Add additional insulation to walkout basement wall cavity. Good idea? Bad idea?

Submitted by hoppyending t3_11dv0dk in DIY

I'm in the middle of finishing my walkout basement. The above grade walls are constructed of 2x6s with fibre glass insulation between the studs, rigid foam on the exterior, and vapour barrier on the interior. The foundation is covered in foundation blanket and vapour barrier. The rim joist is spray-foamed.

Because the foundation is thicker than the wall, plus the additional thickness of the foundation blanket, there is about a 6 inch gap between the back side of the interior framing and the exterior wall. Is it worth filling this gap with insulation? Are there any potential downsides? I live in a climate where highs above 30°C (86°F) in the summer and lows of -30°C (-22°F) in the winter are commonplace.

Framing, above grade wall, and foundation.

Gap between the back side of the two walls.

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codyish t1_jab4mq2 wrote

Some places have specific requirements to meet fire code when filling that space. For example, where I live you can't just place insulation bats between the studs because the narrow gap that would be left between the bats behind each stud is considered an airspace that could allow a fire to transit to the floor above. I do live in fire country and using bats doesn't look practical in your situation, this is just an example. I would just double-check the fire code to make sure that exactly how you are going to do it won't run afoul of that.

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Thrawn89 t1_jacrxbn wrote

The IRC allows this, but you need to provide fireblocking with an approved material (1/2 gypsum board, 3/4 ply, fire spray foam for gaps are most common). The blocking material must be every 10 feet both horizontally and vertically as well as at every horizontal to vertical cavity transition (ie. wall intersects with soffit).

This means that you'd need to add 3/4 ply or something to close the gap at the top plate to the exterior top plate, and you'd need to add this every 10 feet along the wall between the interior stud and exterior stud from the sole plate to the top plate. Though I think stuffing fiberglass in the gap is allowed if it's secured mechanically.

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SpearCatcher1 t1_jab03kk wrote

Where I live we'd be wary of creating a protected path for termites, but sounds like not a threat to you. Any moisture wicking from below? If no moisture issues it sounds like a great opportunity for increasing that R value

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hoppyending OP t1_jab1acd wrote

Termites are only a problem in a few areas in Ontario (Canada) and, thankfully, I don't live in one of those areas.

My house is basically sitting on granite. The builder had to blast to dig the foundation. The foundation walls are waterproofed, and the floor is required by building code to be damp-proofed at minimum, so the basement should be pretty dry.

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Franklin2543 t1_jabd4uk wrote

Off topic, but granite? May want to get an air quality monitor that can detect radon. Levels fluctuate with the weather—my off the shelf test only came back 0.7, but I didn’t trust it could always be that low (we have a lot of granite too). Airthings sensor put it at around 4-5 average, with lows less than 1 and periodically could get as high as 6.

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hoppyending OP t1_jabh9yz wrote

I’ve been looking into it. I keep hoping the AirThings Wave Plus will go on sale, but no luck yet.

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AlienDelarge t1_jabki14 wrote

They had some sales in January for Radon something or other month. I picked up the basic radon meter for nearly half price.

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GoAwayMe t1_jac6jha wrote

Can't comment on a sale but the View Plus is a pretty incredible little air quality tester. I move it around periodically when I'm concerned about certain spots and it almost immediately updates with different data. I haven't authenticated the data but the thing has earned my trust. Great for Radon, CO2, VOCs and Particulate Matter readings.

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Zergom t1_jabd7nl wrote

I live in Manitoba. I would rather spend the money on improving my attic insulation or my windows. Heat loss from your exterior walls aren't going to be as bad as windows, doors and your roof. I also would have tried to figure out a way to install those walls without losing a foot of the perimeter of my basement - like hanging them from the joists and gapping the bottom, which is standard practice out here. So that when your house shifts it's not turning those walls into something structural.

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hoppyending OP t1_jabjbrg wrote

That’s the answer I was hoping for. I thought the money is likely better spent elsewhere, but maybe some Redditor who insulates homes for a living knows better.

The only way to finish the walls and not lose the space would involve having a weird ledge on all of the walls. I’d rather sacrifice the space and have straight walls. My motivation for finishing the basement isn’t more space and I’m only finishing about half of the available space. If the basement wasn’t a walkout, I’d probably never finish it.

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Whane17 t1_jabmiwm wrote

My landlord went with weird shelves, it's actually turned into a boon as we use them as actual shelves (these ones are about 8-12" deep around one side of the house). I appreciate the extra storage space. I know it's a little to late for you but figured I'd chime in my 2c :)

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Zergom t1_jacth09 wrote

Yeah it tough. I’m in a bi-level and had a similar situation.

