Rigid foam vs. spray foam under crawlspace

NiwashikunNovember 21, 2012


We have a two pronged issue we're trying to address.

1. Stale, musty smell, likely coming up through floors

2. Very cold floors in winter

After endless hours researching I'm virtually paralyzed by info overload and conflicting opinions.

I've narrowed it down to two solutions, either:

1. Closed cell spray foam insulation on the underside of the floor

2. rigid, foil-faced polyisocyanurate foam attached to the bottoms of floor joists sealed with foil tape and penetrations filled with spray foam

I realize there are strong opinions on both sides of the sealing/ventilating issue and I don't want to start another thread arguing those sides.

Therefore I'm looking for comments from people who have actually done one of the two solutions and can give their direct experience as to what effect it had on the above two issues as well as related problems.

My concerns/questions :

Both Solutions:

Effect on musty smells

Effect on floor temperature

Effect on dampness feeling of floor

Solution #1:

Issues with leaking pipes

Issues with liquid spilled in house

Issues with access to pipes for repairs

Solution #2

Issues with mold on/under insulation

Issues with leaking pipes

Issues with liquid spills in house

Also curious as to the effects of covering the dirt with plastic. A common practice but according to a study mentioned in a different thread not necessary.

House Facts:

-1948 pier and beam bungalow in the Houston TX, 1250 sf, open crawlspace approx 18" dirt to subfloor.

-Original portion (ca 900 sf) oak floors (with lots of crevices) on top of diagonal shiplap sub floor (parts I've seen have lots of space between boards)

-70's addition (ca 350 sf) has plywood subfloor with various amounts of linoleum and vinyl flooring on top.

-plumbing in crawlspace, most electric and all hvac in attic.


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Very few people actually do what you propose so you might be waiting a long time for actual experience. Also they might be in NC, so the ground is very different.

Your problem is excess humidity in the crawlspace right?

You can isolate like you propose (and honestly both methods seem fine), or you can fix the humidity problem.

Here in NC, if we don't fix the humidity problem, than you have bugs in the crawlspace that come into the house. You also get more rot on the wood parts in the crawlspace. So we generally fix the humidity issue - seal the crawlspace.

So unfortunate that you don't have HVAC down there. So much cheaper to operate...

    Bookmark   November 22, 2012 at 5:56AM
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There are a plethora of companies out there (at least in the south) that perform these services, therefore there must be quite a few people who have had it done (not to mention DIYers).

Our problems are cold floors (from both convection and conduction), odors coming through the floor (convection)

Humidity is less of an issue (according to the study I mentioned-see link below) with open crawlspaces like ours, although Houston is a swamp with heavy clay soil.

>>So we generally fix the humidity issue - seal the crawlspace.I assume you mean including the crawlspace in the thermal envelope of the house. Since we have an open crawlspace that would mean building walls all around the house and then going through all the steps for sealing off the space. Also since we are in Houston there is the potential of flooding so it's best to leave the crawlspace open (that's also why it's better not to have the HVAC in the crawlspace.)

Here is a link that might be useful: Crawlspace Study

    Bookmark   November 22, 2012 at 11:42AM
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Yes - we don't build open crawlspaces here.

My first thought is that 18 inches isn't really a help with floods but I guess 18 inches is better than nothing. Posts here mean 8-10 feet and we still don't foam very often.

Now in more humid areas you can foam although I always doubt the payback. Why not just use batts and then rigid foam. Much cheaper and there is no reason you can't have rigid foam be nearly airtight. Going even cheaper, you don't have to have rigid foam since you really don't need much insulation down there - you just need an air barrier.

When you say cold floors, do you have zero insulation?

Code here with posts puts r-19 batts under the floor with hvac ductwork often in this space. That way the ductwork never deals with the hot attic and is in a sealed space.

I do think spray foam is the easiest way to get an air barrier but since rigid foam was mentioned, obviously someone thinks they can get under there and secure it in place. If you can do that, then you don't really need spray foam.

    Bookmark   November 22, 2012 at 5:50PM
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Louisiana chiming in here.

we have done it both ways...actually 3 ways.

open vented crawlspacew without ductwork
& equipment in crawlspace.

