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Old 10-19-2011, 07:46 PM   #1
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Retrofitting Insulation in 2x4 framing


I've been reading a lot about insulation and am in the process of renovating one room in our house. At the moment there is no insulation in the exterior walls. We blew 10" of cellulose into the attic a few years ago and I've sealed the rim joist with XPS. It seems that exterior walls are the next step.

Rather than repair all the dings, I opted to pull down the drywall on the exterior wall and throw some insulation in the wall while I'm at it. The walls are framed with 2x4s. I've considered framing an additional 2" of wall depth to get the wall to 5.5" (using 2" of spray foam and r-19 fiberglass), just filling the 3.5" void with XPS (I've got a couple of 2" sheets left over from the rim joist and a math error), or using a combination of XPS and fiberglass in the same manner as the spray foam- essentially flash and fill using XPS for the flash. I need to do this one room at a time so I'd prefer to not contract someone to come in to spray each time. I'm OK with the idea of cutting XPS to fit and great stuffing it into place, but I'm not sure if the performance is worth the extra cost or if there is any risk I'm not realizing.

The house is brick and vinyl siding. Eventually I'd like to replace the vinyl with cement siding so I can paint and update the house occasionally. At some point I'd be able to wrap about half the house to help seal things up, but I'm sure the XPS will do an OK job if I am careful in fitting.

Any thoughts or recommendations? I've spent way too much time reading about this and need to get busy with wrapping this up. Thanks in advance for any advice you can offer.

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Old 10-21-2011, 11:23 AM   #2
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Retrofitting Insulation in 2x4 framing


I'm actually doing this in new construction with 2x6 walls. I'm fitting 2 inch XPS in between studs with 1/4 to 1/2 inch of space on the top/bottom/sides (for room for Great stuff), then finishing out the rest of the space with r-13 fiberglass batts. It is fairly time consuming, but it is easy work. Don't underestimate how much Great stuff you'll need, It takes 2 or 3 cans per sheet of XPS installed from what I'm seeing. At current costs, that's around $6 to $9 on top of the $25 per sheet that 2 inch XPS costs at Home Depot (cheaper at HD in my area than Lowes, which is closer to 30 per sheet).

Even with the costs of great stuff, it wound up cheaper to do it this way than with the DIY kits of spray foam that I was finding - you'll save roughly 30-40%. It might save you time to do spray foam for the first two inches + finish with batts (they call that flash and batt), but you'll save money doing it the way you're planning with XPS sheets. I currently have more time than $$$, so that was my deciding factor.

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Old 10-21-2011, 02:16 PM   #3
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Retrofitting Insulation in 2x4 framing


If you're spending all that money on Grwat Stuff....get a Hilti foam gun. You don't have to use a whole can at once. Once you use one, you'll never go back to Great Stuff
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Old 10-22-2011, 08:06 AM   #4
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Retrofitting Insulation in 2x4 framing


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Originally Posted by The other Derek View Post
I've been reading a lot about insulation and am in the process of renovating one room in our house. At the moment there is no insulation in the exterior walls. We blew 10" of cellulose into the attic a few years ago and I've sealed the rim joist with XPS. It seems that exterior walls are the next step.

Rather than repair all the dings, I opted to pull down the drywall on the exterior wall and throw some insulation in the wall while I'm at it. The walls are framed with 2x4s. I've considered framing an additional 2" of wall depth to get the wall to 5.5" (using 2" of spray foam and r-19 fiberglass), just filling the 3.5" void with XPS (I've got a couple of 2" sheets left over from the rim joist and a math error), or using a combination of XPS and fiberglass in the same manner as the spray foam- essentially flash and fill using XPS for the flash. I need to do this one room at a time so I'd prefer to not contract someone to come in to spray each time. I'm OK with the idea of cutting XPS to fit and great stuffing it into place, but I'm not sure if the performance is worth the extra cost or if there is any risk I'm not realizing.

The house is brick and vinyl siding. Eventually I'd like to replace the vinyl with cement siding so I can paint and update the house occasionally. At some point I'd be able to wrap about half the house to help seal things up, but I'm sure the XPS will do an OK job if I am careful in fitting.

Any thoughts or recommendations? I've spent way too much time reading about this and need to get busy with wrapping this up. Thanks in advance for any advice you can offer.
Ayuh,... Fir out the studs to get yer 6"s of depth...
You can't get 6" of insulation value into a 4" space...
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Old 10-22-2011, 08:43 AM   #5
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Retrofitting Insulation in 2x4 framing


Seems like a lot of work to fit xps between the studs for a minimal increase in r-value. if you aren't going to bridge the foam over the studs to stop the thermal transfer through the wood, like Bondo pointed out, fir it out and fill the 5.5 inch bay with batts
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Old 10-23-2011, 01:29 AM   #6
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Retrofitting Insulation in 2x4 framing


Fiberglass batts are the worst insulation you can buy. Use rock (mineral) wool, cellulose, or cotton. Tape and caulk the foam if you use it, but I'd fir out like others said, if possible. (btw: eps is much more environmentally friendly than xps; it is blown w/ air, not yukky gasses). When done insulating, air seal like crazy. I am not a fan of vapor barriers, unless mandated.
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Old 10-23-2011, 07:44 PM   #7
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Retrofitting Insulation in 2x4 framing


I appreciate all the responses. I think they kind of illustrate why I needed some direction- lots of different approaches and some fear on my part that I am setting a mold time bomb if I don't do this right.

I have fit the 2" XPS into the framing. My understanding is that I need r10 to avoid condensation in the wall. It sounds like the XPS is going to start at r10, but reduce over time. If I butt another 1.5" of XPS (or EPS) to fill the 2x4 cavity do I need to use an adhesive between the faces?

In furring out the wall I'm planning on using 2x3 and leaving an inch gap. Sufficient? I'll fill the new void with Roxul. Should I try to fit 1" foam board in the gap between the framing?

As for air sealing- my understanding is that the foam board/great stuff would act as an air barrier.

Last edited by The other Derek; 10-23-2011 at 10:25 PM.
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Old 10-24-2011, 11:30 AM   #8
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Retrofitting Insulation in 2x4 framing


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As for air sealing- my understanding is that the foam board/great stuff would act as an air barrier.
This should do an excellent job of sealing gaps/cracks to the external side of the building, but you also need to remember to air seal any electrical/plumbing/hvac penetrations between floors, which the 2 inch foam/great stuff won't cover.

If being inspected, you'll need to buy the special fire rated caulk or foam for penetrations between floors. To keep things really air tight, you can get electrical boxes that are airtight, or you can caulk up the normal boxes, which have a surprising number of holes/gaps to let air though (even knock outs that aren't knocked out let a significant amount of air through).

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