Air sealing and insulating a poured concrete basement
user-1137156
| Posted in PassivHaus on
I’m hoping to use poured concrete basement walls and wish to know weather I need to & if so how to air seal them. I want to use rigid rock wool as insulation. I also want all insulation on the inside of the concrete. The concrete basement walls support the outer of double stud walls and the roof/ceilings while interior basement walls support floor framing and the inner of the exterior walls. I
realize, of course, the concrete walls need to be designed to restrain back fill without bracing by the floor as is typical.
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Replies
Jerry,
Q. "I'm hoping to use poured concrete basement walls and wish to know weather i need to & if so how to air seal them."
A. Poured concrete walls are airtight. You probably want to caulk the crack at the perimeter of your floor slab, however.
Q. "I want to use rigid rock wool as insulation. I also want all insulation on the inside of the concrete."
A. That's a bad idea. Rock will is air-permeable, so the insulation is unable to stop warm, humid indoor air from migrating to the cold concrete. If you use this type of insulation, you'll get water trickling down the inner face of your wall when the wall is cold.
To insulate the interior of a basement wall, you want to use rigid foam insulation or closed-cell spray polyurethane foam.
Jerry
Have a look at the Thermomass system.
http://www.thermomass.com/
Robert Riversong has used this method and if I recall, he was highly impressed. Insulation is completely protected, and you get some benifit from the thermal mass of the interior wythe. Not sure if this is available in your locale...
Martin,
I' was reading Musings of an energy nerd & came upon one about basements insulation & vapor retarders. Here is a quote:
"Here's the news story: Lstiburek now says that the advice he gave in the Builder’s Guide was wrong. Lstiburek said, “I made a mistake. The insulation just needs to be warm enough to control condensation from the inside. The perm rating doesn’t matter. It’s OK for the concrete to be wet. The concrete doesn’t have to dry to the inside.”
Lstiburek's former recommendation to use vapor-permeable foam was due to a concern that imperfections in the foam installation might allow some warm interior air to contact the cold concrete, leading to condensation. However, experience has shown that most foundation walls, even those with small amounts of condensation, experience some drying activity at the top of the foundation wall. "
What's the problem!?? Water does no harm to concrete or mineral wool & if it happens to make it to anything at or near the indoor air temperature it'll soon evaporate. In my plan I have 3" of 8# density rigid mineral wool ( r 12) between concrete and a 2x4 wall with cavities filled with r 15 mineral wool bats. Even the wood in the stud walls will all be really close to the indoor air temperature as it's only r1/" from the warm side of an r 12 thermal break. A similar argument for for the plate at the bottom as it's resting on concrete that is at room temperature.
Jerry,
Q. "What's the problem!?? ... In my plan I have 3 inch of 8# density rigid mineral wool (R-12) between concrete and a 2x4 wall with cavities filled with R-15 mineral wool batts."
A. Here's the problem: you have chosen air-permeable insulation. Mineral wool batts will not stop the flow of air -- and when moist indoor air contacts the cold concrete, the moisture in the air will condense, run down the wall, and pool at the bottom. You'll end up with a wet, moldly, and eventually rotten bottom plate -- not what you want.
In an actual building, codes would require a perfunctory air barrier on the inside of my walls ie. drywall.
Martin, is your assertion based on actual test results or theory? Air flow doesn't just happen, it requires an energy source, such as temperature differences or wind. Dense mineral wool, as in the 8# density I'd use next to the concrete is claimed to not be vulnerable to " wind washing" which implies a significant degree of air motion friction. One can play a propane torch on one side of a 3" mineral wool board and and not feel a draft on the other side ( PLEASE don't try that with EPS). As condensation runs down the wall it's temperature will rise.
Long before it makes it to the floor it'll be over 45 degrees f, the dew point of 70 degrees & 40% RH & evaporation will begin. Should it make it to the floor it might pool behind the mud sill plate of pressure treated wood, again no harm. Dense mineral wool boards are recommended, by their manufacturer as replacements for vapor permeable foam IN THIS APPLICATION!
Jerry,
My advice is based on theory, not direct experience or a specific study.
The only source of the air between the fibers of your mineral wool is the interior. There is no exterior air involved. Your interior air is warm, and therefore it holds moisture. All of the air between the fibers is this type of air.
As this air gets colder (approaching the cold concrete), the moisture will condense. That's just physics.
As the water dribbles down the cold concrete, it's not going to get significantly warmer. The bottom of a concrete basement wall in February is going to be cold, especially if it is insulated on the interior.
Look, it's your house. Do what you want. But based on my understanding of physics, I would never insulate a basement wall on the interior with mineral wool.
Martin,
I understand and respect your 'position' but also dislike the plastic foam alternative especially EPS.
