January 27, 2016

Question of the Week – 1/22/2016 – Where do I put my air barrier? What about my vapor control layer?

As we seek to increase the comfort and energy efficiency of our buildings by installing higher amounts of insulation, thinking about airtightness is great because airtightness has a disproportionate effect on heat loss and the optimization of the insulation.  But thinking about increased amounts of insulation and airtightness without properly considering moisture in the assembly is dangerous – paying attention to the control of this moisture is absolutely crucial for the structure and durability of the assembly.

Very quickly, we can become confused by where we should put our air barrier, and whether we need one or two – same goes for the vapor control layer.  This week we received a question from a Tiny House builder in upstate New York, and wanted to share with you our response to his question.

The question:

So, if there’s one vapor permeable air barrier on the inside, there doesn’t need to be one on the outside?  Could you reverse that, and put the vapor permeable air barrier on the outside (like with SOLITEX MENTO, a liquid applied product, etc.) and then go without the INTELLO on the interior?

 

The answer:

T,

A few thoughts on the placement of the air barrier/vapor control layer:

 

1) The more you think about the movement of vapor throughout the assembly, the better.  At the end of the day, you need to promote outward drying potential of the assembly, which is why you put a vapor permeable WRB on the outboard side of your assembly, even if you have a class II (low III) retarder as your sheathing (OSB, Plywood, etc.), which will slow down drying to the outside. You need the WRB for protection from intrusion of bulk water (wind driven rain), so when we think about whether this should be vapor permeable or vapor impermeable, we want it to be vapor permeable (climate zones 4, 5, 6, 7 and 8).  Use a system that’s waterproof, like SOLITEX MENTO 1000 with overlaps taped with TESCON VANA tape.

 

2) For many reasons, putting the air barrier on the inside is advantageous, but this doesn’t mean that you shouldn’t have what we call “windtightness” on the outside of the assembly.  Airtightness stops infiltration and exfiltration to and from the conditioned space, windtightness stops windwashing of the insulation.  An airtight WRB will maximize the effectiveness of your insulation, while an interior airtight layer will stop loss of heat and degradation of the insulation from convection, but most importantly keep humid indoor air from moving into and condensing in your insulated assemblies… air carries moisture, so airtightness on the inside prevents that moisture transport.  See this post for more:  An interior air barrier does it better.

 

3) We mustn’t equate an airbarrier/WRB layer with a sole vapor control layer, even though they can be the same thing.  Sure, you can put your airtight layer on the outside instead, but if you equate this to your only vapor control layer, what are you doing to stop moisture from getting to your outside layer of sheathing, and possibly condensing on that cold sheathing?  That’s why you need a vapor control layer on the inside of the assembly, which could be a vapor retarder (class II per IBC), or better yet – a smart vapor retarder.  Because INTELLO Plus serves the function of smart vapor retarder and airtight layer (and densepack netting), it is the ideal solution to improve both the performance of the wall (true installed R-value) as well as providing the best protection against condensation, mold and rot.

 

In summary, an airtight, vapor intelligent layer inboard of the insulation stops a) heat loss through air infiltration and b) moisture intrusion while increasing the inward drying potential of the wall, providing maximum protection from moisture damage; an airtight (or windtight), waterproof WRB layer stops a) bulk water intrusion and b) windwashing of the insulation, while also serving as your outboard vapor control layer (vapor open provides maximum outward drying).

 

 

Some perspective on what might work for you, dear Reader:

Remember, your climate affects your optimal wall assembly.  Have a question about whether the solutions suggested are right for you?  Contact the experts at 475 for help on designing an assembly – for any building size – which will minimize the risk of moisture damage while providing excellent levels of comfort and energy efficiency.

 

Thanks for reading, keep in touch!

To read the other posts in our Question of the Week series, click here.

**Please note that in republishing our question of the week, we sometimes edit the text of the original question and response in order to provide additional clarity to the issue.**

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