In an attic application, you should ALWAYS use a perforated product. Period.

Perforated Radiant Barrier Foil

Why? Solid products like bubble foil insulation are called “Vapor Barriers”.  A vapor barrier basically stops moisture from moving from point A to point B.  Or, another way to view this is that a vapor barrier will “TRAP” moisture.  I’m not going to get into the technical definition of what defines a vapor barrier (perm rating, etc), but here is an illustration of what IS and IS NOT a vapor barrier.

If you take a wet block of wood and put it inside a bag or an envelope made of perforated radiant barrier attic foil the wood block would eventually dry out.  Therefore, perforated radiant barrier is NOT a vapor barrier.  Moisture ALWAYS goes from wet to dry.  If you did the same test with the wood block inside a plastic bag the wood would still be wet months from now.  If moisture in its vapor form cannot pass through or object won’t “dry” then the product IS a vapor barrier.

Why is this important?

Virtually all (over 70%) of home issues are due to mold, mildew, rot, decay, etc. And moisture is the common theme here.  DRY products don’t grow mold, rot or decay. The bottom line is that moisture in wall and ceiling assemblies is not a good thing.  You want DRY walls and ceilings.

In cooler weather, the moisture INSIDE the home is greater than OUTSIDE.  Think cold and dry.  This is why our lips get chapped in the Winter and not in the Summer.  Since moisture will naturally move from wet (inside) to dry (outside) it will pass through sheetrock, insulation and then into a typical attic.  The LAST thing we want to do is TRAP moisture here.  Moisture, attic insulation and wood do not go well together.  A perforated radiant barrier will allow moisture to pass on through into the attic.  We want our attics to be cool and DRY.

Using a perforated radiant barrier will not change the effectiveness of the reflectivity.  Attic Foil has tiny pinholes about every ½ inch that allow for water in its vapor form to pass through (see picture).  These holes make up a TINY percentage of the surface area and will not change the effectiveness of the radiant barrier.

Solid products like bubble foil insulation are usually not perforated and are a recipe for disaster when installed inside an attic.  Solid (non-perforated) bubble foil is a great product when used correctly in applications like metal buildings.  The problem is that solid bubble foil products are often MISUSED in residential attics. This is especially true if the bubble type reflective foil products are laid directly over the attic insulation.  Moisture will pass through the sheetrock and will get trapped in the insulation below the bubble foil insulation.  This moisture will accumulate until it either condensates (turns to water) or freezes (turns to ice).

This is why it is critical to use a perforated tarp-like radiant barrier product. It will give you all the benefits of reflective insulation without the potential for moisture to get trapped.

I've written several other posts on this that you might be interested in. Check these posts below:

How A Convective Loop Works Against You

Radiant barrier is rapidly gaining popularity in cold climates. It’s not just for hot climates like here in Texas. Being a Texas guy, cold to me is about fifty degrees and REALLY cold is when it actually reaches freezing. Admittedly, I don’t have a ton of first hand experience with extended cold periods.

A few years ago when I first started selling radiant barrier I was pretty surprised by how many of my customers were from colder climates. I asked why they were so pleased with the foil and the response was always the same. “Comfort, Comfort…Comfort …oh, and some good energy savings too”. After a while, I would see orders from 3 or 4 houses on the same street. News of the good results would travel fast. Today, some of my largest commercial customers are in colder areas.

I was a little confused on why the word of mouth sales were so strong in cold areas. I started doing some research on everything I could find on radiant barrier installed in cold climates. I found information (limited studies and research) on why radiant barrier was NOT a wise investment in the cold climates. You can do the same research and you generally find that adding more insulation is always the recommended way to go. It seemed like that all my happy customers who were telling friends about their improved comfort and lower energy bills must be wrong.

The Aha Moment.

I was sitting in a building science seminar a couple of years back and the speaker said something like this: “The way we insulate ceilings in the North is just plain stupid. It would be like wearing a jacket WITHOUT the outer wind protection layer on a cold-windy day.”  Aha!!!

He also said that a ceiling should basically be a wall turned sideways. Here are the layers of a typical wall – Outside air barrier, insulation/structural, inside air barrier. The problem is that to save money, many homes have skipped the outside air barrier in the ceiling assembly. You still have the inside air barrier (sheetrock), and insulation but NO air barrier on the attic side of the insulation.
Insulation does virtually NOTHING to stop airflow. Most insulation is made of fiberglass. Think about this, most AIR FILTERS are made of fiberglass. Why? Because fiberglass allows maximum unrestricted airflow. The whole concept of insulation and R-Value is based on “Dead” or “Trapped” air. Ideally there is very little air movement inside a wall, jacket or a down comforter thrown on the bed. Without air barriers and a little wind, insulation can become almost worthless.

Installing a radiant barrier directly over the attic insulation does two things: 1) Reduces Radiant Heat Loss. 2) Minimizes Air Movement – called convective looping inside the attic insulation.

Reducing Radiant Heat Loss – By laying the radiant barrier attic foil OVER the existing attic insulation, it works off the emissivity quality of the pure aluminum foil. This is like wrapping a potato to keep it hot. The foil helps keep the stored energy in the potato (or insulation) from easily converting to radiant energy.

Minimizing Air Movement – We know that air flows easily through attic insulation. The problem is that cold air is heavier and denser than warm light air. Without the top air barrier, the cold dense air literally “falls” through the insulation and displaces the warm air trapped in the insulation close to the ceiling. This is called “convective looping” or the pumping of air through the attic ceiling insulation. It’s a commonly known fact that the R-Value of insulation decreases as the temperature drops. Combining cold temperatures AND air movement dramatically decreases the effectiveness of traditional attic insulation.

Installing a perforated radiant barrier over the existing attic insulation acts similar to the outer wind layer on a Winter jacket. Internal air movement will be reduced resulting in higher R-Value performance of your existing attic insulation.

CAUTION: You MUST use a perforated (heavyweight is best) radiant barrier for this type of over-the-insulation application. The tiny pinholes will allow water in the air (water vapor) to pass through the foil to prevent moisture from collecting inside the attic insulation under the foil and turning to water or ice. Also, make sure the ceiling is sealed airtight. Holes from lights, and other fixtures can leave a direct path for a surplus of warm-moist air to enter the insulation and can condense before being able to dry out.

It’s really a pretty simple installation. Just lay the radiant barrier foil out directly over the existing attic insulation. Get as much as you can and don’t kill yourself if you can’t get every spot. You can do-it-yourself or hire a professional installer.

Finally, don’t forget that radiant barrier is like shade in the Summer. Even “Cold” areas need some relief from the Summer heat.

I guess my cold weather customers were right after all. Year-round comfort and savings for a minimal investment is definitely an Aha moment.

Please let me know your experiences in cold weather applications.

I've written several other posts on this that you might be interested in. Check these posts below: