# I think I found the best way to frame and insulate an exterior basement wall...



## rdr8887 (Jan 28, 2012)

I spent some time over the past few days researching framing against a poured concrete or block wall. As many of you probably already know, there is a wide difference of opinions on this one. Air gaps, rigid vs fiberglass, free standing vs secure to foundation....the list goes on. 

This is what I came up with. I'm zone 3 for most of my work as far as IECC is concerned. 

Majority of construction

1-2" air gap between studs and exterior wall. Then, they fill the cavities with r-13 (typically faced). Also, pressure treated lumber for sill with no capillary break. 

This presents a couple problems. The air gap can allow for warm air to come into contact with the foundation wall causing sweating and possible mold in the future. Not using a capillary break can have water wick through the sill and into the studs. Over time could cause rot. The air gap is also a fire hazard without a plywood return back to the concrete at top of wall. 

What I will do in the future

I will install rigid directly to concrete wall. Haven't decided between 1/2" and 1". Sealed seams. Then frame a wall touching face of rigid using 2x4 with 16" centers. Pressure treated bottom plate over felt layer for a break. Wall not tied to concrete. Only floor and joists above. Cavities filled with I faced batts and covered in Sheetrock. No air gap. 

I never like following the block or concrete so I can ensure my walls are plumb. 

Any thoughts on this? It seems like the best install.


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## TRMolnar (Mar 19, 2012)

I like it. 

I wonder why you wouldn't just use spray foam and save time on labor installing the ridgid foam and batts?. 

I personally haven't used it yet but a lot of guys around here spray foam everything now. 

If its a cost issue - I could see your method being very effective. :thumbup:


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## rdr8887 (Jan 28, 2012)

There is always a cost issue when bidding work. Maybe I'm old shool, but I don't like the idea of spray foam. I could imagine the pain factor if I ever had to demo a wall with that stuff for any reason.


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## TRMolnar (Mar 19, 2012)

That's a good point! I couldn't imagine any future modifications would be fun... moving an outlet could turn into a career.


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## kiteman (Apr 18, 2012)

One concern we had been having with an air gap was draft/fire stop. If we left the normal 3/4-1" gap in between, the gap at the top was difficult to seal off. We started nailing a 2X2 tight to the sill plate and then the top plate tight to that. We were also required to sister up a stud every 10' with a treated 2X4 pushed tight to the concrete for lateral draft stop.

On occasion we have hung poly on the foundation walls prior to framing, but I don't see that much anymore. Now the builders are required to put 1 1/2" foam on any unfinished foundation walls. Some are using a blanket type insulation instead of foam.


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## SLSTech (Sep 13, 2008)

2" foam against the walls & no other insulation is needed per the codes there - don't forget to seal the seams

Personally as most basement walls are just that little bit off, I would say frame up the walls about an 1.5 to 2" away & spray closed cell behind it - that takes care of sealing, moisture, and insulation concerns while leaving the bays open to run wires & pipes as you want


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## GO Remodeling (Apr 5, 2005)

Seal any foundation cracks, first. Glue rigid foam to wall and start framing 1/2" off foam. Seal seams. No other insulation. Ptl for bottom plate.


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## Morning Wood (Jan 12, 2008)

I built my walls on the floor fastened 1" XPS to the back of them and taped the seams. Then stood the floors up and got them as close to the foundation wall as possible. Good treatment in my book. I also happen to have 4" of foam above and below grade on the exterior of the foundation wall.


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## bwiab (Mar 17, 2006)

Since the wall is non bearing, if you local ahj allows it, you can run a strip of 3/4" ply as a fire block prior to setting the top plate. This is a code issue so you have to check what the city allows.


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## Spencer (Jul 6, 2005)

I just sprayed closed cell foam in my new basement. IMO you won't find a better basement insulation. It also allows you to put in on a little heavier up higher where more insulation is needed. As far as demoing it someday, why would you ever be demoing the outside walls. There are an extremely small number of cases where you would ever do this.

I framed my walls about 1-1/2" off wall first, then ran utilities, then foam. Makes the wall rock solid.

Downside is that code here is that you have to cover any spray foam as it emits a carcinogen in the event of a fire. If you just use blue board you don't have to cover.


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## KennMacMoragh (Sep 16, 2008)

I always wanted to try these http://www.ovrx.com/installing-wall-panels.html you get a few more inches of space than you would building a stud wall. Except they are made in Canada, so I don't know if they abide by code around here. Not sure about your idea of mold building up in that gap between the wall and concrete. Since the gap is so small I don't think it would be an issue. That's the reason we vent roofs but not every wall cavity in a house, there is not enough room inside a wall for moisture to be a problem. Sprayed insulation would work good if cost is not an issue.


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## SLSTech (Sep 13, 2008)

Ken, I can't see why it wouldn't meet codes as long as the r-value meets the minimums. If you are worried about moisture I would look into putting something up like Delta Dry or some other dimpled membrane & making sure it runs into a French drain, etc...

Spencer, it depends on the fire & smoke rating of the foam - some do not need covering but as it sounds like you are completing the basement, the drywall more than meets the needs


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## KennMacMoragh (Sep 16, 2008)

I've posted about it before, I thought someone said running electrical through it wouldn't work. But I haven't looked into it that much.


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## DaveWolfOC (Apr 17, 2013)

Hi rdr8887, Dave Wolf here – I’m new to Contractor Talk and just wanted to first introduce myself. I’m the senior research and development program leader at Owens Corning. I’ve worked extensively on air sealing research and building science – so I’m happy to provide some thoughts. 

Regarding the assembly that you're considering for future installations, I have some suggestions. 

First, you didn't mention the type of “rigid” insulation that you were considering. I’d recommend extruded polystyrene because of its good moisture resistance. 

Second, you mentioned taping the joints. That’s a good idea because you want to avoid the passage of moisture-laden air through those joints, but remember to also extend the rigid insulation and taping horizontally back to the sill plate. This will avoid the passage of moisture-laden air over the top of the rigid insulation, which could condense on the top of the foundation wall. 

Third, you mention using faced batts in the framed wall cavity. This is risky because the facing is designed to resist vapor movement and the rigid insulation (if extruded polystyrene or similar) has the same type of resistance. This means that your framed wall cavity will have a hard time drying out, if it ever gets wet, because it would have vapor-restrictive materials on both sides. Another approach would be to use an unfaced batt in the wall cavity, but use a thicker rigid insulation (more R-value) to avoid condensation. 

Finally, all of this assumes - of course - that there is effective bulk water management on the exterior side of the foundation wall. Any insulation strategy is vulnerable with bulk water moving through the foundation wall. 

I hope this helps.


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## EthanB (Sep 28, 2011)

Welcome to CT Dave. Good point about the batts. If you already have the rigid foam then you should use unfaced batts to avoid to dreaded vapor barrier sandwich.

A couple other things that can be nice in a basement wall assembly are to stop the drywall just below the top of the moulding height and add a nailing strip at the bottom to prevent the paper face of the gypsum from wicking water in the event of a flood. If you're doing that you should also add an extra block of rigid in the stud bays and stop your batts a few inches above the floor as they will wick too. I've been trying to upsell DensArmor for the bottom sheet on basement walls.


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## renov8r (Feb 16, 2013)

I think you have a good plan there. 

I do similar sometimes. 2" foam on the wall, tuck taped on joints, framed with 2x4 @ 16" O/C and then filled with fiberglass insulation, SPF studs and plates with sil-gasket for barrier between concrete and wood, bolted to both floor and joists.


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