# Basement Wall- Insulate or Ventilate?



## Rob D. (Mar 26, 2009)

I'm in the middle of a mold remediation project and I'm trying to decide whether I should close-up the top of a 1.5" air gap that is between the existing wood stud basement interior walls and the concrete foundation. Here's a cross section of the construction as is:









We dry-walled, painted and poured a gypcrete floor over a 3 week stint in November and outside temperatures dropped under 0F for days at a time December-January. Not surprising that mold started to take off behind the ill-advised fiberglass batting we installed in the rim joist areas. 

I just had the rim joist area sprayed with closed cell foam making sure to leave the 1.5" gap open. Before I close-up the ceilings again, I'm wondering whether it would be better to leave the gap open for circulation (which could be enhanced with floor and ceiling vents) should moisture from the foundation wall or icing on the foundation wall from inside humidity ever become a problems. Or should I close-up the gap as far down as I can with low-density door and window foam to reduce the cold that transmits through the 10" cement foundation and reduce the chance of icing at least at the part I can still access. Half of the foundation wall on one side of the house is not covered with earth, so the cement in that wall gets pretty cold here in Wisconsin.

Any opinions about how to work with what I have? 

You can see that I have drain tiles on both sides of the foundation wall and the exterior wall is coated from cap to foot with TK2000 waterproofing with 1.5" foam sheathing. The soil is sandy-stone on three sides but heavy clay on another. I suspect my bigger risk is the temperature of the foundation wall dropping so low that some ice might form on the wall. If I left the gap open and installed vents, I could even mechanically move air through to keep ice from forming when its really cold. I also have windows that are 10" below the top of the foundation on the walk-in side. I could definitely fill in the air gap above those.

Thanks so much for your thoughts. Rob D.


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## Gary in WA (Mar 11, 2009)

Got a few minutes, read this: 


http://www.buildingscience.com/docu...rol-for-new-residential-buildings?full_view=1 

Be safe, GBAR


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## Winchester (Aug 27, 2008)

I think that gap is supposed to be there from my research in my future project of finishing my basement.


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## Rob D. (Mar 26, 2009)

GBAR in WA said:


> Got a few minutes, read this:
> 
> 
> http://www.buildingscience.com/docu...rol-for-new-residential-buildings?full_view=1
> ...


Thanks for the excellent resource. I've been reading this and other moisture control publications today. 

The only interior basement wall plans I've seen that are sort of like mine with an air gap and no insulation/vapor seal against the cement foundation were in an article by Tismusk and all of these designs seemed to have drawbacks. The designs that use a rigid or spray foam vapor seal/insulation layer against the inside of the cement wall seem to abide by the moisture control principles I understand. I don't understand how the air gap design with the double-sided- foil polyisocyanurate vapor barrier under the gypsum is supposed to work. Maybe after the new construction moisture is gone next winter it will work fine? I can see that there was some icing at the top of the foundation this last winter and it seems likely that without some insulation against the cement foundation wall that is only half covered with soil, I'll not be able to keep the humidity in the air gap low enough. I can understand how installing some vents and moving air through the gap might work (with room humidity under 30% when its really cold). Its would be a heat/energy loss for sure, but not as bad as mold or ripping all the finished walls out and starting over.

I guess I'm trying to determine if the basement wall system that was designed for me is "sound" or faulty. The same advisor recommended fiberglass batting in the rim joists which did not work out. Its my fault for not getting other opinions then, so now I'm moving slow and trying to inform myself. I guess I could just leave my access openings in my ceilings open through all of next winter and see what happens with my interior humidity and icing and then decide what might work best. Rob D.


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## Rob D. (Mar 26, 2009)

Winchester said:


> I think that gap is supposed to be there from my research in my future project of finishing my basement.


Hi-- Can you point me to the design you are referring to? Thanks! Rob D.


