Adding interior insulated walls

warmwxrules · 02-03-14, 10:24 AM

I have an idea to add interior 2x4 stud walls (basically a double stud wall) with a 2 inch gap filled with R23 mineral wool (24oc). The walls are plaster (painted). No insulation in the exterior stud cavity (2x4) currently-- I would dense pack cellulose from the outside.

Can i leave the painted plaster wall in place? I guess i could rip the currently walls out and just extend the studs and dense pack the whole cavity with cellulose from the inside, but that seems like a lot of work/mess.

I like the idea of mineral wool because i do deal with some road noise (excellent for noise). I would only do the 3 bedrooms to begin with. The entire house (1story ranch) would get the dense pack cellulose.

Thoughts? Anyone ever do this? What about the gap that will be (2 inches) behind the studs...do i fill this somehow?

Daox · 02-03-14, 10:57 AM

I would take out the plaster wall. Id tear it out and then extend the wall like I did with my office, or a mooney wall type setup. There will be much less thermal bridging this way. You can seal things up nice and tight before you button up the wall. You can fix any electrical. There are many benefits, but it is more work of course. Still very much worth it IMO.

If you did keep the internal plaster wall I would definitely dense pack from the inside before putting the new wall in. Why cut holes from the outside if you're already covering up the inside?

AC_Hacker · 02-03-14, 11:26 AM

Quote:

Originally Posted by Daox

...If you did keep the internal plaster wall I would definitely dense pack from the inside before putting the new wall in. Why cut holes from the outside if you're already covering up the inside?

I agree with Daox, once you decide that the plastered surface is not valuable, just create access holes in the plastered surface so you can blow in the cellulose (dense pack).

Leaving the plaster in place will not only reduce the chore of debris removal, but it also keeps the mass of the plaster to help with sound reduction.

Also, since you are building IN, why not go for 2x3 instead of 2x4 and leave a gap between the outer wall structure and the inner wall structure... this would leave you with the same room for insulation, would be a bit cheaper for materials, but most important, it would eliminate acoustic coupling between the outside wall and the inner wall.

Less debris removal problems
Less material cost (slightly)
More sound deadening mass
Same insulation value
No acoustic coupling

-AC

warmwxrules · 02-03-14, 12:13 PM

Good idea on the 2x3... I almost was tempted to rip 2x4s in half after reading that a lot of old homes were (interior) framed with them. There is no weight on these walls, so as long as they are attached firmly, they aren't going to go anywhere.

The first 2 rooms i would do are 10x14 (just exterior walls) and 12x14 (just exterior walls). The 3rd bedroom is just one wall 10ft. The kitchen would be a pain (lots and lots of cupboards/sink and sliding glass door) and the bathroom is like 4ft on the exterior end...living room has one long wall, but over half is picture window and door. Maybe at some point when i get the funds to reside the house (the siding is ugly/old vinyl), i could wrap it with polyiso.

That is a good thought on the plaster. I live near a busy highway and depending on the wind, we can get some good road noise at times. I would imagine cellulose would be cheaper? I haven't priced out materials yet.

Small bedrooms here so we are losing valuable space, but then again...its mainly for sleeping, so the kids can deal with it.

AC_Hacker · 02-03-14, 01:25 PM

Quote:

Originally Posted by warmwxrules

..Small bedrooms here so we are losing valuable space, but then again...its mainly for sleeping, so the kids can deal with it.

The rooms in my house are about the same size as yours. I also did some 'in-building' on my place by adding 2" to all the 2x4 studs. That was before I realized the large effect of thermal bridging. So I lost 18% of the insulation value I would have had otherwise... not a minor amount.

By the way, I am now equipped with IR non-contact thermometers, and you can definitely measure the difference in temperature of places in wall structures that allow thermal bridging vs. places that do not. It's not just an energy geek concern, it really happens and it is measurable.

I even think differently about the thermal realities of a wall structure now. Now I realize that the sheetrock is a fairly good conductor of heat energy in a lateral direction... and when you have thermal bridging, you are not just losing heat energy from that one stud-width strip of sheetrock, rather the thermal energy of surrounding sheetrock also flows toward the thermal bridge. I now visualize a wall in the same way that I see a sink full of water, with all the water flowing in the direction of the drain. The water flows in the direction of least resistance. Just so with heat energy.

Since these insights, I have done something similar to what you are considering, with interior framing not coincident with the old framing. Your choices around windows are limited however.

And I do not miss the reduced space. I think that the idea of reduced space is much worse than the actuality.

-AC

MN Renovator · 02-03-14, 09:16 PM

This is partially why I'm looking towards rigid foam outsulating because that thermal bridging issue gets mitigated. Then again I don't know drywall and siding is easy for me and I need to do that work anyway.

AC_Hacker · 02-04-14, 10:20 AM

Quote:

Originally Posted by MN Renovator

This is partially why I'm looking towards rigid foam outsulating because that thermal bridging issue gets mitigated. Then again I don't know drywall and siding is easy for me and I need to do that work anyway.

Don't know drywall
Siding is easy for me
I need to do that work anyway

...three good reasons to outsulate. But you forgot other major advantages:

No disruption of the dwelling space
No disruption to electrical outlets

I can tell you from personal experience, these are definitely issues worthy of consideration.

-AC

doug30293 · 02-04-14, 06:45 PM

Warmwxrules;

Having spent 30 years in the frozen north I am a firm believer in the Mooney wall approach. However, if your plaster is good and your siding is bad you might be better off using this method on the exterior. Either way you reduce thermal and acoustic bridging.

