View Full Version : Basement refinishing- how to keep mold away

02-29-2012, 10:43 AM
Well doing research on my basement project (1300 sq ft in Northern Va) I hope to reduce the chances of future mold issues. Here is my scenario:

House is 4 years old, poured concrete walls and the basement is a walk out where the back and half of the sides are above ground. I first put up the basement walls keeping the rolled bat insulation and built the stud walls about 3 1/2 inches out from the concrete. While the basement walls have no water infiltration and is dry as a bone in the winter, I noticed quite a bit of water condensation in the summer sweating and soaking the bat insulation. It got soo humid from this the hvac sweat at the main trunk for the first couple of feet. So I decided to read up on how to fix this before going to drywall. i realized bat insulation is about the worst thing I could have used regarding thermal break.

Reading Dana's posts here and a couple sites listed, I have ripped down the bat insulation and purchaed 2" XPS foam board that I will glue, tyvek tape and spray foam the seams to make it tight... There might be a 1" gap though from the foam board to the stud walls...

I wish I used that cool BLU wood you see now on all of the DIY programs, but too late I already used regular lumber with the treated lower plate. Anyway I read about this Boracare Mold care spray treatment I can put on the framing.. Seems like for 130 bucks not a big deal to do... Not sure how effective this might be

Last but not least I decided to go with the Mold Tough drywall that has the treated paper face on both sides and treated core. I do not want to use a fiberglass based board or paperless that needs skim coats so this product looks to be a great compromise over regular drywall. it is only a couple bucks more over regular drywall. Not sure why they sell the regular stuff anymore lol

Any insight on where I am going wrong or any ideas before I close it to drywall? Thanks.

02-29-2012, 02:28 PM
To be fully effective and minimize air-transported moisture the batts need to be snugged up to both the XPS and the gypsum, and it needs to be air-tight. If there's still a gap between the studs and foam you might want to use gap-filler 1-part expanding foam to fill it at each stud (and at the top plate) to make the cavities air tight, and use unfaced R19s to fill up the more-than-3.5" depth. R13 batts have the same amount of material as R19 batts, just lower loft. If you compress an R19 fully into a true-3.5" cavity it'll be R13. If you actually have 4" or 4.5" it'll be higher R. But if you leave a gap without a snug air-barrier air convects randomly in & out of the R13 batt, and it's performance is undercut substantially. Any where that air can move unimpeded is a thermal-bypass, and a likely contributor to the problem.

You may have been experiencing humid summertime outdoor air infiltration into the gap between the batts & foundation wall too, or if you had kraft faced batts with the facer on the conditioned-space side it may have been trapping ground moisture in there, causing condensation on the interior side of the cavity. Foam-sealing the foundation sill & band joist to limit air movement could be a necessary part of the fix.

Only unfaced batts will do- the facers are too vapor retardent, and the cavity can't dry to the air-conditioned Latex paint on air-tight drywall has about 6-10x the drying capacity of a kraft facer. At 2" XPS still has 1.5-2x the drying capacity of a kraft facer, and it's on the humid side of the assembly in summer, when most of the issues are likely to occur. With latex on the finish surface and XPS on the foundation side, moisture passes more readily out of the studwall toward the interior. In winter the moisture drives are the other direction, but the R-value of the XPS is sufficient to keep all of the wood & fiberglass above the dew point of the interior air (unless you keep it down right tropical by adding a lot of humidity with a humidifier- not recommended.)

Going with a mold-resistant drywall is also moving in the right direction.

Getting serious about air-sealing both the basement and the attic will vastly reduce the infiltration drives that are probably contributing to the problem, and lower both the heating and cooling bills to boot. With lower infiltration rates in to the basement the sweating of AC ducts will also be much reduced- the dew point of the basement air will then track that of the fully conditioned upstairs, which is the same as the air INSIDE those ducts. Under those conditions the ducts won't/can't be much be lower in temperature than the dew point of the basement air.

If you don't have a poly vapor barrier under the slab, you can reduce the amount of groundwater vapor diffusion through the slab with masonry sealers. (On a 4 y.o. house it SHOULD have poly under the slab though, but just because it should doesn't mean that it does, always.)

02-29-2012, 03:31 PM
Thanks Dana for responding. Your advice is really helping with my basement decisions.It is much appreciated.I can only imagine myself ripping the basement apart in a few years due to mold if I did not read your posts.

I will look into using unfaced insulation in the wall cavaties and foam spray between the xps and studs. the sill to the stud walls are enclosed by a 2x4 needed by code here for fireratings and is caulked with fire rated caulk.

looks like I forgot the band joist area also. thanks I will seal that up.

