More Musings of an Energy Nerd
Basements and crawlspaces can be damp. That’s why many homeowners assume that a basement or crawlspace needs at least one vapor barrier.
They’re right. But where does the vapor barrier go? Do you need one for your walls? The ceiling? The floor? And if some of these assemblies include insulation, you probably have another question: on which side of the insulation does the vapor barrier belong?
These are all good questions.
Two preliminary points
Before providing advice on vapor barrier placement in basements and crawlspaces, it’s worth answering two questions: Why are basements and crawlspaces damp? And what type of performance can we expect from a polyethylene vapor barrier?
There are three basic mechanisms that explain moisture accumulation in basements:
- Water can enter a basement or crawlspace in liquid form—through cracks in the walls, cracks in the slab, or the crack at the perimeter of the slab.
- Water vapor can enter a basement or crawlspace by evaporating from the interior surface of a damp wall or slab.
- Moisture in the indoor air can condense on cold surfaces in your basement, forming drips and puddles.
For more information on these three mechanisms, seeDealing With Basement Water From the Inside.
接下来的问题是:你能实验性能ect from a vapor barrier—for example, 6 mil polyethylene?
- You can expect polyethylene to greatly limit the migration of water vapor (that is, evaporation from a damp surface).
- If warm moist air ever contacts cold polyethylene, you can expect condensation to form on the surface of the polyethylene that faces the warm air.
- You can’t expect 6 mil polyethylene to stop liquid water, since water will almost always find a small hole or seam to seep through.
For example, let’s say you have an old-fashioned damp basement with uninsulated…
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9 Comments
It seems like this is mostly talking about new construction. Retrofits can be trickier--you might need a vapor barrier above the slab, followed by insulation and flooring. And for retrofit walls, there's an added incentive to keep the concrete dry at the top, because you might have wood resting directly on the concrete with no capillary break, or even have wood embedded in the concrete. Some people recommend some level of vapor permeability for interior foam insulation for that reason, even open-cell spray foam. One concept is to use open-cell spray foam for the top few feet and foam boards or ccSF for the lower parts. Any comments on that scenario? See for example this previous question of mine://m.etiketa4.com/question/crawl-space-open-cell-spray-foam
One strategy is to use mineral wool boards as exterior foundation insulation above grade to keep the wall warmer in winter and promote drying to the outside--at least in a cold climate. That could potentially backfire in a hot, humid climate, where that insulation would help keep the foundation cold in the summer, and allow humid outdoor air to condense on the concrete.
字符lie,
Thanks for your perceptive comments. You're right, of course, that in an older house that has no vapor barrier under the basement slab, it's impossible to install the polyethylene where it belongs without demolishing the slab. The usual solution is to install a layer of polyethylene above the slab followed by a continuous layer of horizontal rigid foam, followed by plywood or OSB. For more information on this approach, see"Installing Rigid Foam Above a Concrete Slab."(After reading your comment, I edited my article to include information on this issue. Thanks for the suggestion.)
Concerning your second concern -- an older house with an untreated mudsill resting direction on top of the concrete wall, or an older house with embedded joists -- I have provided several recommendations in my article,"Insulating Basement Walls With Embedded Joists."The best solutions tend to be site-specific, and depend on an analysis of the relevant risk factors.
I wish I could afford to "lay some sheets of OSB" in my crawlspace - or as I used to throw one into the mud on site to keep my boots clean. Now I'm considering piling a few sheets under my bed as an inheritance for my nephews when I'm gone.
Joking aside, a very useful blog.
Malcolm,
Use shipping pallets. That's what I have in my dirt-floored basement. After 10 years, you can replace them if they get a little soft (assuming, of course, that you know a local business that gives away old pallets).
Martin,
Several of my neighbours have started using them for infill in the walls of their pole built sheds.
However recently, rather than ask whether I minded if they left the pallet with the materials for me to dispose of, my lumberyard charged me a deposit which would be refundable when I returned the pallet to them. Crazy times!
Great article, but I am curious what the process and assembly is when using Roxul rigid insulation under slab and exterior of poured concrete basement walls? Also curious if there are other foamless solutions for new and existing basements and how to achieve moisture control with those?
User 7568882,
GBA has published at least three article focusing on sub-slab mineral wool insulation. Here are the links:
"Sub-Slab Mineral Wool"
"Insulating a Slab with Rigid Mineral Wool"
"Urban Rustic: Prepping for a Basement Slab"
I have written at least two articles with ideas for foam-free foundations and foam-free houses. Here are the links:
"Foam-Free Insulated Foundations"
"Building a Foam-Free House"
Great! thank you Martin.
What about retrofitting a old rubble basement? Specifically, isn't there value of an air barrier putting down a poly layer on the basement walls to tie into a new subslab poly layer bringing it up to tie into an interior air barrier up through the house?
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