Do I still need to use vapor barrier before drywall?

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Posted by: from Saint Catharines
11/14/2018 at 4:11:11 PM

Starting a basement reno soon. Got new windows installed and want to start insulating. my question is when i use 1 on 1.5 inch rigid foam bard on the basement walls i plan to use batt insullation between the studs. do i still need to use vapor barrier before drywall. Live in st.catharines Canada. humid in the summer.



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Date/Time11/14/2018 at 8:57:30 PM

Yes you do

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Ryan from DOJ contracting in Brantford
Date/Time11/14/2018 at 9:10:45 PM

Yes you should always use a vapour with batt insulation. Spray foam it is not needed

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Date/Time11/14/2018 at 9:12:21 PM

No vapour barrier needed

Board on the concrete frame in front

Leave some space at the bottom maybe 6 to 8 inchs

Another Tip

Lay a sub floor (drycore)

I prefer foundation membrane and plywood and frame all your walls on top of the subfloor so air may pass from the walls under the floor from end to end

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Robert from ElecTriLight Ltd. in Oakville
Date/Time11/14/2018 at 9:21:13 PM

vapour barrier must still be used on all your electrical boxes and holes passing from hot to cold areas.

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Date/Time11/14/2018 at 9:26:09 PM

Yes, all exterior (cold) walls require vapour barrier, the basement walls that have earth behind them are considered to be exterior walls. This prevents condensation from forming where the warm inside air meets cold exterior air, even under ground.

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Date/Time11/14/2018 at 9:28:25 PM

Hi Mike Dedicated vapor barriers have no place in a basement wall. If the basement wall is insulated on the outside, then the vapor barrier is superfluous; otherwise, is it dangerous, as it will trap moisture against a moisture-sensitive element, as I indicated. No. Basement wall systems should never include any polyethylene.

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Date/Time11/14/2018 at 9:50:29 PM

Yes you should mike. Always apply poly on the warm side of the wall before drywall installation. Overlap joints by 3 and tape with blue poly tuck tape. That is code. Just not with 2 pound closed cell vapor barrier infused spray foam.

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Date/Time11/14/2018 at 10:17:18 PM


I always glue R-10 ridged foam panels to the concrete walls. Then use tuck-tape on all the seams

This is now your vapor barrier with R-10.

Then frame all the walls. Be sure to run a roll of foam underlay beneath all your bottom plates in the walls. Add all your electrical runs, gas lines or water lines. Then install R-20 batt insulation in all exterior framed walls. You now have R-30 insulation.

I always use sub floor panels. 2x2 foot with a plastic membrane on the floor side. You now warm up the floor. And if you ever have a water issue in the basement, it will run under the panels towards the drain. And air will always move below it. Leave a quarter inch from your walls when installing it. Enjoy your new basement!

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Thomas from Trusted Trades in Mount Forest
Date/Time11/14/2018 at 10:52:43 PM

Hey Mike,

I think this depends on a couple of factors but there is no great answer in my opinion.

The code board installed on the concrete walls gives roughly R5 or R7 and will act as a vapor barrier. The issue is if moisture comes in at this point it gets trapped between the foundation wall and ridgid insulation. Potentially causing mold. This has been a very common practice though--all through the 90's and 2000's and works well but can breed mold issues in older homes without a weeping tile system or homes experiencing a crack in the foundation.

Adding a vapor barrier on top of the batt can aid in the overall performance and I dont believe it can cause any additional issue.

My recomendation though would be to look into Rockwool insulation systems. Previously known as Roxul, they offer a 2 tier insulation system that has a board mounting directly to the concrete followed by the batt. The board allows for a thermal break between the stud and concrete wall but unlike the ridgid insulation, allows moisture transfer. (This is good)

The batt insulation can then be covered with a permeable vapor barrier. A vapor barrier that allows safe moisture transfer. This allows the whole wall assemble to breathe. You can expect a total system cost to be 30-40% more costly than the current products you have spec'd but it is a superior mold resistant product- In my opinion.

Hope this helps.


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Date/Time11/15/2018 at 2:12:48 AM

Hi Mike,

Some of the answers are good, others not so much.

What I will say is this, when I did finished basements in the GTA for Great Gulf, Mattamy Homes, and a bunch of different builders this is how they all required them built and were passed on inspection.

Typar against the concrete (you fasten it to either the sill plate or floor joists, whatever you have access to - staples are fine) if you're concerned about energy star ratings you then tape every single seam with Blue Tuck Tape. Once this is done you measure out two points at each end of every exterior wall that will have a finished wall built against it, these points are ~5" from the concrete and mark a line from point to point (I use a laser level these days - faster and easier) this gives you roughly 1" air gap between Typar and wood. Plastic ALWAYS between wood and concrete, so your bottom plate of the wall will have poly under it. As will every interior wall that terminates against the exterior finished wall (poly the end stud of that wall between the stud and the other 2x4 wall - it'll be sandwiched wood to wood). You build your walls as you normally would (top plate to floor joists and plastic to concrete bottom plate on the floor (obviously) level and square. Once the exterior walls are built you insulate with regular bat insulation (I recommend Roxul Safe'n Sound) and vapour barrier the wall as normal.

This is a tested and true method to building basement exterior walls an inspector will pass. Scrap the rigid foam insulation, it's only complicating the process for you.

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Date/Time11/15/2018 at 6:01:07 AM

Yes u do for code

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Date/Time11/15/2018 at 8:54:25 AM

Drywalls are often strengthened with vapor barriers to make them more durable. However, the presence of vapor barriers among drywalls isn't a standard practice. Many of the contemporary residential spaces are also built without them. Further, vapor barriers aren't recommended for all types of drywalls. Homeowners should be equipped with the basics of vapor barriers and to recognize the need to install them.

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Gerard from Betterbuilt in Burlington
Date/Time11/16/2018 at 4:19:41 PM

Hi there...

The combination of styrofoam...preverably blue and batts is strange.

Why not put 20 lbs. black felt on the poured concrete or block walls .......put 2x4 studs up and finish with R12 batts and six mil poly before the drywall.

Old fashioned but (cost) effective.


Gerard Schoeman

Betterbuilt (2525)

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