The floor does have the poly vapor barrier. I noticed it when I jackhammered the floor to move the sump pit and bathroom roughin.

I almost forgot to mention....do you think adding a return in the basement will help with the basement air issues I had in the summer? I am planning a return about 20 ft from the HVAC unit in the basement.The house has 2 separate HVAC for the first and second floors and the first floor HVAC was installed with a finished basement in mind when sized.
The basement door to the first floor currently has a 2" gap and you can feel the returns from the first floor sucking air from the basement coming up the steps. This is why I decided to put in a return. It also currently has a 6" fresh air intake tied to the return trunk.
the basement currently has 6 registers for the basement.

03-01-2012, 09:02 AM
The band joist & foundation sills in most homes will have several times the air leakage of all windows & doors in the rest of the house (even if built with a foamy sill gasket), and is often the largest single untreated air leak. Sealing it is high priority! If you seal both the basement and the attic it kills the stack effect, and the overall natural air infiltration rates can plummet.

A duct system has to have balanced supplies & returns, otherwise the air handler creates pressure differentials between rooms AND with the great-outdoors, which drives air infiltration far beyond what the natural stack effect and wind-washing infiltration would be. If you add either a supply or a return to the basement, at the very least cut one of these (http://www.greenbuildingadvisor.com/blogs/dept/green-communities/perfect-balance-makes-cut) into the door leading to the basement. Without a supply to the basement adding a return will de-pressurize the basement, causing it to suck more outdoor air in at every air leak.

Take a look at how supplies & returns are located elsewhere in the house relative to doors too. You can rebalance with jump-ducts if door-grilles don't work aesthetically for some reason or other.

Other things that depressurize basements pulling in outdoor air can be open flues, plumbing & electrical chases that run from basement to attic, etc. If you can air-seal between the basement & the first floor (which can get complicated), adding door sweep & weatherstripping to the basement door and sealing all of the ductwork can limit the active & passive infiltration drives too. It's usually easier/better to air-seal the at the exterior walls, and treat any passive or active flues that might be depressurizing the basement with stack effect.

Last & not least, setting up a 70pint room dehumidifier to auto-drain into a sump or drain and limiting the basement humidity to 60% RH will usually keep it dry enough, and can keep up with at least some incidental air infiltration.

03-01-2012, 04:44 PM
You can buy all of the fancy materials you want, but at the end of the day, if you keep any material DRY, you wont have mold issues.

Buying blue lumber and letting it get soaking wet will net you the same result, it just may take longer.

03-01-2012, 05:02 PM
WRT adding a return, there may be code issues. You don't want a return to reverse draft a nearby flue. Reducing air changes in a cool basement can reduce humidity, particularly if the replacement air is humid.

03-01-2012, 06:23 PM
Good stuff guys. Thanks

I actually have a 6" fresh air intake pulling in outside air to the return of the 1st HVAC unit.. (2nd unit upstairs has no fresh air intake) This is not a high efficiency unit, (80%) so here I am supposedly needing to stop air infiltration yet I am purposely pumping in outside air. I was told since todays houses like mine are tight, air quality inside dictates the need for fresh air intake. I am sure this is also hurting the performance of my unit and shortening it's life, but I was told it is needed by 2 HVAC people, others have said to disconnect it from the return and let the 6" fresh air hang in the enclosed HVAC room for combustion air. Soo many different opinions on this.

I have a 4" schedule 40 type of pipe from the basement directly to the attic space wide open that could be used for Radon remediation if it were needed. I guess I should close that up lol

The proposed return in the basement is 20ft away from any combustibles and the gas hvac and heater is in it's own closet which will have a grilled door.

The basement already had returns cut out of the trunk and all I am doing is covering the existing holes and running them out 6-8 feet off the trunk.

I read I should use the Polyisocyanurate foam product that gives a better R value for the sill areas and rim board areas and use spray foam around the perimeter to seal it up.

I am glad Dana you mentioned the dehumidifier. I specifically setup an outlet in the basement and will have a dehumidifier hosed directly tied into the sump in the summers to run constantly.

i will look into the hvac balancing issue with that added return

Thanks for the advice guys, I am actually having fun with this project and learning a ton.


03-01-2012, 06:53 PM
A tight house needs CONTROLLED fresh air, not what you have, which is very inefficient. Most homes have enough air leaks where it isn't a big deal, but as people tighten then up, or if it is actually built nice and tight, then yes, you should have some. But, it should be done in a controlled manner with recovery units that don't waste the conditioned air you've paid for and also manage the humidity and dust/pollen issues as well.

