There is some debate in the radiant barrier business whether to use a perforated or a solid radiant barrier product in an attic application.
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:
Tags: Attic Insulation, foil installation, perforated radiant barrier, r-value, radiant barrier foil, radiant heat
I live in Michigan, I want to use your Radiant Barrier in my attic. I want to staple to the bottom of the roof rafters. I am going to leave 3 inch space on the bottom and three inches on top. I like to use non-perforated barrier, I think it does a better job. Since both sides will be ventilated and the same temperature, do I have to worry about condensation, or can I use it?
The perforated and non-perforated radiant barriers work exactly the same. The holes are tiny pinholes about 1/2″ apart. The difference in effectiveness of the two products – (reflectivity and emissivity) is so small it’s not even measurable. Vapor barriers should be used with caution regardless of location. The general theory is that you NEVER use a vapor barrier unless you are SPECIFICALLY trying to stop/trap moisture in it’s vapor form. In an attic, you WANT air and moisture to flow freely from from maximize the drying process. Therefore, it is NEVER suggested to use a vapor barrier in a vented attic application. If the effectiveness is the same, why risk introducing something that can slow/stop moisture flow out of the attic?
Also, you CAN get condensation on ANY surface if the temperature is below dew-point and you have a source of warm-moist air. Ventilation helps, but you REALLY want to eliminate the source of warm-moist air.
Ed-
I obviously made the mistake of installing bubble wrap in my attic to the bottom of my rafters. My friend told me to get it not because it was more effective but because it has less chance of it ripping. I am not too hep on ripping it out and starting over. What should I do to try to eliminate water vapor from being trapped and constantly evaporating, condensing etc until it breaks down my sheathing? I have a pretty tight install and I now know it wasn’t needed or preferable. Please give advice on any action that I should take.
Signed,
uninformed consumer….
Uninformed,
No worries. A simple solution to this problem would be to cut some holes in the bubble foil or to make some slits – your goal is going to be to make sure that air can flow freely on both sides of the bubble foil.