# Using fiberglass batts in basement



## joecaption (Nov 30, 2011)

There has to be several thousand post on this one subject if you check the search funtion.
#1, Is this basement 100% dry?
#2, In your area there needs to be at least 2" of foam insulation glued to the foundation before the walls are built, if not your going to have a mold nightmare.


----------



## Windows on Wash (Aug 30, 2011)

http://buildingscience.com/document...-foundation-8-xps-2x4-framing-with-fiberglass


----------



## Bud9051 (Nov 11, 2015)

Basement foundations have to deal with both the moisture vapor passing through the concrete as well as the humidity in the heated basement air. From the outside the moisture vapor is always passing through and if you leave a gap it is circulated to the top area which is colder and forms condensation. On the inside, warm basement air passes through the fiber and reaches a colder foundation wall. Either way, you get moisture where you don't want it.

The rigid foam insulation solves both problems by maintaining an inside surface warm enough to not form condensation from the inside basement air and it slows the migration of moisture through the concrete. Whatever gets through is easily handled with a conditioned basement.

The link WoW provided shows the rigid foam and depending upon your local building codes you can determine the required thickness. 2" as Joes suggest would probably be sufficient, but you might also be able to use 1" rigid with r-13 batts in the stud walls.

Bud


----------



## harpua728 (Jul 13, 2015)

joecaption said:


> There has to be several thousand post on this one subject if you check the search funtion.
> #1, Is this basement 100% dry?
> #2, In your area there needs to be at least 2" of foam insulation glued to the foundation before the walls are built, if not your going to have a mold nightmare.


I've never had issues with water in the basement other than one time where my humidifier's drain hose came undone and I had a leak. However, that was not water coming from the outside in.

The area I'm in drains pretty well and most basements around here are relatively dry. However, that doesn't mean that moisture is not coming in through the foundation walls.

Have people had mold issues by not using foam insulation boards on their walls?


----------



## SeniorSitizen (Sep 10, 2012)

joecaption said:


> There has to be several thousand post on this one subject if you check the search funtion.


Is this member to read several thousand posts and or replies to not find the answer he needs as opposed asking a direct question?


----------



## Windows on Wash (Aug 30, 2011)

PSK insulation does have the potential to allow warm moist air to get through the poly blanket and condense on the cold cement wall. The fact is that it probably happens much more than people know about because it is concealed within the insulation. The most effective way to insulate a concrete wall is as indicated in that link that details out how to use rigid foam and standard batt insulation for a high-performance wall and airtight wall. 

Be sure to use the airtight drywall approach.


----------



## harpua728 (Jul 13, 2015)

Thanks for the help thus far. Does there need to be a gap between the foam boards and the studs/batts?


----------



## Bud9051 (Nov 11, 2015)

The only reason for a gap would be to keep the wall straight and then the batt insulation would be used to fill any extra space to limit air circulation.

Bud


----------



## harpua728 (Jul 13, 2015)

Bud9051 said:


> The only reason for a gap would be to keep the wall straight and then the batt insulation would be used to fill any extra space to limit air circulation.
> 
> Bud


Do I want to limit air circulation? Some moisture will still get through the foam boards, correct?


----------



## Bud9051 (Nov 11, 2015)

Air circulation increases the heat transfer effectively reducing your insulation value. The amount of moisture vapor that passes through the rigid insulation is very small and has no problem continuing through the fiber insulation and drywall, effectively drying to the inside.

Bud


----------



## harpua728 (Jul 13, 2015)

Thanks, Bud. Would rigid insulation on it's own (with a higher R value) be good enough to use without batts, or do I need both?


----------



## harpua728 (Jul 13, 2015)

In the pic below you'll see that in the corner of my basement I have my breaker box and gas meter. The alcove has the washer and dryer and I plan on building a wall a few feet off this wall (basically where the window starts) and coming straight across so that there is a separate laundry room and "utility corner." How would I insulate this corner area?? Would I just put the rigid foam boards along the foundation walls in the corner and then put the batts in the studs on the new wall that would be a few feet off of the foundation walls? I would put drywall around the laundry area but can I leave the foam boards in the utility corner "exposed" within the laundry room/utility closet?


----------



## harpua728 (Jul 13, 2015)

anyone??


----------



## throrope (Oct 30, 2015)

harpua728 said:


> So I've been reading a lot of articles saying NOT to use fiberglass batts in basements as the moisture from the foundation walls will transfer into the batts and cause mold. The overwhelming majority say that fibergalss batts cant touch the foundation walls. However, if I frame my studs a few inches off the foundation walls, then am I safe to use the fiberglass batts within the studs as long as they don't come in contact with the foundation walls? The gap should leave enough room for the moisture to evaporate off the foundation walls, correct?
> 
> Thanks for the help.


joecaption and Bud9051 provided good information.

Water does what it wants and the the constraints Bud9051 notes apply not only to the insulation, but all surfaces in a basement. Basements are heat sinks.