Since you are also concerned about noise it might be a good time to look at your windows as well. I ripped out cheap aluminum double panes and replaced them with high end vinyl. I knew they would have thermal benefits but was amazed at the improvement in acoustic insulation. Aluminum is used to make tuning forks. Vinyl is used as sound damping material.

The possible drawback to starting on the outside is that it won't impress the neighbors if you only do one wall at a time.

MN Renovator · 02-04-14, 07:11 PM

The mooney wall is good in some ways, definitely better than stacking on top of existing framing members but you get a small bridge at every cross section and you don't have studs in the up-down direction so if you hang heavy things on the wall you need to screw in at the height of the stud rather than just lining up the width. If you go with 24" centers and space the new section from your existing wall just a little bit(or a bigger gap for even more insulation if you aren't going with batts and use cellulose instead) you could be better off on the thermal bridging front and give yourself more insulation room. I think getting to R40 or close in climate zone 6 or higher makes sense if you can insulate the attic to R60 and mitigate losses through glass through either a framed double sheet plastic shrink wrap or if you need new windows to go with U value .2 or lower. I figure if you are already insulating the walls to go all the way.

02-03-14, 10:24 AM	#1
warmwxrules Helper EcoRenovator Join Date: Nov 2011 Location: La Crosse, WI Posts: 78 Thanks: 0 Thanked 3 Times in 3 Posts	Adding interior insulated walls I have an idea to add interior 2x4 stud walls (basically a double stud wall) with a 2 inch gap filled with R23 mineral wool (24oc). The walls are plaster (painted). No insulation in the exterior stud cavity (2x4) currently-- I would dense pack cellulose from the outside. Can i leave the painted plaster wall in place? I guess i could rip the currently walls out and just extend the studs and dense pack the whole cavity with cellulose from the inside, but that seems like a lot of work/mess. I like the idea of mineral wool because i do deal with some road noise (excellent for noise). I would only do the 3 bedrooms to begin with. The entire house (1story ranch) would get the dense pack cellulose. Thoughts? Anyone ever do this? What about the gap that will be (2 inches) behind the studs...do i fill this somehow?

02-03-14, 10:57 AM	#2
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	I would take out the plaster wall. Id tear it out and then extend the wall like I did with my office, or a mooney wall type setup. There will be much less thermal bridging this way. You can seal things up nice and tight before you button up the wall. You can fix any electrical. There are many benefits, but it is more work of course. Still very much worth it IMO. If you did keep the internal plaster wall I would definitely dense pack from the inside before putting the new wall in. Why cut holes from the outside if you're already covering up the inside? __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

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02-03-14, 12:13 PM	#4
warmwxrules Helper EcoRenovator Join Date: Nov 2011 Location: La Crosse, WI Posts: 78 Thanks: 0 Thanked 3 Times in 3 Posts	Good idea on the 2x3... I almost was tempted to rip 2x4s in half after reading that a lot of old homes were (interior) framed with them. There is no weight on these walls, so as long as they are attached firmly, they aren't going to go anywhere. The first 2 rooms i would do are 10x14 (just exterior walls) and 12x14 (just exterior walls). The 3rd bedroom is just one wall 10ft. The kitchen would be a pain (lots and lots of cupboards/sink and sliding glass door) and the bathroom is like 4ft on the exterior end...living room has one long wall, but over half is picture window and door. Maybe at some point when i get the funds to reside the house (the siding is ugly/old vinyl), i could wrap it with polyiso. That is a good thought on the plaster. I live near a busy highway and depending on the wind, we can get some good road noise at times. I would imagine cellulose would be cheaper? I haven't priced out materials yet. Small bedrooms here so we are losing valuable space, but then again...its mainly for sleeping, so the kids can deal with it.

02-03-14, 09:16 PM	#6
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 939 Thanks: 41 Thanked 116 Times in 90 Posts	This is partially why I'm looking towards rigid foam outsulating because that thermal bridging issue gets mitigated. Then again I don't know drywall and siding is easy for me and I need to do that work anyway.

02-04-14, 06:45 PM	#8
doug30293 Too Many Projects Join Date: Nov 2013 Location: georgia Posts: 91 Thanks: 0 Thanked 6 Times in 6 Posts	Warmwxrules; Having spent 30 years in the frozen north I am a firm believer in the Mooney wall approach. However, if your plaster is good and your siding is bad you might be better off using this method on the exterior. Either way you reduce thermal and acoustic bridging. Since you are also concerned about noise it might be a good time to look at your windows as well. I ripped out cheap aluminum double panes and replaced them with high end vinyl. I knew they would have thermal benefits but was amazed at the improvement in acoustic insulation. Aluminum is used to make tuning forks. Vinyl is used as sound damping material. The possible drawback to starting on the outside is that it won't impress the neighbors if you only do one wall at a time.

02-04-14, 07:11 PM	#9
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 939 Thanks: 41 Thanked 116 Times in 90 Posts	The mooney wall is good in some ways, definitely better than stacking on top of existing framing members but you get a small bridge at every cross section and you don't have studs in the up-down direction so if you hang heavy things on the wall you need to screw in at the height of the stud rather than just lining up the width. If you go with 24" centers and space the new section from your existing wall just a little bit(or a bigger gap for even more insulation if you aren't going with batts and use cellulose instead) you could be better off on the thermal bridging front and give yourself more insulation room. I think getting to R40 or close in climate zone 6 or higher makes sense if you can insulate the attic to R60 and mitigate losses through glass through either a framed double sheet plastic shrink wrap or if you need new windows to go with U value .2 or lower. I figure if you are already insulating the walls to go all the way.