Insulate: Inside or Outside

[Train]

So we're planning a large reno. Virtually every room will see a change in our old 2x4 framed house. Since the drywall is coming off anyway, would you fir out the inside to accommodate R20, or go with 2 inch foam on the exterior, which is going to get new siding anyway?

 

[The Cobbler]

how does adding 2" to the outside affect the windows, doors, soffits etc?

 

[Pluribus]

I'd add exterior insulation in a heartbeat, since you only have 2x4 framing, unless you live somewhere with a really narrow temperature band like Hawaii. Depending on where you live, you might consider going with mineral wool (Comfortboard) which is better if there's any potential fire danger.

Matt Risinger (

) has a great Youtube channel with multiple videos on how to do exterior insulation, including various options for dealing with windows, doors, etc.

 

[Bert_]

Fiberglass in the stud bay then foam sheeting over the studs is what I did. I put horizontal strapping on the wall then cut the foam to fit between. If I were doing it again I would tack up full size sheets and then strap over the top with 1X.

I also taped every seam and filled gaps around outlets and wall penetrations with expanding foam.

 

[DC73]

I'd add exterior insulation in a heartbeat, . . .

I agree. In addition to the Matt Risinger info, I'd visit BuildingScience.com and GreenBuildingAdvisor.com for more information.

DC

 

[matt_i]

If you are going 2", I would consider furring with additional 2x4s out to ~2x8 thickness. Not exact as 3-1/2 * 2 = 7" whereas an "8" is 7-1/4 finished thickness. You could Roxul to R-30 like that.

You'll be able to build up your wall insulation tremendously. But... there are a lot of details to consider...moving out electrical outlets & switch boxes where it would likely require moving them "up" due to the lack of a wire-stretcher Can't bury any electrical boxes so that would be a big change. It may also affect HVAC vents. Definitely door and window frames would require changes as well. Probably a good time to invest in a biscuit-joiner to edge-join 1x stock.

It would be worthwhile to take a look at attic insulation as that could easily become the weakest-link.

 

[Kaizen]

Fir out to 5.5 inches and spend the extra in rockwool if you can. Foam fill every single hole esp in stud bay top and bottom.
Would also add zip sheathing in the outside and tape it well. This will stop all air movement. They do make zip with foam attached but as said all windows and doors will need to be worked on. Not sure if I’d do that unless I was replacing all of them.


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[TurnipTruck]

I just finished gutting my old house to the 2x4 studs. I took out all wiring, too.
In my climate, I would have needed to add 4” of foam outside to keep the sheathing from freezing. So we replaced the R11 with new R15, vapor barriered, horizontal 2x3 straps every 2’, 1-1/2” deep electric boxes and wires inboard of the vapor barrier, and closed cell foam board between the straps. Sheetrock then went on the strapping with zero perforations in the vapor barrier. Sort of a Mooney wall.
We went from 6000cfm to 900 cfm in a 4600 ft sq house.

 

[ard]

both

 

[MushCreek]

Continuous foam panels, either inside or out. Thermal transfer through framing reduces the R-factor of insulation by some 25%. The foam breaks that thermal transfer, and is an easy surface to seal to prevent air infiltration, the other major factor in an efficient building envelope. I don't know enough about it to recommend inside or out. Vapor control comes into play, and you don't want to trap it in the walls.

 

[acer66]

I just tackled my last untouched room in a 1901 farm house.
Framing was 2x4’s 24” ish oc, furred them out to 2x6 size for r21.
I foamed all the gaps I could find and are planning to use zip panels on the exterior, maybe even the one with insulation.

 

[rjacobs]

flash and batt inside.

2" foam outside.

 

[bczygan]

Both

Bill

 

[bullnerd]

Neither. Just re-insulate the 2x4s and move on to the GARAGE!

How did we ever survive with 2x4 walls!

 

[Bretny]

Are you also changing the windows out?

 

[Train]

Lots of (unfortunately, conflicting) opinions. This will be a full gut. New windows, new wiring, even new heating ducts. (Currently the ducts are on inside walls) One thing I hadn't thought of was the exterior foam creating a thermal break of sorts for the studs. The cavity insulation will be replaced also. This house was done once before, very poorly. Can't wait to see what surprises I come across.

 

[MattT]

Lots of (unfortunately, conflicting) opinions.

From folks in lots of different climates. There isn't a one size fits all correct solution for insulation and vapor barriers. So where do you live?

 

[56Safari]

Depending on your local codes you can add 1” or polyiso inside over the cavity insulation then Sheetrock it to meet code.. GAF has installation instructions for doing it this way... It gives you the thermal break, only downside is that it’s a PItA when hanging stuff on the walls after the fact. I did it in one room because I didn’t want to lose my 1948 brick exterior but really needed something better than R13... it made a huge difference... (use adjustable boxes though, pricey but worth every penny.) and

But you could get A pretty solid R value from spray foam, which I would do next time.







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[Train]

Southern Alberta, it gets to -40C occasionally, and we get a lot of wind too. Spray foam here is insanely expensive.

 

[homebuilt burner]

When I built my shop, I use 2x6" for the walls. 1 1/2" of closed cell spray foam and 3 1/2" of fiberglass. The ceiliing is 18" of blown in cellulose.

Our house has 1" of blue foamboard on the outside before siding. Then when remodeling I used 3" of closed cell spray foam inside where we have remodeled.

Both work very well. The shop insulation was much cheaper.

I have been reading a bit about spray silicone chaulk. It looks interesting for a sealing application. Maybe someone know more on that subject. I think it would work good with fiberglass.

 

[Bretny]

Use new construction windows and exterior foam. You get the best of both worlds this way. Furring strips to pad the framing out to the edge of the foam to nail your windows on. Doing it this way makes for less bending of aluminum flashing and less of a chance for leaks.

 

[MoonRise]

Just how crazy do you want to go?

Minimal restrictions on $$ or 'size'? Do both, exterior foam 4" (double 2" panels) of polyiso foam boards on the outside (gives a thermal break over the studs and adds insulation and air tightness, ALL seams taped) and furr the inside walls to change from 4" stud (3-1/2" actual) to 6" wall thickness (5-1/2" actual) to get a nominal ~R20 insulation in the walls.

Pay attention to air infiltration and also drainage planes.

https://www.buildingscience.com/doc...l-assemblies/advanced-frame-wall-construction

https://www.buildinggreen.com/blog/how-much-insulation-enough

https://www.buildingscience.com/document-search?topics[0]=708

https://www.buildingscience.com/sit...rinsulation Retrofit Details and Analysis.pdf