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## Gary in WA (Mar 11, 2009)

In new construction the interior insulation and finishing approach must take into account the moisture migrating up through the footing. This is best accomplished by installing vapor semi-permeable rigid foam insulation on the interior of the assembly to protect the interior finishes and to release the capillary water to the interior in a controlled manner – at a rate that does not damage interior finishes or lead to mold.
The best foams to use have a perm rating of greater than 1 perm for the thickness used. This means limiting extruded polystyrene insulation to* less* than 1-inch thickness for walls (*more than 1 inch thick and they do not breathe sufficiently*) and making sure that the rigid insulation is not faced with polypropylene skins or foil facings. Additionally, since foams need to be protected from fire, and this is often done with gypsum board only latex paint should be used on interior gypsum finishes (since it breathes).
Avoidance of the installation of vapor *barriers* such as polyethylene vapor barriers,* foil faced batt insulation* and reflective radiant barrier foil insulation on the interior of air-conditioned assemblies – a practice that has been linked with *moldy *buildings.


*Figure 21: Concrete Basement With Interior Insulation*






_Applicability_ – all hygro-thermal regions
The key to this assembly is the use of non-water sensitive rigid insulation on the interior that still permits drying to the interior. The recommended permeance of the interior rigid insulation layer is approximately 1 perm. This typically limits the thermal resistance of the interior rigid insulation layer and an insulated frame wall assembly can be located to the interior of the interior rigid insulation. No interior vapor retarder is located within the frame wall permitting inward drying. All interior concrete surfaces are wrapped with the rigid insulation layer – particularly at the top of the wall and at foundation “step downs.” Exterior rigid insulation is located at the rim joist floor framing to control summer condensation. When insulating sheathing is not used, rigid insulation should we installed to the interior of the rim joist or an air impermeable insulation be applied at the rim joist assembly. Note the capillary break at the top of the footing. * Be safe, GBAR


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## Rob D. (Mar 26, 2009)

Thanks for getting back GBAR,

The same principles and construction details jumped out at me too. My existing, fully-finished walls are very different. 

I need to figure out how to best manage what I've got. I can't afford to tear out the walls in order to put 3/4" foam in there.

How do you like my idea of leaving the access openings in the ceilings open for a year to help the remaining construction moisture get out and to observe what happens in the extreme cold next winter? Maybe I should install a floor and ceiling vent in one section of the coldest wall and see how that works when it gets really cold. I certainly would appreciate management ideas. 

The diagram of my walls that I posted is accurate except the soil only comes half-way up the wall on one side of the basement (South).

The basement walls have excellent drainage. We'll see. Three sides are sandy and the clay side has a lot of extra gravel with a membrane on top. All exterior walls were coated with TK2000 water-proofing. I have 4" drain tile both inside and outside of my foundation footings. The inside tile system drains separately. Rob D.


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## Gary in WA (Mar 11, 2009)

The gap is picking up the moisture off the concrete wall. If the ceiling is left open to watch--- the results would not be accurate because of the open ceiling. Maybe visqueen it?

But..... the top gap needs to be fire-stopped, with no space greater than 100sq.ft.? If an electrical outlet sparks in the basement, it would spread into the joist space very fast, hence the whole floor above. The fan would spread a fire faster.

I would think the moisture wouldn't be able to reach the interior, because of the foil. Do some research on loose-fill cellulose insulation, fireproof- ? When it gets wet? Any commercial applications, etc.? Sorry I can't be of more help, GBAR


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## Gary in WA (Mar 11, 2009)

1 more time! The interior drainage system you have sounds good. If you have any problems, I think it would be leaving the lid open for warmed, moist interior air to reach the cold foundation wall. I would seal the gap -top of wall, check on it -3 months. Plastic the ceiling until later inspection. Over and out, be safe, GBAR


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## Rob D. (Mar 26, 2009)

Thanks! You've given me several good ideas to pursue. I'll also see if I can track down some more info on how the wall design I have is _supposed_ to work. Maybe it will work with some tweaks. Rob D.


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