I, personally, have had to replace EPS used to insulate a spa's cover more than once due to water saturation! The spa covers became too heavy to lift and, no doubt, were seriously degraded as insulation. Had the spa's cover been insulated with mineral wool the condensation within the insulation would have, harmlessly, drained away.. Any moisture permeable insulation will experience condensation within the insulation under some circumstances! How and how soon that, now liquid, water gets out of the insulation depends on several properties of the material. If the insulation allows "drainage" at least some of the water can escape by that route, without 'drainage' the water's only escape route is vapor diffusion a much slower process that requires specific conditions, which in the case of my spa covers never occurred. Based on my personal experience with water saturated plastic foam, I come to exactly the opposite conclusion! Isn't it better to drain the liquid than hold it for who knows how long?
I don't think Martin is suggesting that you use EPS on the basement walls. Closed cell spray foam or XPS rigid foam are the only materials recommended for basement applications. I have no experience with rigid rock wool so cannot speak to its use as a basement insulation.
I have no experience with XPS as a spa cover but. Manufacturer data for XPS shows some, all be it small vapor permeability so I'll avoid it where I can use drain able rigid mineral wool. In the case of under slab insulation the compressive strength pretty well excludes mineral wool and suggests HD XPS but it is installed over polyethylene and is not near as likely to encounter internal condensation conditions, so it should have a long life. BTW the EPS in the spa covers was wrapped in 6 mil polyethylene with taped seems so the vapor was getting through the poly vapor retarder slowly & continuously for several years..
Jerry,
The vapor drive from a 104 degree F hot tub towards the lid (especially in winter) is entirely different from the vapor drive from a basement towards a cold basement wall.
Either XPS or EPS will work fine to insulate a basement wall, as tens of thousands of successful installations have shown. However, I still maintain that mineral wool insulation is risky because it is air-permeable.
FWIW my hot tub story took place when I lived near Portland Or. The spa temperature was 102f. most of the time outdoor temperatures were 50 to 70 f with rare excursions below freezing. The spa cover is always subject to conditions conducive to condensation within the insulation & I have observed it. Moisture trapping by foam Is not an attribute of plastic foam that's often considered but water absorption & eventual saturation definitely happens.
Why don't you try an experiment and replace the insulation in your hot tub lid with rigid mineral wool and see what happens?
I have cellars with glass, EPS and XPS and in dry cellars all are fine. Frame with PT framing and paperless drywall; might work. Try it and let us know.
A dry cellar has few problems. A wet cellar is nothing but problems.
Jerry:
You asked if your assembly needs air sealing. Building Science 101 and an overwhelming majority of really smart people with a ton of real-world experience think it's a good idea to keep warm, moist air out of wall assemblies. Your reply: "What is the problem?" The problem is that basements with liquid water behind the wall assembly are damp, smelly and inferior to dry ones.
You can't assume that using rock wool and treated wood will prevent a musty, moldy basement. There are houses with basements that are conditioned and only raw concrete yet are damp and thus musty and moldy.
You are unsatisfied with your spa cover because it uses foam insulation that absorbs water via vapor drive, yet a product with 10X the perm rating would be a favored alternative? You do realize that the perm rating determines the rate that moisture can enter/exit a material? Moisture will continue to enter the insulation until equilibrium is achieved. For high perm material (rock wool) this will happen faster than low perm (foam).
How much water does a sponge hold if you don't wring it out? Capillary action keeps the liquid water inside the sponge (rock wool). It does not necessarily all drain out the bottom.
I don't think you can directly associate a material's perm rating with its capacity (or lack of capacity) to hold bulk water. Some materials experience an increase in perm rating when they get wet--it's not always intuitive.
If you air seal and use low-perm insulation, you don't have liquid water in your wall assembly and your basement is dry.
Bill,
I do not disagree with "You asked if your assembly needs air sealing. Building Science 101 and an overwhelming majority of really smart people with a ton of real-world experience think it's a good idea to keep warm, moist air out of wall assemblies" Air sealing the interior via ADA or similar is of even more importance if the chosen insulation is mineral wool. The question I asked still hasn't been answered!
Your statement "Moisture will continue to enter the insulation until equilibrium is achieved. For high perm material (rock wool) this will happen faster than low perm (foam)." Is half right and all wrong in the rock wool case because you neglect drainage. The statement "If you air seal and use low-perm insulation, you don't have liquid water in your wall assembly and your basement is dry."
while possibly true some of the time conveniently ignores the liquid water trapped in the foam!
From what little I know, once air sealing is adopted ,even with a moisture permeable air barrier, on the inside the air "tightness" of the insulation becomes irrelevant. The choice is air seal and use drain able insulation or ignore air sealing, depend on airtight foam and live with liquid water in the foam.
