment. Are you really sure you will be living there that long?

Other things to think about. Almost anything in the outer wall now will require custom work to make work with an additional wall. Any electrical outlets, switches, etc. unless whomever wired it the first time around left generous amounts of extra wire looped in the cavity will mean pulling lots of new wire. Any door or window will require custom work to extend jams and such. Any plumbing in the wall will need to be moved to the new cavity to be closer to the warm side of the house. Things like toilet drain lines and such are a set distance from any wall so if you have stuff like that on an outside wall now, that will have to be moved. Any thing like ducting in the wall will require metal work. If it is in the floors, often times the registers are close to the existing walls meaning you will have to shorten all those runs to properly position them.  What about existing flooring? Are you tearing it up or building the new walls on top of it? If the latter, it could create problems if say it is hardwood and now it can shrink and expand properly. It also creates a headache if you want to remove existing floor later and put in a new surface. Any thing that penetrates the surface like exhaust vents, etc are generally designed for 2 x 4 or  2 x 6 walls. Your probably won't be able to buy anything off the shelf to use and will have to modify everything to fit your new wall thickness.

With all these potential headaches and the knowing it might be a long payback, it might be cheaper and easier to just insulate from the outside and reside the house to accomplish the same goal.

[NaN]

Gah, the OP said " There are no rafter ties, so the 30' exterior walls are beginning to spread apart". It sounds to me the structure was not fine to begin with and the original motivation here was to improve the structural condition. For all the love of unholy and holy construction spirits, can I get a second on recommending a certified structural engineer to sign off on this? If not for the OP, for the next poor soul that comes to the house later?

Just saying "oh, a 2x4 wall can take 3000 vertical lbs" ignores all the other direction vectors. Popsicle sticks carry a lot of compression, too. It is the complete system, from foundation to roof, and every connection point withstanding snow, wind, and temperature gradients.

My original point on just building new walls was based on the well articulated point by @lthenderson with how much work building any new interior walls would be.

[nereo]

. For all the love of unholy and holy construction spirits, can I get a second on recommending a certified structural engineer to sign off on this? If not for the OP, for the next poor soul that comes to the house later?

I second!

We're doing out own remodel of a c. 1900 home, with a primary goal of improving energy efficiency, but also to improve DECADES of shoddy workmanship and ad-hoc additions.

A few thoughts (in no particular order)
1) consult a structural engineer for your structural needs.  No need to go beyond 2x4 for structural reasons.
2) two layers of exterior insulation is your best bet for making the walls more energy efficient, but requires removing the cladding and extending the window jams.
3) spray foam is a miraculous alternative for all the reasons zolotiyeruki mentioned
4) insulate the hell out of hte attic with blown-cellulouse once the roof is fixed.  R-60 or higher.
5) air seal, air seal, air seal.
6) don't re-invent the wheel.  Lots of people have taken old drafty buildings and fixed them up. 

I'd polke around green building advisor (someone linked it upthread).  Requires a subscription, but more than worth it if you are doing something so extensive.  TONS of really great advice on there.

[Papa bear]

Gah, the OP said " There are no rafter ties, so the 30' exterior walls are beginning to spread apart". It sounds to me the structure was not fine to begin with and the original motivation here was to improve the structural condition. For all the love of unholy and holy construction spirits, can I get a second on recommending a certified structural engineer to sign off on this? If not for the OP, for the next poor soul that comes to the house later?

Just saying "oh, a 2x4 wall can take 3000 vertical lbs" ignores all the other direction vectors. Popsicle sticks carry a lot of compression, too. It is the complete system, from foundation to roof, and every connection point withstanding snow, wind, and temperature gradients.

My original point on just building new walls was based on the well articulated point by @lthenderson with how much work building any new interior walls would be.

That’s fair. I thought you were suggesting upthread that a 2x4 wall may not be able to handle the trusses, not that this particular house may have additional issues.  If the walls are out of plumb and square, it may take additional work to get that back. 

In any case, we’re taking about a 20x30 building, it’s basically a 3 car garage.  Figuring out how to build this properly, or fix it properly, isn’t rocket surgery.  Basic framing experience should be more than enough knowledge here.  Sounds like the original builder had no idea how to build a roof which caused the other issues. 


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[BudgetSlasher]

Deep window sills are nice (I have some in our newly-finished basement). However, if the primary goal is improving energy efficiency, and you're already willing to rip out the drywall, then I'd suggest a different alternative:  rip out the drywall and existing insulation, and replace with spray foam.  In a house that old, air infiltration is going to be a huge contributor to energy costs.  Building a second wall inside the first, and then making sure every gap and seam gets properly sealed, is going to be a huge amount of effort/time/expense.  Spray foam is expensive, but it may be cheaper/easier/faster/more effective than the exterior insulation approach or the double wall approach.