That said, we hung twelve foot wide 6 mil plastic in a continuous sheet on the basement walls by wrapping furring strips a few times and screwing them to the top of the wall with concrete fasteners. The remaining width draped to and over the floor. We then framed on 24" centers (non-bearing) on the plastic (pressure treated studs for bottom sill plate), ran our electrical and insulated with paper faced four inch R13 thick batts. We than ran more 6 mil on the concrete floor and seamed with 3M™ Polyethylene Tape 483. On the floor we used a felt under layment and floating floor. This provided some insulation and give. Solid concrete will eventually do your leg joints a disservice. All walls, ceilings and sofits covered with standard drywall. No issues since install three years ago.

We used two 100 ft rolls of 6 mil at about $70 each. The tape was about fifteen bucks. The 6 mil is more robust than 4 and IMHO easier to work.

This configuration not only limits the moisture penetration from the surrounding earth, but also inhibits the effects described by Bud9051. We went one more and installed a 70 qt. dehumidifier on a shelf with drain to safe waste. This makes for one less thing to do and keeps the humidity down in summer months. We notice a marked difference between a 50% relative humidity setting and 60%.

I configured for a mini-split ac unit, but since in the dead of summer, the basement never gets above 78, I doubt it will ever be installed. For heat I ran fin tube off our 75 gallon water heater with a domestic water rated circulator and across the line digital thermostat. In spite of many naysayers, the basement is the coziest place in the house.

Water flows downhill and basements are the lowest level. You will want to manage sources like water heaters, drains, humidifiers, sumps, pipe penetrations and similar to avoid collateral damage. We have battery powered moisture alarms purchased in a three pack that helped during a long rainy week where many had flooding.

Hope this helps and good luck.

We have a similar area as you show in your picture. I left as is. Since the near all the walls and floor is covered, that little bit is easily captured by the dehumidifier.


----------



## Gary in WA (Mar 11, 2009)

A good read, download this; http://buildingscience.com/document...ygrothermal-modeling-basement-insulation/view 

“The lowest risk approaches for interior insulation of basement walls use non-moisture sensitive
semi-vapor permeable materials at the interface between the concrete and the insulation
(Lstiburek 2006). Alternately, in foundations known to leak liquid water, insulation assemblies
specifically designed to safely drain this water can be retrofitted to the interior.” From pp3of25

*“Goldberg *has tested a variety of interior basement insulation configurations at a Minnesota test
facility. After testing frame walls with fiberglass insulation (Goldberg and Huelman 2001), she
recommended an assembly with polyethylene on both sides of the stud bay, as adopted by the
Minnesota building code. However, a 2002 addendum provided warnings against using this assembly
in “superficially dry” basements, which would accumulate moisture behind the exterior polyethylene.
Tests were also run using a variable permeability vapor control layer made of polyamide-6 (PA-6)
(Goldberg and Gatland 2006). The PA-6 wall experienced minimal condensation during the
summertime, and was able to dry inwards, unlike similar polyethylene walls. The PA-6 had similar
monitored performance to Kraft-faced batts. In addition, some walls were constructed with a latex
elastomeric waterproofing on the concrete surface. These walls had noticeably higher wintertime
condensation than the uncoated concrete walls (which largely showed only surface dampness), due to
the elimination of the storage capacity of the hygroscopic material, by the hydrophobic
dampproofing layer.” From pp. 4of25 -------- So don’t ever use Drylock or similar product inside, unless you have an interior drainage system.

“The plot shows that moisture accumulation roughly corresponds to the vapor resistance of the
assembly. The polyethylene roll blanket shows the highest sustained RH levels: near saturation in
fall, spring, and summer, dropping to ~90% RH in winter. The XPS wall has vapor permeability an
order of magnitude higher, and shows RH levels in the 80-90% range. The frame/latex paint wall
has permeability an order of magnitude higher than XPS, and shows the lowest humidity levels,
indicating drying of the assembly. In this wall, RH values are at their peak during winter when the
concrete wall is coldest. The frame/polyethylene wall shows relatively low calculated RH levels; after
replacement of the damaged RH sensor, this relationship will be verified.

*Framing Moisture Content: *When assessing the risk of a wall assembly, the danger is not
necessarily condensation _per se_, but mold growth and damage to moisture-sensitive portions of the
wall, such as the framing. The moisture content of the framing at the upper portion of the wall is
plotted for the monitored period in *Figure 6*. Moisture contents were measured at the inboard and
outboard edges (3/8” or 9 mm from the faces) of the stud. The summertime increases in the
frame/polyethylene wall are seen at the interior side, similar to the moisture content wafer; however,
peaks are roughly 17% MC, which is still in the safe range. A similar rise in wintertime moisture
content is seen at the outboard side of the frame/latex paint wall; again, the MC remains within the
safe range (15% peak). Overall, moisture contents at all of these upper framing locations presented
minimal concern for mold growth; based on over a year of data, these cycles appear to be stable.” From pp 11of25

“In the frame/fiberglass/latex paint wall, there was no visible staining or mold growth on the
insulation, framing, or wafers. Moisture contents were in the 9-11% range throughout the wall. One
concern with omitting a polyethylene layer in assemblies is that inward vapor drives might cause
mold growth on the back of the drywall, due to accumulation at that location. Monitored data
showed that there was negligible accumulation; a visual inspection of the exterior side of the drywall
showed no damage, discoloration, or mold growth.” From; pp 12 of 25. ------------ So use some non-paper backed drywall.


