More Q&A Spotlight
Member “otinkyad”—let’s call him Oti—has been reading about managing moisture and insulating in basements, and he has some questions, which are posed in this Q&A post. He thinks he understands the premise of “How to Insulate a Basement Wall” i.e., polyethylene shouldn’t be used as a vapor retarder anywhere in a basement.
But then Oti read a more recent article, “Vapor Retarders for Basements and Crawlspaces,” which recommends installing poly over uninsulated, vapor-porous slabs that are often found in older homes. He’d like someone to explain why a vapor retarder is recommended for a basement slab but not for basement walls.
Commenters point out the difference between a wall assembly’s ability to dry to the outside vs. to the inside, and the influence a vapor retarder such as poly would have on those processes in a concrete basement. DCcontrarian explains, “Moisture moves from warm to cold and from wet to dry… To the extent a basement wall can dry, it’s to the interior only, so it has to be vapor-open to the interior. At the same time, it needs to be vapor-closed to the concrete, otherwise you’re trying to dry all the moisture in the soil through your walls.”
He continues, “Placed on the wrong side of the wall assembly, poly can trap vapor, where it can condense on the cool concrete wall surface.”
There is some discussion, too, between how to handle vapor retarders in basements vs. crawlspaces. Typically, the latter is at least partially above grade, and therefore is likely to have fewer moisture issues due to greater air circulation and drying.
That said, the question for this Spotlight is: Why is it okay to install a vapor retarder over a basement slab, but not on the…
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10 Comments
I see that I didn't pay close enough attention to the specific question regarding slabs, of placing a vapor retarder OVER a porous slab. In his article (https://www.greenbuildingadvisor.com/article/vapor-retarders-for-basements-and-crawlspaces), Martin says you can install poly over the slab, but he also says you need foam insulation and a plywood or OSB subfloor over it. I fully agree; poly alone would likely result in condensation, but the foam prevents condensation. Foam needs to be covered with a fire-resistant layer and it also needs to be something you can walk on, which I'm sure is why he recommends plywood or OSB. Andy Engel's classic article explains the finer points: https://www.finehomebuilding.com/2005/03/01/the-no-mold-finished-basement.
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My basement has high-humidity, but no water-intrusion or condensation issues that I've noticed. Is the only safe way to dampproof from the interior to use impermeable insulation? Or is there (ideally) a durable vapor-retarding coating for the slab and walls? Such a coating should reduce the humidity and make condensation even less likely...assuming the floors/walls are the dominant sources of vapor.
I suggest reading these articles--there's a lot to say on the topic!
https://www.greenbuildingadvisor.com/article/installing-basement-waterproofing-from-the-negative-side
https://www.greenbuildingadvisor.com/article/interior-waterproofing-of-basement-walls
https://www.greenbuildingadvisor.com/article/dampproofing-and-waterproofing-foundation-walls
The foundation blanket in the picture above, where the ploy is attached to a roil of fiberglass, is code compliant here and most commonly used due to low cost. I've seen it soaked with moisture and water during construction before it's covered with drywall. The code is focused more on providing a continuous insulation layer on the foundation walls rather than moisture issues.
It looks like an insulation blanket, or as Dr. Joe Lstiburek likes to call it, diaper insulation. But in the caption Martin Holladay used in the article author Chuck Bickford grabbed it from, he assumes that it's plain poly sheeting, which could also be the case, as it's available in white.
The 2021 IRC has some updates on insulating basements: https://codes.iccsafe.org/content/IRC2021P1/chapter-11-re-energy-efficiency#IRC2021P1_Pt04_Ch11_SecN1102.2.8, compared to earlier codes--updates are in blue--but I am not aware of specific installation requirements, which would be great to have, since there is so much confusion on the topic.
AC200,
Those foundation blankets are the most common way to insulate crawlspace walls here. I would never recommend them, but I've moved lot of them over the years and never seen either moisture or mold problems. That good fortune is probably unique to our temperate PNW climate.
But then Oti read a more recent article, “Vapor Retarders for Basements and Crawlspaces,” which recommends installing poly over uninsulated, vapor-porous slabs that are often found in older homes.
For the record, Joe Lstiburek and BSC are *not* proponents of (1) Wet existing slab + (2) polyethylene + (3) new finish floor, as covered in this article.
BSI-082: Walking the Plank
https://buildingscience.com/documents/insights/bsi082-walking-the-plank
So what do you do if you don’t have a low w/c concrete and you don’t have time? And even worse, what do you do if you don’t have an effective vapor barrier and capillary break? Well, you go back to the old way of doing things. You install a topside vapor barrier. But you do it smart with new materials.
The smart way is a fluid applied vapor barrier (Figure 7). Epoxies work well. The dumb way is to use a plastic sheet.
Why is using a plastic sheet a dumb idea? Folks on the Internet love it. It has got to be the right way to go if it is on the Internet. If the vapor barrier layer is not completely fully adhered condensation will occur in air pockets. If these air pockets communicate with the interior you will grow mold and algae. And the floor will smell. What are the odds that a thin sheet of plastic will be installed in an absolutely airtight manner on the top of a concrete slab where the perimeter of the plastic layer is sealed in a continuous airtight manner to the concrete and where the thin sheet of plastic will survive the construction process and not have any holes in it such as holes necessary to fasten the flooring?
That the polyethylene plastic sheet recommendation still comes from many in the wood flooring industry including wood floor trade associations is flabbergasting and disappointing. You would think they would know better. The old guys knew better with the bitumen layer. But apparently they all died off and didn’t tell the new guys. Come on guys, put the plastic sheet under the concrete not on the top of the concrete.
(NJ here) Here’s what I tell my clients about each of these questions:
Basement walls: I’d prefer that the walls can dry to the inside so I don’t spec any vapor barriers for the walls. My concern is what all that moisture behind the wall will do to the will plate especially if it’s just wood on masonry (old home). If you seal the wall from the inside much of the vapor drive will go up and hit that plate. On my 1945 home the basement walls were already sealed from the inside with a fluid applied VB like Drylock or similar; so what I did was cover the exterior of that wall with a rainscreen so that it doesn’t get more wet from rainfall but is always allowed to dry to the outside.
Basement slab: Of course we want poly below the slab but what do you do when it’s an old slab in good condition but has no VB below? In my 1945 home I applied a fluid-applied VB membrane (redgard) as a on top below the finish/cork floor underlayment. I would have gone with poly but I was concerned that I might experience condensation forming in the small air pockets and cavities between the VB and slab surface. Because the fluid applied VB is painted on, there is no space for condensation to form below it. Next time I will specify drylock which is much less expensive and should perform just as well.
When I-Joists are the FF Deck: I ran a framing business for 15 years and now am a building science nerd / architect. I can tell you that if I see I-joists at the basement ceiling then that means the bandjoists or rimboard is made of an OSB material. I do not trust OSB as a First Floor deck rimboard for longevity which is why I prefer to only use I-joists above the First Floor deck. All that plastic on the walls makes it so that the moisture really only has one place to congregate and that’s at the plate near all that structural OSB product. I don’t love it.
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