Jerry,
You wrote, "In the rock wool case ...you neglect drainage."
Yes, rock wool insulation will drain. If you install it against the interior side of a cold basement wall, the moisture will drain to the bottom plate and pool on your basement slab.
"The question I asked still hasn't been answered!"
Wouldn't it be fairer to say that your question has been answered repeatedly, you just don't like the advice you are consistently receiving?
Never saw wet rigid EPS or XPS foam sheeting. Rigid polyurethane kept submerged in water does over time become saturated (dock builders have learned this too often the hard way). Starting with a dry cellar humidified and climate controlled and rigid foam will stay dry. End of debate.
""Wouldn't it be fairer to say that your question has been answered repeatedly, you just don't like the advice you are consistently receiving?
NO, the question I asked was both weather to and how to air seal a concrete basement having rigid mineral wool and stud wall also with mineral wool as interior insulation. I got half an answer sort of in (with no mention of air sealing) there WILL be liquid water "running down the wall" due to air permeability.. I also was told that my choice of materials was "BAD". and an alternate material was strongly suggested. Later in the discussion it was admitted that liquid water running down the walls was hypothetical, not based on any test or study etc. When I pointed out an experimentally proven weakness of the suggested alternative material, the materials were defended only with statements of their wide acceptance. How many "wet foam" lawsuits will there be in future years?
The simple fact is that air permeable insulation dominates the building industry and works really well, with proper air barriers. Plastic foam may not need an air barrier but drywall is required for fire resistance. Mineral wool may not need drywall for fire resistance but it's prudent (maybe necessary) as an air barrier. As there are thousands of foam insulated basements, I'll wager there are also at least hundreds, if not thousands, of basements insulated with rigid mineral wool.
Jerry, glass is used in cellars. None of us recommend it. You certainly will use it and you will have to find others that know how to guarantee success. And whatever you do may be fine. Let us know.
Here:
http://www.buildingscience.com/documents/reports/rr-0202-basement-insulation-systems
Malcolm,
Thank you for an excellent reference document. From this document an answer to my initial question appears to be: install Tyvec or equivalent on the outside of the stud wall yet inside the rigid mineral wool boards and detail it as an air barrier. This meets the second bullet's requirement despite the rigid mineral wool's air permeability and provides the capillary break mentioned in bullet 3.. For reference here are the reports requirements.
"Requirements for Interior
Basement Insulating Systems
Any interior basement insulating wall system must have the
following properties:
• It must be able to dry to the interior should it become wet
since the below grade portion of the wall will not be able to
dry to the exterior during any time of the year. This
precludes an interior polyethylene vapor barrier or any
impermeable interior wall finishes such as vinyl wall
coverings or oil/alkyd/epoxy paint systems.
• The wall assembly must prevent any significant volume of
interior air from reaching the cool foundation wall. Thus it
must have an effective interior air barrier or a method of
elevating the temperature of potential condensing surfaces
(such as rigid insulation installed directly on the interior of
concrete or masonry surfaces).
• Materials in contact with the foundation wall and the
concrete slab must be moisture tolerant; that is the
materials should not support mold growth or deteriorate if
they become wet. However, moisture tolerant materials are
not necessarily capillary resistant. That is, some materials
may tolerate being wet without blocking the passage of
liquid water through the materials. A capillary break must
be placed between these materials and moisture sensitive
materials."
Jerry,
Joe Lstiburek has already changed his mind on at least one piece of advice in that report you quote -- where he says, "An interior basement insulating wall system ... must be able to dry to the interior should it become wet since the below grade portion of the wall will not be able to dry to the exterior during any time of the year."
To read about more on this issue, see Joe Lstiburek Discusses Basement Insulation and Vapor Retarders.
Martin,
Yes, see post #3 in this thread. The change of mind was not in the "Requirements for Interior
Basement Insulating Systems". Joe now says "However, experience has shown that most foundation walls, even those with small amounts of condensation, experience some drying activity at the top of the foundation wall. "
Jerry,
One caveat I might add is to design your wall systems for the life of the building and whatever eventualities may occur. One of my concerns with both walls that rely on drying to the interior or those that rely on exterior sheathing as an air barrier is that they make certain assumptions about the way the future occupants will use the house. A cursory look at the houses around me shows that people paint their rooms with whatever they want without regard to its perm rating and tend to poke holes in their sheathing for everything from TV cables to vents for their marijuana show-ops - and this behaviour is unlikely to change. A forgiving system, even if a bit less efficient, beats one where the effects of common changes cause real problems everyday.
Jerry, the Owens Corning basement wall system is by far the most forgiving cellar wall. Why, because the panels are easily removable to do whatever. And buying from them gives you warranty and liability by others.
Gettin time to build Jerry. Grab your hammer.