# fact or myth: leave an air gap between insulation and exterior wall



## demandrew

A lot of people have been telling me that when installing insulation it is necessary to leave a bit of an air gap between your outer wall and your insulation in order to allow for air flow. How much of a gap has never really been specified, which is also confusing because I understood that bat insulation was sized to fit the entire 3.5 inch depth of your standard studded wall. 
If it is true that this space is needed, then why is it no longer the case when installing rigid or spray foam, as presumably these would be applied directly to the wall leaving no space at all? Is it because these materials are moisture resistant so the space is no longer needed?

What is your opinion?

I'm about to install a bunch of bat insulation and I'd prefer to not f*** it up too much. Should I be trying to "pull it out a bit" so an air space remains at the back? Would it be terrible if there was an air space at the front under the drywall?

Thanks for all your advice


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## concretemasonry

demandrew -

Above grade or below? What part of the world are you in and what is the climate?

How much faith do you have in the primary moisture barrier on the exterior of the wall and the quality of the installation of doors and windows if it is above grade? Moisture is a problem and some of the cheap pink (or other color) batts do not really absorb moisture, but hold it to supply moisture that can cause mold and deterioration and moisture does not go away unless the insulation is agitated and air is forcibly circulated.

Usually with batt insulation, your vapor barrier is on the surface of the insulation and an air gap there does little good.

I saw hundreds of homes after Katrina that had to be redone when they did not toss the wet batts and dry out the wood.

Dick


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## demandrew

This house is in Toronto, Canada.

The room in question is above grade. It is a kitchen on the top floor of the house and there is only one window. 

The exterior walls are all brick. I'm not sure what that means in terms of its primary moisture barrier, but after removing all the interior walls there don't appear to be any overly worrying signs of moisture. 

I do not have too much faith in the window, as it is pretty old, probably 20 years, but nevertheless the brick on the inside shows little signs of any issue. Of course this could be because it was never insulated in the first place so whatever moisture was present always had an opportunity to flow away? 

My whole situation is complicated by the fact that the walls in question which need insulating are of varied depths (see my other post on that: http://www.diychatroom.com/f103/insulating-walls-varying-depth-127134/)

Do you think it might help if I place down some rigid foam board instead of an air space?


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## Grampa Bud

Think about your question and the fact that your wall stud cavities are bordered by vertical studs and both top and bottom plates. Where do you think air flow would be in a wall covered on the inside with drywall, plaster, metal or wood, and on the outside with plywood, fiber board, siding, brick, stone, or metal? How about no where. You do need a moisture barrier (usually on the surface of the insulation batts) or 6 mil plastic indoors and the current application of Tyvek on the outside helps your building keep moisture outside so that you can control humidity inside.
The only places in your home that need air flow behind the insulation would be in a vaulted ceiling that follows your roof rafters to help keep the roof cooler and any exterior wall or floor that has concrete or brick/stone behind or under it to help flush out the condensed humidity from the surface of the concrete/brick/stone/


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## Yoyizit

A dead air space is a good insulator unless convection currents cause it to move.


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## demandrew

Grampa Bud said:


> Think about your question and the fact that your wall stud cavities are bordered by vertical studs and both top and bottom plates. Where do you think air flow would be in a wall covered on the inside with drywall, plaster, metal or wood, and on the outside with plywood, fiber board, siding, brick, stone, or metal? How about no where. You do need a moisture barrier (usually on the surface of the insulation batts) or 6 mil plastic indoors and the current application of Tyvek on the outside helps your building keep moisture outside so that you can control humidity inside.
> The only places in your home that need air flow behind the insulation would be in a vaulted ceiling that follows your roof rafters to help keep the roof cooler and any exterior wall or floor that has concrete or brick/stone behind or under it to help flush out the condensed humidity from the surface of the concrete/brick/stone/


I thought it sounded strange too which is why I was posing the question here on the forums. Lots of people who I've met with have told me that you need to leave an air gap behind your insulation but this idea didn't always make sense to me.

This question is of particular importance on one wall, where I will almost certainly have to press the insulation right up to the wall due to an extreme lack of depth in the wall cavity. I just wanted to make sure that having no air gap here would not have a detrimental effect on life expectancy of my insulation or house


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