1-air sealing of large openings in floors..under tubs
around plumbing & elec penetrations.
R-25 batts in floor joists. 2x6 joists
not R-19 because it settles creating an air
space between insulation and bottom of floor.
this air space condensates and rots floors.
thus the upgrade to R-30, slightly compressed
to overfill joist bay, resulting in actual R-25 value.
1" extruded polystyrene sheathing with foil facing.
nailed foam sheathing to undersides of joists with
button cap nails. taped all seams. at perimeter
of house foam board was caulked at exposed edge.

this install allowed insulation to perform as it
was no longer wind washed & reducing R-value at
exterior of walls. the foam sheathing added R-7 to
the R-value of floors and moisture could not enter
floors from crawlspace.

this turned out to be the highest performance floor
insulation job.
labor intensive..very labor intensive. thankfully the
house was raised 3 1/2' off the ground making access
not as difficult.
this install was south central La.

after hurricane Katrina in New Orleans we did this
install recommended at that time.

2-air sealed floors from inside crawlspaces..caulk,
flashing, backer rods & more caulk.
no batts in floors.
dow blue board 3/4" installed as above with button cap
nails, seams caulked and caulked at exterior walls.
floor insulation value (again 2x6 joists) were value
of foam sheathing plus minimal R-value of air at rest.

labor intensive, to me..less satisfying results.
pro was..if flooding ever occured these joists with no
insulation would dry quickly when foam sheathing was
cut to allow circulation. once floors were dry again..
then the sheathing could be put back in place.

3-the last type of insulating floors over open crawlspaces
is closed cell foam. again large openings..like plumbing pipes under tubs have to be sealed first. any opening over
3" should have some type of filler prior to install of foam.
install depth of closed cell is 3".
note this is not 'average' fill of 1/2 up to 3" but
3" minimum.

with this install air leakage reduction was achieved,
and insulation value was approx R-20
this performed as well as install one.

here is a link to a study that came out well after
we had been doing installs. but it is nice to have verification.

keep in mind with spray foam insulation that floor coverings & types of finishes for wood floors can't form
interior vapor barriers.

best of luck.

    Bookmark   November 22, 2012 at 7:14PM
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Hey David

Thanks for the reply.

>>Why not just use batts and then rigid foam. Much cheaper and there is no reason you can't have rigid foam be nearly airtight.

Actually using rigid foam was one of my solutions. Batts don't seem to be a good idea due to moisture trapping. The study I mentioned above also found that, "fiberglass batt insulation (was) not reliable for preventing summertime moisture accumulation in subfloors."

>>When you say cold floors, do you have zero insulation?

Yes, it's common here to have no insulation under the house in older homes. Some people have tried adding batts and ended up with moisture and mold problems. I've also heard that the batts don't help much with cold floors which makes sense because if batts allow moisture vapor to pass they also allow cold air to pass.

Hi Energy Rater LA,

I found that study thanks to one of your earlier posts and that's how I arrived at my two "solultions".

Your #2 is similar to my #2 but you used 3/4 Dow Blue Board which seems to be R-4. The study suggests using 2" foil-faced polyisocyanurate which is R-13. That seems like a significant difference. Perhaps with the 2" Iso your #2 would have compared better to #1 and #3.

The study didn't mention using batts inside the rigid insulation. It seems like that would take care of the problem of moisture with batts since ..."Foam board faced with aluminum foil is essentially impermeable to water vapor." (quoted from the study)

I'm a little skeptical of your claim that R-30 batts in 2x6 joist bays would result in R-25. R-30 batts are approx. 9 1/2-10", R-25 batts are 8-8 1/2" and R-19 batts are 6 1/2". Since it's the trapped air in the insulation that gives the insulation it's r-value, it seems that compressing the R-30 by approx. 60% would drop it well below R-25 (perhaps even below R-19). How did you come to that figure?

Were you living in any of the houses with those insulation installs? If so I'd like to know your experience with any of the issues from my original post.

Thanks for your info and that link.

    Bookmark   November 30, 2012 at 1:30PM
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How did you come to that figure?
the software I use for ratings does the
calculations for me.
even if R-value is R-19, it is a fully
performing R-value not affected by
wind washing of unsealed batts at
edges of floors.

none of these houses were mine.
the batts with foam board is a house nearby
that I have worked with the homeowners for
several years overseeing & sometimes doing
the work myself.
antibellum home with baloon framing, floors
insulated as posted, 7" of open cell at
roofline of attic. furnaces are both 96% gas
the floor method was due to cold floor
complaints & winter time discomfort.
there were no mold issues except in the
ductwork in the attic. this was due to
lack of mastic seal of ducts combined with
hot temps in attic. ductwork & furnaces
were installed prior to foam install.
no issues with smells from floors prior to
any work done @ floors.

in my personal house on piers with open crawl
space..I'll do closed cell foam.

best of luck.