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SnowFlakeUsername2 t1_jabfv90 wrote

The vapour barrier is behind the interior wall? If so there is some rule of thumb ratio that I can't really remember about adding insulation on the warm side of a vapour barrier. Guessing 1/3rd r value of the insulation on the cold side.(look it up) IMO you don't want anything touching the warm side of the plastic that would trap condensation and grow mold. An air gab between would be a good thing.

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hoppyending OP t1_jabfzok wrote

Good to know. I was concerned about it being on the wrong side of the vapour barrier.

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woodprefect t1_jace031 wrote

You don't want to use a vapour barrier at all if you will hang drywall. The moisture will move through the drywall but not the plastic and now you have mold conditions.

Keep the vapour in or out or allow it to pass all the way through.

​

>Incorrect use of vapor barriers is leading to an increase in moisture related problems. Vapor barriers were originally intended to prevent assemblies from getting wet. However, they often prevent assemblies from drying. Vapor barriers installed on the interior of assemblies prevent assemblies from drying inward. This can be a problem in any air-conditioned enclosure. This can be a problem in any below grade space. This can be a problem when there is also a vapor barrier on the exterior. This can be a problem where brick is installed over building paper and vapor permeable sheathing.

https://buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

If you _have_ to use it for code use certain tweed's which allows the interior to dry.

https://www.certainteed.com/insulation/vapor-barriers/

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CrispyBacon_87 t1_jack1am wrote

This is so ass backwards.... You're contradicting the building codes of countries around the world.

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Thrawn89 t1_jacss96 wrote

You must always follow local code. They require vapor barrier on the interior side of the wall above grade. Also your links just say incorrect use will cause problems, they don't say don't use vapor barriers at all.

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UncleBobPhotography t1_jabkaae wrote

Makes me wonder, what is the warm side in OPs case when there is such a big temperature difference between summer and winter? I guess the inside is the warm side since the delta is larger during the winter than in summer?

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SnowFlakeUsername2 t1_jabnm5h wrote

Could have the terminology wrong. Where I live we typically call the interior the warm side. I'd also guess it is about temperature differential. The condensation problem is much worse going from humid 20c into dry -30c.... plus the moisture on the cold side isn't warm enough to dry the insulation and foundation. So vapour barrier goes on the inside with it's most important job being to keep warm interior air from condensing into a cold wall. In the summer, any moisture inside a wall can dry out and escape out the non-barriered wall.

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UncleBobPhotography t1_jabozqs wrote

I think you've got the terminology right, we say the exact same thing here in Norway. The difference is just that in Norway, the inside is always the warm side. I'm pretty sure you would do it the other way around in Sinagpore.

Especially if you live in a fairly dry area I guess 30c outside won't make too much of an issue in the summer since it's just 5-10 degrees warmer than the inside which is not much compared to the 50c difference you could get in winter.

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JMJimmy t1_jabkf37 wrote

For insulation, you need to add exponentially more for it to make a difference. Basic insulation should achieve 90% heat retention. Doubling that gets you to 94% and returns diminish from there.

I would just air gap it

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r7-arr t1_jab4cgb wrote

One big downside is your framing isn't correct. You should have fire blocking all around the top plates. And also at intervals along the wall length.

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hoppyending OP t1_jab4x15 wrote

There are no such rules for interior walls where I live. They’re just there to hold drywall.

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MinkOWar t1_jabkg3s wrote

The issue they are pointing out is that the air gap in the wall is connected to the concealed space in the floor assembly. If you fill the studs with insulation, you are excepted from the fireblocking requirement.

Otherwise, you can block the space between the framing with lumber or plywood or drywall.

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hoppyending OP t1_jabkopz wrote

Yeah. I understand what they were saying now. In my jurisdiction Sheetrock is an acceptable fire block, so there’s nothing wrong with my “framing”. I haven’t even ordered any drywall yet, so that’s why that space is open right now.

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r7-arr t1_jab582c wrote

I find that hard to believe. It's been in the IRC for years. https://codes.iccsafe.org/s/IRC2015/chapter-3-building-planning/IRC2015-Pt03-Ch03-SecR302.11

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MinkOWar t1_jabi4nw wrote

Canada doesn't use the IRC.

Also, fireblocking materials in that reference include glass fiber batts... so OP's literally proposing to fill the wall with approved fireblocking by that reference.

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r7-arr t1_jach3u6 wrote

I guess fire is different in Canada. There should be a solid block of plywood or 5/8 drywall at the top of the cavity that he has created to create a block between that and the joist bays. The reason is that any fire that starts in that wall space will rapidly spread into the basement ceiling. Fiberglass batts are not fire blocking. Mineral wool batts are acceptable.

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randomn49er t1_jabaeab wrote

I have only been required to do this for seperate living quarters in the basement. In my experience this is not required if one family has free access to all levels of the home.