You have to be careful with spray foam in older homes. That air movement through the wall you mentioned also acts as a drying method for any water that get into the wall. Things like small flashing leaks, splash back from the drip line, ect that have been drying perfectly for decades suddenly stop drying ... and "if it can't dry it will die" rot can soon follow.

From what I have read, double stud wall (with studs offset) also has the added benefit of reducing noise transmission.

[fubarcamry]

Guys, Thanks for all the awesome input! I didn't expect so much so quickly.

I thought my original post was getting long, so I omitted some details I thought might distract from the question. Here is some more detail that I see now is relevant to some of the comments and questions you brought up.

In regards to the overall main objective of the project, it may have been unclear in the original post, since I was focusing on the wall detail only. What prompted the decision to remodel was a desire for a house layout that faces south, which also happens to have the best view at our place. An increase in efficiency would be a nice benefit, but is not the main objective. I am actually somewhat surprised at how efficient our house already was with R13 walls and R19 attic, but it's also small. Part of the remodel is an addition. The house will go from 20x30 to 20x50. My original plan was to gut the ceilings and interior walls (done), try to pull the bowing walls in, and add rafter ties. I also would have had to sort of build the rafters into trusses in-place in order to be able to remove some load bearing walls to change the layout. Once gutted, I got a better look at the condition of the rafters and sheathing and decided to go with new trusses over the existing part of the house, and then they can continue nice and straight into the addition with the same roof line. This would also make it easier to straighten the walls. And to give an idea, when I say the walls are bowed out, in the worst spot, they are bowed out about 1.25" per side, so about 2.5" from side to side. The decision to replace the entire roof structure only adds about $3,000 of materials cost to the project since the roofing needs replaced anyway.

I did not have a structural engineer involved, but I do have a BSME, and have some decent experience with buildings and additions, but nowhere near on par with the pros. I have a builder/carpenter/woodworker/licensed plumber/etc mentor with 30 years experience who has been mentoring me through some of this, and I do have a lot of confidence in his advise. That being said, from a structural standpoint, once we get the roof off and walls straight, there's not much funny business going on, just 20' span trusses designed for 24" on center for the loads in our area, sitting on the old 2x4 walls. As Papa Bear said, it's basically a 3 car garage. However, it does have a full basement with block walls, so the exterior walls are sitting on the subfloor, sitting on the joists, etc. Doubled walls would also sit on the subfloor.

If I still choose to go with a doubled wall, my intention is that the new innermost walls would be the service cavities. There is electric but no plumbing or venting in the exterior walls, and since the house layout is changing, I'll be reworking most of the electric anyway. In regards to the complexity of adding interior walls, my house is pretty darn basic. The walls in question would have a total of 2 doors and 4 windows to work around, and electrical to add.

In our area, we can do all our own plumbing and electrical. You just have to pay your $20 for a building permit, and you're ready to rock. And that's inside the city limit, where I am. Outside the city limit, seems like just about the only thing you need a permit for is a septic system. This sounds crazy if you don't live in an area like this. It's nice for DIY'ers that do things properly. It's bad in that you end up with some junky places scattered about.

I agree that strictly from an investment standpoint, this is not an optimal investment. However, the house will be worth more than the total of original purchase price and materials for the renovation, which all added up is still way less than your average house. If you include my labor, that's probably another story, but the challenge and learning is fun too. We hope to end up with a place we really like, and it's all paid for. I'm entering a phase where I still can't help but consider if something is an optimal investment, but in occasional cases, like this one, we decide the trade-off is worth being less-than-optimal. I think I am getting relatively close to semi-retirement, but not sure when I'll really be confident enough to draw the line. I guess you could say my wife has been for about a year, from managing a corporate store to working PT as a bartender at a local brewery. It's all good.

Thanks so much for the help so far!

[jpdx]

At what point do you just build a new house?

I wouldn't go for the double wall trick. Make the existing structure sound and well-sealed, then focus your attention and money on the things you can see and touch.

[Papa bear]

I see no reason you need to have a 2x4 service cavity for electrical.  If you needed to run ducting up, I can understand building a chase, but that wouldn’t need to be the entire perimeter of the house.

If you are concerned about air infiltration with the electrical, spray foam around and behind all of your junction boxes after you’re roughed in. 

To the post above about adding closed cell spray foam and not letting the house breathe - I’ve seen adding baffles on the inside of the exterior sheathing and then spray foaming over that. It creates an air cavity to allow the structure to dry from the outside. 


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