Is it ok to add more insulation to existing insulation with vapour barrier?

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Posted by: from Toronto
7/18/2015 at 8:40:24 AM

Hi pros,

My question is if it's ok to install insulation batt over existing insulation and vapour barrier? Or they need to be removed?



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Date/Time7/18/2015 at 12:14:58 PM

If you have room without cutting or perforating the vapor barrier. You can certainly add insulation on the inside of a wall.

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Date/Time7/18/2015 at 1:57:53 PM


Short answer is no. He vapour barrier prebents the moisture from penetrating the house and if it is between two layers on insulation, the inside layer of insulation isn't doing the job it is intended to do.

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Date/Time7/19/2015 at 7:40:25 PM

The vapor barrier must be on the warm side.

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Date/Time7/21/2015 at 11:01:13 AM

Hello Olivia,

This is usually the way that insulation is installed,and this might quickly answer your question,and give you the tool to make the right decision on whether to add or simply remove your insulation.

* Warm side

* the Vapor or Moisture barrier

* the Insulation

* Cold side

You only would want a moisture barrier between the warm side and the insulation, because warm air holds onto more moisture than cold air, and when warm air meets cold , the air has to let go of that moisture as it cools, forming and that will form condensation.

having the moisture barrier will prevent the moist air from getting into the insulation where cold and warm meet.

If you put insulation on the wrong side of the moisture barrier,It could lead to some problems,leading to the moisture forming in the insulation, resulting in the creation of 2 layers of barrier, which will create a potential moisture trap.

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Date/Time7/28/2015 at 2:31:40 PM

Hi Olivia,

I am uncertain from your question exactly what you are referring to but I will try to answer for both scenarios.

If you are referring to an "attic" that has a 6mil vapour barrier on top of the ceiling drywall or plaster below and then insulation on top of that, then YES you can absolutely add as much insulation as you wish on top of that.

If you are referring to a wall assembly, particularly the typical basement wall where a builder met the minimum code requirement of fibreglass insulation that is attached to the foundation with plastic vapour barrier (like a large bag) to 2' below exterior grade and you would now like to finish the basement. Then NO, absolutely not. The vapour barrier must be removed prior to installation of new insulation.

The "bagged" insulation that meets minimum code is terrible anyway and you will usually find it full of water.

To clarify and what I think was being said in previous responses, is that the vapour barrier must be on the warm on winter side of the assembly and is there to keep warm, moist air from INSIDE the home from condensing in the insulation or against the exterior wall. It has nothing to do with moisture or water entering the home through the foundation. That is a completely separate issue.

The vapour barrier must be air tight to be effective. Slapping it up with a staple gun and rough cutting it and not overlapping or sealing joints is actually worse than not having one, particularly in a basement. You absolutely will wind up with mould in the walls and rot if you are using wood framing.

Also keep in mind that fibreglass insulation has a stated R-Value or Resistance to heat transfer. That is in optimal conditions and only when installed correctly and at its full thickness. Foundation "bagged" insulation is greatly compressed around the areas where the plastic is attached to the foundation rendering its value worthless.

Another key area to address is the areas where the joists (floor support) meet the rim joist or outside of the wall. This area is notoriously hard to insulate and even harder to seal with vapour barrier. Although it costs more, I would highly recommend having these areas sprayed with closed cell spray foam insulation.

Moisture condensing at the structural ends of floor joists leads to a lot of problems in the years ahead and mould in the shorter term.

Keep in mind if this is for a basement then you will require a permit from your Municipality and insulation is one of the inspections. The City Inspector can be a valuable ally and can often catch areas that are not properly addressed saving you a lot of money in damage and protecting the health of your family.

Lastly, in the City of Toronto, an Insulation contractor or any Builder / Renovator is required by law to have a Business License. While this does not assure you that work will be done correctly, it is definitely a step in the right direction.

I hope this helps. Best of luck.

Jason Irving

Cedarfalls Building Consultants Limited

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