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Spoona1983 t1_jabgvrv wrote

You're assuming USA i personally would have assumed canada as op used C denoting celcius and also the 60°C temp difference is kinda normal for canadain cities

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Thrawn89 t1_jactz7x wrote

You'd be assuming north America, Europe also uses C.

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geshie t1_jabd15h wrote

Yes there are - fire blocking requirements are in OBC 9.10.16. https://www.buildingcode.online/1506.html

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hoppyending OP t1_jabe909 wrote

I think I misunderstood u/r7-arr’s comment. Yes I’m planning to put fire blocking between the interior wall and exterior wall at the top plate. I don’t think any other fire blocks are necessary in Ontario for interior walls.

Edit: My understanding is that fire code Sheetrock is acceptable, so the comment about my framing being incorrect didn’t make sense.

Edit #2: 1/2” Sheetrock is apparently acceptable according to 9.10.16.3. I don’t even have to buy the fire code stuff.

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r7-arr t1_jachv1o wrote

The sheetrock or plywood should be between your top plate and extend to the sill plate. The seam between the sill plate and the sheetrock or plywood should be caulked with fire blocking caulk. Once you finish your walls and ceiling, you will have created a massive chimney for any fire unless you put up the blocking.

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MinkOWar t1_jabhz29 wrote

9.10.16.2 is the relevant article, and they are filling the wall with insulation, so fireblocking is not required between the wall and floor inside a dwelling unit.

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ZhanZhuang t1_jabfm7x wrote

I thought you only had to have that for walls higher than 8 ft.

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ZhanZhuang t1_jabfq6d wrote

I would say go for it. Insulation is cheap and it's better to not have to go back and put it in later. As long as the insulation isn't going to trap moisture behind your wall and you should be fine.

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markusbrainus t1_jabjqx2 wrote

I think it's already insulated well enough and extra insulation isn't needed. How cold is it right now in the basement?

I've never considered how walk-out basements require this secondary interior framing inside the foundation walls. You're already done framing, but you might have considered only framing a pony wall in front of the foundation and having a step back to the exterior framing. You regain 4" room depth above the foundation walls and avoid reframing around all the windows etc. The shelf it creates in the wall may be a plus or minus depending on your aesthetic.

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hoppyending OP t1_jabjznv wrote

I thought about the shelf/ledge approach, but then I thought about all of the extra drywall work, and having to dust my walls until the end of time. Dusty baseboards are bad enough.

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Traveling_Carpenter t1_jaclpqd wrote

You have to have a certain ratio of interior and exterior insulation in order to keep the sheathing above the dew point. The more insulation you stuff in the cavities, the more continuous insulation you have to add to the outside to compensate for the reduction in heat energy making it to the sheathing. There are tables in the IRC that should help. You can also use your search engine of choice to search for wall insulation ratios for your climate zone. https://codes.iccsafe.org/content/IRC2018/chapter-11-re-energy-efficiency

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Thrawn89 t1_jacv6ha wrote

The insulation values given in the prescriptive compliance path of the IRC are minimum values. Could you please help me find where the ratios are specified in the code?

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Sluisifer t1_jae5jep wrote

Vapor barrier is an issue.

VB is placed on the interior side for cold climates because this will be the warm area. It will not provide a cool surface for vapor to condense. When you insulate over the VB, you cool it off. It is now insulated from the conditioned interior. Now the cooler VB can act as a condensing surface for warm and moist interior air.

This is not some killer issue, but it's not good practice and could certainly lead to some issues. In practice, it's very hard to say what will and won't lead to failures.

Just keep in mind that many people are incredibly confused about moisture issues and will very confidently say all kinds of bullshit. Even - really especially - professionals that understand some things well and other things poorly. In reality the problem is simple, but the solutions are complex. Empiricism rules the day.

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TechJunky1 t1_jae802g wrote

Do not put insulation there.

With the poly already on the wall you would create a warm spot there and could be trapping any moisture that needs to evaporate.

The walls are a little far off the walls but you will be fine with leaving the drywall on. The air space will actually help a bit as well.

Modern insulation on buildings are leaning to exterior insulation and air gaps inside to create another barrier.

I would leave it as is and board

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Suppafly t1_jabnua3 wrote

Insulation is cheap, if the walls are already open it seems like a good idea. I'm not sure if there are any concerns with that needing to be open for air/vapor reasons though. Seems like maybe insulation might be better than leaving that cavity open, for fire blocking reasons.

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bluGill t1_jacxofp wrote

Ask your local building inspector. What they will pass or fail is always the first consideration. I'm in particular concerned about vapor barriers as what you described sounds wrong, but I'm probably just concerned. You do not want the problems of wrong vapor barriers down the road.

If the inspector will allow it I'd put insulation in these walls, but no more vapor barrier. It may not meet the letter of local code, but it should meet the intent so I think your inspector will allow it - but your inspector should know more than me, so if he describes reasons not to listen.

