# R20 in 2x4 walls framed inside exterior walls?



## teejus (11 mo ago)

I'm remodeling my bathroom and framing 2 new non-loadbearing walls against exterior walls. I have a brick home that is 130 years old in Colorado (zone 5). The code I've found says I need to insulate framed walls to R20. I'm trying to maximize my space, so I'd like to use 2x4s (which I've already bought).

Firstly, I'm wondering if these walls still require that high of R-value considering they're not true exterior walls. But my house is just brick, some spots with plaster and lathe on the walls.

Secondly, what are my options for insulation with such limited space? I've found 1/2in R3 foam board that I could stack, and spray foam seems like another costly alternative.

Any guidance here is appreciated.


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## Nealtw (Jun 22, 2017)

As it is an old house built with out insulation I would think you would be fine with R 13 in a 2x4 wall.


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## chandler48 (Jun 5, 2017)

With 2x4 framing you will be limited to R13 or R15 (Rockwool). You can't compress insulation as it would reduce the quality of insulation. It must remain lofted.


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## SPS-1 (Oct 21, 2008)

teejus said:


> Firstly, I'm wondering if these walls still require that high of R-value considering they're not true exterior walls. But my house is just brick, some spots with plaster and lathe on the walls.


If the great outdoors are on the other side, thats an exterior wall.

XPS is common and has an R value of R5 per inch, so you would need to space out the finished wall half an inch.
Polyiso is common and has an R value of 6.5 per inch.

Either would be readily avalable in 1.5" or 2" thickness.

Not exactly cheap, but its a bathroom --- how big can the wall possibly be ?


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## teejus (11 mo ago)

SPS-1 said:


> If the great outdoors are on the other side, thats an exterior wall.


Well, technically I'm building false walls that will have brick walls on the other side 

Ideally, I'd be able to use a 1.5 + 2 inch board like this: RMAX Pro Select R-Matte Plus-3, 2 in. x 4 ft. x 8 ft. R-13.1 Foam Insulation Board 637898 - The Home Depot

But the 1.5 isn't in stock within 100 miles of me. That's why I'm leaning towards the .5in R3 stuff, which I'd have to stack 7 layers of per cavity: Owens Corning FOAMULAR 1/2 in. x 4 ft. x 8 ft. R-3 Square Edge Rigid Foam Board Insulation Sheathing 36L - The Home Depot

I'll have about 200sq/ft of wall to cover.



chandler48 said:


> With 2x4 framing you will be limited to R13 or R15 (Rockwool). You can't compress insulation as it would reduce the quality of insulation. It must remain lofted.


I believe my plan to use rigid foam would circumvent this issue, right?

It seems like the rigid foam is an unconventional approach but still valid, and likely the best DIY option for me.


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## SW Dweller (Jan 6, 2021)

The ONLY way to get R20 is either foam sheets or spray foam. 2x4 insulation values were stated previously.

A hint I would use the best insulation you can afford. It is a constant in today world of rising costs. Insulation is usually the cheapest cost vs the energy you will spend over the life time of the home.


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## SLSTech (Jan 19, 2021)

You are reading "new construction" requirements when you are dealing with an existing building / alterations - per the codes all you need to do is "fill" the cavity
N1109.1.1 Exception 2
With that I would install the most R-Value I could - so at least R15 batts or go for the 20
Now not all inspectors read the code the same way so it can get interesting, or they may say once you go past X% everything is considered new but that is something you would have to talk to your local AHJ about.


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## XSleeper (Sep 23, 2007)

On a sort of related topic, there was a post some time ago about a guy who built new interior walls inside of his load bearing exterior walls and he got into trouble with his inspector and they had a big argument about it. I wish I could recall where / whose thread that was, as the whole thing was an argument over insulation and dead air space and supposed mold etc, etc. Everything seemed fine but the inspector had a problem with it. And we all know those type of great area arguments with an authority figure usually lead nowhere. It was an interesting read. Wish I could find it again.


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## SPS-1 (Oct 21, 2008)

teejus said:


> I'm leaning towards the .5in R3 stuff, which I'd have to stack 7 layers of per cavity: Owens Corning FOAMULAR 1/2 in. x 4 ft. x 8 ft. R-3 Square Edge Rigid Foam Board Insulation Sheathing 36L - The Home Depot


I expect that is actually R2.5 and somebody in the marketing department decided to round up. Its XPS, and if you look at related products, the 1" thick board is R5 and the 1.5" thick board is R7.5. Just don't figure you are getting better insulation by buying 3 sheets of half inch instead of 1 sheet of 1.5 inch.


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## Old Thomas (Nov 28, 2019)

You are building new walls so they have to conform to the code for new construction. If you were adding insulation to an existing wall you could get away with whatever will fit. I would use 2x6s, R19 wall insulation, 1/2” GWB which is about R.45 and brick which is around R.5 per layer. Or use rock wool, which is a little higher R value.


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## Elmer-Dallas Texas (9 mo ago)

I like Foamular 250. At 4" it is R20. Maybe you could bridge over the 2x4 with 1/2 polysol and that gives you terrific insulation performance. 
I know the spray in has great R value but sometimes it offgases and some people are allergic to that. I would not use spray especially in a small enclosed space.
I think you can have anything delivered from many online stores. Even HD and others do special orders too.
Reading another thread and question regarding code and whether foam is permissible in certain wall assembly. Better check with inspector as to what they accept.


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## SkyM (7 mo ago)

I want to chip in because I recently posted a similar question here. I have 4" clearance and need to insulate recessed fan under roof plywood with 2x6 joists. So here's what worked and what didn't.

First and foremost, the space only allows for effective use of R-13 fiberglass. Anything thicker will be compressed. Fiberglass retards heat/cold by acting like a sponge, so if it's not proper fit and compressed it will actually act more like a heat blanket in hot weather and freezer in cold.

I tried combination of R-10 2" XPS. Double layer didn't work with my space to give me R-20. But with your walls, it might. I ended up with a single layer of R-10 flush to roof's plywood (non-vented roof) and stuffed fiberglass in remaining 2". It was better than R-13, but once temps started climbing past 90F it was a miserable fail and wasn't worth the effort or the $$.

In-between I tried radiant barrier (bubble wrap stuff) which was sealed off with vapor tape at the joists at 2" gaps. Shockingly it actually worked better than R-13 fiberglass alone and combo of 2" XPS and fiberglass. But it still wasn't enough - fixture was getting very hot (enough to make me worry that it would hit 140F for electric not to be safe).

Anyway, my last and final solution is to be tried out, but seems like the only option for me, so hopefully will be for you: aerogel blanket. I reckon that if this stuff is good enough for Rover on Mars, it's gonna solve the issue. It's already used in builds, but of course it's super expensive and you've got to treat it like you're working with asbestos. Each 10mm sheet has R-4 rating. So you achieve higher R rating by layering the blankets. 

Again, super expensive, but sometimes you don't have many options (like myself). Spaceloft dominates the market in US, but you can always source from China (AliExpress - search for Nano Aerogel Felt Thermal Insulation - they come in as much as 20mm thickness there wholesale).

Good luck. I don't envy your situation.


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