03-01-2012, 07:21 PM
Polyiso board will give you the greatest value per inch, but you wont find it at your local home centre.

03-01-2012, 07:49 PM
Polyiso board will give you the greatest value per inch, but you wont find it at your local home centre.

I was looking at this stuff from Home Depot and it is Tuff R Polyiso board.

03-01-2012, 07:53 PM
A tight house needs CONTROLLED fresh air, not what you have, which is very inefficient. Most homes have enough air leaks where it isn't a big deal, but as people tighten then up, or if it is actually built nice and tight, then yes, you should have some. But, it should be done in a controlled manner with recovery units that don't waste the conditioned air you've paid for and also manage the humidity and dust/pollen issues as well.


Is this an example when you are mentioning when bringing in fresh air in a controlled manner? I just googled it.

03-01-2012, 09:05 PM
I was looking at this stuff from Home Depot and it is Tuff R Polyiso board.

Interesting, I guess it is me who wont find it locally.

03-02-2012, 08:44 AM
Foil faced foam should NOT be used unless:

A: There is a membrane (or metal) capillary break between the footing and the foundation wall (unlikely, unless you can find it in the building documentation)

B: There is a capillary break between the top of the foundation wall and the foundation sill. (Foamy sill gaskets are better than nothing, but metal or membrane is orders of magnitude better.

The moisture content of the foundation wall will be quite high unless you allow it to dry at least somewhat to the interior, otherwise you put the foundation sill at risk of rot. Better to use XPS- pink, blue, green or gray, just something rated greater than 1 perm @ 1" thickness, and keep the thickness under 2". Foil facers are rated under 0.01 perms- they're true vapor-barriers.

If foil faced goods are the only option, stop the iso 18" from the floor.

It doesn't matter how far away the combustion appliances are from the HVAC return, if they're in the same pressure envirnoment (yup!) depressurizing the basement will backdraft the flue, creating a very serious indoor air quality problem (potentially lethal, if the appliance isn't burning cleanly.)

If your house was truly "tight", the odds are you would not have experienced visible condensation in the basement. If you're serious about it you'll get a blower door test done before you close it all up. The HVAC units that do outdoor air exchanges are a mixed blessing- humid outdoor air doesn't improve air quality if it's raising the indoor humidity levels above 50%RH @ 75F, which it very well CAN in summer. Pressure balanced ventilation (ERV or HRV) is a much better approach, and you can set the actual ventilation rates independent of the HVAC systems as-appropriate for the season & occupancy. (The outdoor air exchange on the HVAC at my office is miserable- it over-dries in winter, and increases the latent loads in summer, but I don't own the building.)

On the basement dehumidifier, a smaller one would still be able to keep up with the latent-loads, but even the EnergyStar versions smaller than 70 pints are pretty inefficient. If you ever plan to fully finish out the basement it's probably better to use a balanced supply & return of the existing system, or install a small mini-split to heat & cool it as it's own zone, and get a version with a "dehumidify" mode.

03-02-2012, 01:04 PM
I only mentioned using foil faced foam on the band board area, not on the foundation walls because it has a higher R value than poly.

Reading your post on the condensation and fresh air, I realized I caused part of the problem by running the blower at all times in the summer so fresh air entered without being conditioned. oops.

Looks like I am adding the HRV-ERV to my list of things to do.

Thanks again.

03-02-2012, 01:25 PM
I only mentioned using foil faced foam on the band board area...

Is that because it will remain exposed? Around here, they shoot a stucco mixture over the exposed foam board with air so as to fire rate it.

03-02-2012, 02:32 PM
HRV (heat recovery ventillators) are the thing you want to consider if you have a truly tight house. They run the air by fancy heat/vapor exchangers so the incoming air is warmed by the exhausting air (or vice-versa in the summer) and helps to maintain the relative humidity levels in the process. These are much more common in Canada, which has mandated much stricter house building regulations than the USA. They also help to maintain a balance in the house pressure to account for both over and under pressurizing the house which will affect inflitration and the drafting of burner equipped appliances, and things like bathroom and kitchen exhaust fans.

03-02-2012, 02:39 PM
In terms of foam R/$ you're usually better off with low-density EPS rather than iso.

On foundation sills and band-joists the ignition barrier requirements are eliminated in local codes, per the IRC, so long as:

* The maximum thickness of the foam plastic shall be 3-1/4 inches
* The density of the foam is between 0.5 & 2.0lbs per cubic foot
* has flame spread index ≤25
* has a smoke developed index ≤450

(Which would be true of most rigid foam products.)

See: http://publicecodes.citation.com/icod/irc/2012/icod_irc_2012_3_sec016_par015.htm