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hoppyending OP t1_jad0kff wrote

I was describing the existing exterior wall insulation and vapour barrier, installed by the original builder, and inspected by the town. I know that part is built correctly.

I think the general consensus in the comments is that I'd see minimal gains while complicating compliance with the Ontario building code, so I'm just going to leave it as is.

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pomoh t1_jadjxsy wrote

How thick is the exterior rigid insulation and what type? I ask because it appears you created a vapor trap in the above grade portions. You want the wall assembly to be able to dry out in one direction.

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hoppyending OP t1_jadl5dz wrote

I didn't build the exterior wall. It kind of came with the house. I think it was 1-inch foam. It's pretty standard here. Every house in the subdivision was built the same way.

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Routine_Left t1_jae88kv wrote

My house's basement looks just like yours, and I also live in Ontario (south). Do you have to add a new set of studs in front of the existing ones? Can't the drywall be just placed on the existing studs?

just asking for when I'll go about finishing that basement (prolly never).

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hoppyending OP t1_jae8vxp wrote

The drywall can be affixed to the existing studs, but you'll end up with a step or ledge somewhere part way down the wall where you have to frame around the foundation. I wanted to avoid that, so I framed new walls in front of the existing walls. The downsides are the loss of about 9–10" of space on every exterior wall and r/AbsoluteUnits window jambs.

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Reeheeheeloy t1_jabgc1k wrote

I'd be concerned about moisture, that said you could restrict the convection potential in the airgap and that'd make it more insulated without the moisture risk.

Mount some horizontal plywood strips in there (kinda like shelves). That keeps more of the heat in the airgap at the same level, rather than it pooling at the top. Then it can radiate back into the room and keep the heat in the walls longer. Kinda like bubblewrap insulation.

Diagram:
Inner Wall I
Plywood strips E (because that's what it'd look like)
External wall L (you leave some vertical air gap still, just less)

IEL

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MinkOWar t1_jabjd8w wrote

Even if air were gelatinous enough for convection to be meaningfully affected by making baffles that way, you're mostly just making the air gap smaller, allowing more thermal bridging.

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YJMark t1_jac3q8z wrote

What I’ve seen done is that you don’t need to frame the walls. You use foam board insulation glued to the concrete walls. Then you can actually glue the sheetrock directly to the foam board. All your electrical would then be inside your exterior framed wall.

Of course, it is too late now since you already put in the interior framing. At this point, the only potential risk is a fire path behind your wall. Not sure about codes in your area. You might want to just double check that.

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Thrawn89 t1_jaczeco wrote

Glueing sheetrock to the foam board I think is against IRC, which requires backing, either 2x framing members or furring strips (or other structural members like metal).

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Tebasaki t1_jaciyf4 wrote

The gap should be between your outside wall and plastic. Outside to in should be wall, gap (1/2" ok) vapor barrier, studs with insulation l, then acoustic barrier for fun, then drywall. If there's nothing between your foundation and insulation then there's nothing to prevent the concrete from soaking up water and insulation following suit.

Also, your bottom plate should be greenboard and I'd put a vapor barrier under it as well.

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bloomingtonwhy t1_jaddwzj wrote

Windows in load bearing walls should have a proper header

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hoppyending OP t1_jadej6n wrote

It's not a load-bearing wall. It's just to hold drywall. The outer wall (the one with insulation between the studs) is load bearing.

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bloomingtonwhy t1_jadfdgl wrote

Ah, the ceiling joists are anchored into the outer wall?

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hoppyending OP t1_jadhafe wrote

Yeah. The floor joists sit on top of the exterior wall. I didn't build any of that. Load-bearing walls are not something I play with.

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Any-Smile-5341 t1_jadmh6j wrote

Adding additional insulation to the gap between the framing and the exterior wall can be a good idea, as it can help improve the energy efficiency of your basement and reduce heat loss. However, it's important to make sure that the additional insulation is installed correctly to avoid any potential downsides.

One potential downside of adding insulation to this gap is the risk of moisture buildup, which can lead to mold growth and other moisture-related problems. Moisture can accumulate in the cavity between the framing and the exterior wall, especially if there are any air leaks or if the insulation is not properly installed or ventilated. To avoid this, it's important to make sure that the insulation is properly sealed and that there is adequate ventilation in the cavity to allow any moisture to escape.

Another potential issue is that adding insulation to the gap may reduce the amount of space available for wiring or plumbing, which can make future renovations or repairs more difficult. If this is a concern, you may want to consider using thinner insulation or alternative insulation materials that take up less space.

Overall, adding insulation to the gap between the framing and the exterior wall can be a good idea, as long as it's done correctly and with proper attention to moisture management and ventilation. Consult with a professional contractor or insulation specialist to ensure that the insulation is installed correctly and to help you choose the best insulation material for your needs.

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