IMHO, if the basement is dry, add some building paper with the fiberglass insulation. Interestingly, there have been no follow-ups on this cheap solution, squashed by foam manufacturers or others, possibly…. ------------- “*Swinton and Karagiozis (1995) *examined the phenomenon of condensation on the exterior
side of the polyethylene vapor barrier during spring and summertime, at the above-grade portion of
the wall. This problem is caused by inward vapor drives from the damp concrete; when there is an
inward thermal gradient, the moisture moves inwards and condenses on the polyethylene. They
replicated this problem using two-dimensional hygrothermal modeling in a Montreal climate and
demonstrated that using *semi-permeable materials (building paper) on both sides of a fiberglass batt*
*cavity had the best overall performance*. Although removing the interior polyethylene layer
eliminated the summertime problem, it resulted in moisture accumulation at the concrete-insulation
interface during the winter.” From pp 3of25. ------Bold is mine.

Pages 20-23 mention IF you have AC, you may have different summertime results and possible future mold problems… I thought the paper was code in Canada before they switched to poly... lol.


Gary


----------



## Davejss (May 14, 2012)

If you have moisture getting through your foundation you need to address it before you do any remodeling. Stop the moisture infiltration and then you can use whatever insulation you choose.


----------



## Bud9051 (Nov 11, 2015)

@throrope I have difficulty recommending all of that plastic and don't think I did with my posts. The more recent advice from the research groups will generally avoid plastic vapor barriers in favor of allowing some drying to inside through the rigid foam. 

You may not be having problems with your installation, but it is still an example of one and that is why we like to follow the advice of organizations that have tested and measured their results on many homes in many climates.

A true vapor barrier as in a layer of plastic as opposed to a vapor diffusion retarder allows zero moisture to pass, as water or as a vapor. The result is, everything to the moisture side will slowly equalize until that side of the VB is just as wet as everything outside of it. In some soil conditions that may not be a problem, but in many it is.

Some reading: http://buildingscience.com/documents/digests/bsd-103-understanding-basements?full_view=1

Bud


----------



## Gary in WA (Mar 11, 2009)

A follow-up on my previous post; "
The use The use of building paper instead of polyethylene
sheetisheeting as a vapour barrier in case B5 provided the
opoptimum results. The permeability balance on both sides
of tof the cavity resulted in a steady, low moisture content
*which fell below all other strategies"* from;https://www.cmhc-schl.gc.ca/publications/en/rh-pr/tech/95-207.pdf

Gary


----------



## harpua728 (Jul 13, 2015)

harpua728 said:


> Thanks, Bud. Would rigid insulation on it's own (with a higher R value) be good enough to use without batts, or do I need both?



Still looking for help on this....Instead of buying both rigid foam and fiberglass batts, could I get the same result by just using rigid foam boards with a higher R value?


----------



## Bud9051 (Nov 11, 2015)

Hi harpua,
I didn't go back and reread the entire thread so I risk some confusion, but in your climate zone (4) link below a foundation wall (2009 IECC codes) says r-10 continuous or r-13 cavity. Now that isn't the best advice IMO because we shouldn't be installing batt insulation directly against the concrete. But you question about using more rigid is yes. Now, to qualify that, I'm looking at those 2009 codes and some areas have moved on to even the 2015 codes and I have not looked at the 2012 or 2015 requirements. Thus, you would need to determine what codes are applicable at present.

If you want to leave the rigid exposed, then you could use Dow Thermax only. It has a foil facing that has been approved to meet requirements without the drywall. Not cheap, but it can eliminate studs and drywall. Their 1.5" sheets are rated at just under r-10 close enough to meet most codes. $41 a sheet last I priced it.

IMO, the hybrid approach is still best, 1" rigid, stud walls against that, then fill with batt insulation. the minimal amount of moisture coming through will easily be handled on the inside and you are retaining some drying to the inside.

If I missed anything, let me know.
Bud
https://energycode.pnl.gov/EnergyCodeReqs/?state=New%20York


----------



## Gary in WA (Mar 11, 2009)

Check the codes, for sure as the 2012 allows you to count both insulation types for a total, earlier does not. Also, using the 3 coldest months for your location (first county- second is colder); http://longisland.about.com/od/neighborhoods/a/Long-Island-Ny-Climate.htm, with basement at 68*F the foam (1") could condense water vapor from dirt on the inside face at 39% relative humidity as the temp there is 43*F. So you would have condensation at the wood stud edges next to the foamboard for 3 months--- above grade and to "frost line"; Jan., Feb., and Dec., hopefully the HVAC can handle it to not cause mold. 

Gary


----------

