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Air barrier on top of Roxul insulation

From everything I read, I need an air barrier on top of the roxul insulation we have in a vented attic space we are renovating. I put the roxul down and am struggling to find a cheap, easy way to create an air barrier without causing moisture issues.

I'd love to buy some rigid foam to help the Rvalue, but am worried about creating a moisture issue.
Tyvek is expensive

Are any of these cheap ideas viable?

I had been thinking of going to an appliance store and begging for free sheets of cardboard, but never got around to it.

I bought the red paper which goes under flooring, but it doesn't seem durable.

I have a few pieces of sheetrock (with some mold spots on them because they were in a workshop) I am ready to put at the curb, along with several old doors.

Any thoughts would be appreciated...
This is a dormer area and there might be alot of air movement to get to the baffles under the cathdral ceiling in the neighboring roof on it's way to the ridge.

Asked by Nick H
Posted Oct 23, 2012 5:58 PM ET
Edited Oct 23, 2012 6:59 PM ET


9 Answers

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Insulation installed on the floor of an attic generally doesn't have a top-side air barrier.

It is usually cheaper to just add more insulation on top (if you are worried about R-value) than to install any air barrier products on top of the insulation.

Answered by Martin Holladay
Posted Oct 23, 2012 6:55 PM ET
Edited Oct 23, 2012 6:57 PM ET.


Martin, do you have any information on why it's ok to skip the air barrier in this location?

Answered by Michael Maines
Posted Oct 24, 2012 8:29 AM ET


For some reason, this question is coming up a lot lately. There have been several recent comments on the topic posted on another Q&A thread: http://www.greenbuildingadvisor.com/community/forum/energy-efficiency-an...

Here's the basic story: Unlike all other locations for insulation in a house -- places like wall cavities and cathedral ceilings -- most attic floors have few, if any, restrictions on total insulation thickness. It's usually easy to pile on a little more, especially if the attic floor is insulated with blown-in insulation (ideally, cellulose) rather than batts.

There is a slight degradation in the performance of insulation on an attic floor due to wind-washing and internal convection currents, especially in very fluffly fiberglass insulation at very cold temperatures. Newer formulations of blown-in fiberglass insulation don't suffer as much of a degradation as the older, fluffier fibers of 30 years ago, and high-density batts perform a little better than low-density batts.

The cheapest and easiest way to improve the performance of insulation on an attic floor is simply to blow a little more insulation on top of what you have. That's cheaper than a layer of Tyvek. That's easier to install than a layer of Tyvek. And that will perform better than a layer of Tyvek.

If you are worried that your 12 inches of insulation only perform like 8 inches of insulation when it's really cold outside, the solution is simple: just install 16 inches of insulation!

Answered by Martin Holladay
Posted Oct 24, 2012 8:47 AM ET


That makes sense, thanks for the explanation!

Answered by Michael Maines
Posted Oct 24, 2012 9:00 AM ET


Roxul batts are high-density and fairly air-retardent compared to low-density fiberglass, and would only get modest benefit from a topside air barrier. But they WOULD benefit from an over-blow of cellulose to fill in all gaps (and add more R.)

In a cathedral ceiling the baffle can be the air barrier. If you want to use rigid foam as the baffle an inch of unfaced EPS or half-inch XPS are sufficiently vapor permeable to have PLENTY of drying capacity (about the same permeance as latex paint) and adds a modest amount of R-value, but it's time consuming.

Answered by Dana Dorsett
Posted Oct 24, 2012 11:08 AM ET


I was trying to digest your explaination Martin. I won't have guessed for you to say that.

Maybe I didn't describe my attic well enough because it isn't a flat attic - those ones shown in all the pictures of "how to insulate your attic." Or maybe I'm just worring to much because I know the insulation isn't perfect between the layers. That's why I had been leaning originally towards rigid insulation on top of the hole thing.

But to put more insulation on would cost more than the tyvek, so that is why I was thinking a cheap air barrier would be good.

I didn't "super insulate" because I am a cheapskate right now. Money is tight.
About R45 using the Roxul on the top and R30 where the living room wall is exposed.
And I'm in northern NJ.

Answered by Nick H
Posted Oct 24, 2012 5:24 PM ET


On any flat open-attic situations you'll do much better with blown than with batts. It's an easy DIY with a low-end rental blower (often free if you go through a blue or orange box-store for the cellulose, but have them order up "stabilized" cellulose designed for wet-spraying, since it's guaranteed to have no sulfates in the fire retardents like most of their off-the-shelf goods have.)

Air seal as much as you can BEFORE you install the insulation though- take your time, be thorough. The less air leaks through the ceiling into the attic, the less moisture there is purge in winter.

Answered by Dana Dorsett
Posted Oct 24, 2012 6:19 PM ET


I wish my attic was the flat, typical type...would make it much easier. DIY blown in wouldn't have made sense for this small area (blower would not have been free) and I still would had to figure out the "wall" area. So batts seemed better at the time.

I did try to air seal the living room ceiling as best as I could at this point.

Answered by Nick H
Posted Oct 24, 2012 6:58 PM ET


Nick H,
It's hard to imagine the attic you've described.
But I think Martin and Dana are offering some good general advise...

Sounds like you've already upgraded the insulation some - which is most likely better than not having done anything...

If it seems tough to blow cellulose in there, it'll probably be even tougher to install any kind of decent air barrier that is worth the time and money invested.
Unless attic access is impossible after renovation, consider blowing in a cellulose cap down the road.

Answered by Lucas Durand - 7A
Posted Oct 25, 2012 12:31 AM ET

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