    Bookmark   November 30, 2012 at 1:58PM
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Hi Energy Rater LA

>>the software I use for ratings does the calculations for me.

Good to know. I assume that software's a little too pricy for a non professional;-)

>>>there were no mold issues except in the ductwork in the attic. this was due to lack of mastic seal of ducts combined with hot temps in attic.

We have this issue as well. We have mold in the vent grills and into the boxes a bit. I had been thinking it might be the AC being oversized and was considering having a J-Heat Load Calculation done but then thought I should wait till after we do insulating and repairing window and door problems (presumably that would change the calculation quite a bit). We had the HVAC installed 6 years ago when we moved in and it seemed to be a pretty good install (to my unprofessional eyes). They certainly seemed to use a lot of mastic on the duct to box connections. However our attic insulation is very poor and I've heard that can also be the cause.

>>>no issues with smells from floors prior to any work done @ floors.

I finally got my wife to do a smell check (she's the one that can smell these odors) and she said under the house was a totally different odor from the ones that bother her inside. So that odor from below might have been a wild goose chase. Although I'd still like to get these floors warmer.

>>>in my personal house on piers with open crawl space..I'll do closed cell foam.

Since you are a pro and I'm assuming will do it yourself, I'm curious why you would choose the spray foam??

Thanks again!

    Bookmark   November 30, 2012 at 4:11PM
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love this forum that you can do a little diy

ok the mold in the supply boxes and on the
grills probably isn't from the duct takeoffs
@ the plenum. it takes a long time for mold
to travel that far..and if there isn't moisture &
food source it won't spread to that area.
it isn't always mold..sometimes it is just dust.

so mold/dust buildup at the ducts @ plenum is lack of proper
air seal. mold in supply boxes & on grills
is lack of proper air seal at the cut in the
sheetrock ceiling where supply grill is attached
to the box.

nine times out of ten..you can solve the issue
by sealing supply boxes to sheetrock.
see attached picture.

hardcast brand #1402 mastic tape is used
to seal supply box to sheetrock.
this tape is 3" wide. I split the 3" into
1 1/2" lengths. stick to sheetrock and fold
into supply box.

as the tape adheres to many surfaces you need
to be careful to keep it within the area covered
by the supply grill. once it sticks to the
sheetrock..you'll lose paper if you try to move it.
surfaces have to be clean & dry for tape to adhere.

and once in place you'll want to press it in place
as it is somewhat pressure sensitive.
the tape is strong enough to keep the box
tight to attic floor on the attic side when
installing the supply grill.

wash the dust off the grills, wipe inside the
supply box & into the duct.

reinstall the supply grill.
oh and when you remove the supply grills..take
a sharpie and mark what direction they faced
(throw to window..dirveway side whatever note
you need to put them back in same position)
this way you'll put them back as they were.

I've tried several other brands of mastic tape.
hardcast 1402 is the best. the others are 2" width
and mastic is thinner, resulting in less strength
and useful life.

why would I chose spray foam for my floors?
I've sealed floors several ways, foam is the easiest
albeit more costly, once prep work is done.
there isn't enough room to physically install
foam sheathing to the bottom of the floor
joists. I'll seal any big holes..like under tub
then do 3" foam. I've looked at froth packs like
tiger foam..but having worked with so many foam
companies locally, will just have them do
my floor when they are in my area.

the software is pricy. we lease it yearly.
once upon a time we bought the cd..but too many
people abused it. so now it is a download & upgrades
to original install.

a lot of mastic isn't always an indication of
a good seal. it doesn't take a lot if you put
it in the actual leaks. you can feel around the
duct take offs on the plenum while unit is running
and feel some of the leakage. also checking plenum
connections to the equipment at the same time.
I use mastic tape to connect plenums (after screwing
them in place) much easier to seal under the plenum
to equipment with tape than painting mastic upside down.

    Bookmark   November 30, 2012 at 7:25PM
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It is amusing to hear that Houston does not have a humidity problem.

I guess that's all relative.

I understand that you know it is winter time in Houston when the temperature finally drops below the humidity.

    Bookmark   December 3, 2012 at 5:10PM
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I guess that's all relative.

you can say that again..all relative

    Bookmark   December 3, 2012 at 7:59PM
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