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.
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:
16 thoughts on “Perforated vs. Solid Radiant Barrier Products. What is a vapor barrier and why does it matter?”
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.
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.
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.
Ed, I have been contemplating making shades utilizing your product ( actually they would be more like shutters). We have a home made with icf’s so our window sills are approx 8 inches. constructing a bi-fold shutter with some material on the exterior and using attic foil on the interior. I guess my main question is, do you feel the attic foil will still perform being covered by cloth, wood, etc. Any info is appreciated.
As long as the foil is open to an air space on one side of the foil, then it will work. However, I don’t recommend using the foil in an application where it will be continually folded and unfolded. Constant moving or reshaping of the foil might make it flake off and then it wouldn’t be as useful.
Ed,hi. We have indoor swimming pool in Minnesota, no problems. We are installing denshield in the interior they claim has a built in “vapor barrier” but only a perm rating of 1.0 should we use an additional vapor barrier behind it? currently 2×4 walls r11 batt in the wall cavity cinder block building exposed exterior.. The room is heated with hydronic baseboard heating, ventilation and de-humidification in place. We would like to use reflective insulation but understand the air place rule / multiple vapor barrier rules. What do you recommend in this situation. Thanks , Tim
Any material with a perm rating of 1.0 or less is considered a vapor retarder (meaning it doesn’t allow vapor to pass though), so it will work as a vapor retarder/barrier. If you want to help keep the heat in the floors with the radiant heating tubes, you can install a layer of radiant barrier below the tubing, again as long as there is an air gap between the tubing and the foil. Take a look at this application: Radiant Heat Flooring Above a Non-Conditioned Space
Just stumbled upon your blog here and the foil website. I do ice dam prevention systems on the top side with radiant roof edge panels. I do understand the importance of air sealing and insulation. Some of my customers need to address these issues first before considering heat cable systems. In the past I have referred this work to other contractors, but am concerned it is not being handled properly. I plan on offering these services. I like the radiant barrier idea, not only for winter, but also for summer conditions. I will pour over the information you provide. Thanks for the service.
Thanks for the informative site!
Which type of radiant barrier, perforated vs non-perforated, is more suitable for installation on *top* of the roof sheathing when installing a batten/counter-batten stone coated steel roof? (~3″ air gap between deck and steel)
Does the answer change depending on whether you use an underlayment under the foil? (I understand that traditional felt breathes, whereas most synthetics are quite impermeable.)
If it matters, my potential install is in Colorado.
You are correct that it does depend on which underlayment is used. If you are using a vapor barrier underlayment then the foil should be perforated so you don’t have two vapor barriers on top of one another. Otherwise, you can use the solid version of the foil to block radiant heat AND moisture. If in doubt, go with the perforated to avoid any potential problems.
Ed, I plan to add 3 1/2″ insulation under my house crawl space to the existing R19. My wife say’s floor in cold. We live in Myrtle Beach, South Carolina. Will perforated by OK. It will be placed under the existing R19 with has a vapor barrier on the top side under the plywood. Thank you
Yes, just staple the foil across the floor joists in the crawlspace – it will keep the heat that radiates from the house into the crawlspace from leaving the floor because the foil will retain 97% of that radiant heat. Check out the photos of this type of install here: Radiant Barrier Foil in a Crawlspace
I am wanting to keep the heat down in my attic and i began installing bubble wrap type radiant barrier just below my galvanized roof. I then put insulation below it. Since it is not perforated, will i have moisture issues? My ventilation is through the ridges all the way down both sides of my house coming from the ridge vent up top. I need to know before continuining my install. Thank you. Paul
Paul, we need more info to provide a good answer. Insulation below the bubble foil on the roofline? Are you venting the space below? Could you send us some pictures to support(at)atticfoil.com? With more details on the “layers” you are proposing and where and how air will flow.
I will be building a house in Tennessee in the next year or so and am looking to put a Radiant Barrier on my roof and walls. I will be using the ZIP R system on the walls and the roof if I can get my engineer to approve it. Saw it done in Texas. Now for details. The wall will be exposed Zip R water sealed etc with a .75 in air gap and Hardie Board Lap Siding. The Roof again will be Zip R also with an air gap and a standing seam metal roof. What I was wondering is, is it advisable to use and non vapor permeable radiant barrier on the roof or the walls or both. I was looking at vapor permeable on the walls since it would NOT be subject to much added moisture but if it did with Zip behind it I need it to dry. I would imagine the roof puts me in the same position but was wondering your thoughts. The attic of the house will be part of the conditioned space.