Comments on: Will Open-Cell Spray Foam Insulation Really Rot Your Roof?

By: frank prasek

frank prasek — Sat, 21 Mar 2015 03:44:12 +0000

I live on long island ny and
I live on long island ny and want to get me rafters in my ranch sprayed with iceiline foam will it rot my roof or create any problems?

By: Skye Dunning

Skye Dunning — Thu, 12 Mar 2015 00:48:32 +0000

Oh Allison, you wanted me to
Oh Allison, you wanted me to actually READ the article? LOL!

Alright my bad but something else is going on because I keep hearing this (high RH levels in the attic) as a problem that is particularly bad in humid climates. Maybe because an indoor RH in the low 50's is still higher than in other climates? Doesn't sound right to me.

David, believe me when I say, just like our sealed crawls, this is a problem that requires a mechanical solution in our area even in new construction jobs with very tight envelopes - including the attics.

This business I keep hearing about returns in the attics though, is madness! It just sets people off because of the fire code issues and it's unnecessary in our experience. We're only supplying a small amount of air for drying potential.

Good conversation as always everyone. Next time I'll try to read the piece as well as the comments!

By: David Butler

David Butler — Sun, 08 Mar 2015 00:15:53 +0000

@Allison, I think you missed
@Allison, I think you missed my point. No doubt retrofit projects very often present challenges that upend the calculus. You do the best you can to mitigate moisture at the source and, when necessary, apply an appropriate mechanical remedy to get you where you need to be. Jim Coler posted an excellent overview of mechanical DH options in his June 29, 2014 comment.

But to my point (and with all due respect to Skye), mechanical dehumidification should not be the baseline assumption when designing/building homes with encapsulated attics (or crawls). It's in that context that I responded to Richard that mechanicals shouldn't be blammed for Doug's failure, or for rotting SPF roofs in general. These problems are most likely caused by poor building envelope designs and/or installation practice (infiltration, insulation, vapor control).

In new construction, I think it's best to address moisture at the source, and as you said, it's a lot easier (and much less expensive). That's not the same as saying that mechanical systems should never be part of the solution in homes with defective envelopes.

@Jim, great point about diffusion versus air leakage as a transport mechanisim for moisture. Here's BSC's familiar diffusion vs. air leakage graphic that illustrates this point. Those interested in this should check out Allison's 2012 article: You Don't Need a Vapor Barrier (Probably).

By: Jim Coler

Jim Coler — Sat, 07 Mar 2015 06:50:20 +0000

@Richard: Mr. Beyer -
@Richard: Mr. Beyer - moisture transfers primarily through air movement. There is a difference between mass water shedding, capillary action, water absorption, condensation and hydraulic pressure.

Mass water shedding is the deflection of water droplets which is supposed to occur at the tar paper, roofing or house wrap layer. This prevents the wall or roof assembly from getting wet to begin with.

Capillary action is the wicking of water up small cavities or offices. A tree wicks water up through to the top through small capillaries around the tree just under the bark and even inside the tree. It can move vertically over 30-45 feet or more in some cases.

Water absorption is what occurs with a sponge is water. Yes, some open cell foams can and will absorb water but that's not the environment their designed to be in. But even closed cell foam can and will absorb water and it can even wick up open or closed cell through capillary action. Wood is a great example of a product used for building which can absorb water but it's still used and limited to non water contact applications. If it was in water contact applications it would need to be pressure treated or something like this to prevent moisture absorption.

Condensation is when a cold surface comes in contact with warm moist air. We see this regularly on our soda or beer cans in the summer. But, if you take the same cold soda and put it in a styrofoam cup, it won't condense on the outside. The 1/4" styrofoam cup is just enough to prevent condensation because it keeps the outside of the cup at about the same temperature as the outside air. Condensation occurs when warm moist air deposits it's heat into the cold cup and water but the moisture which was entrained in the air can't transfer so it just accumulates on the outside surface.

Hydraulic pressure or hydrostatic pressure as you call it, is when you have liquid water build up and contained. This again is not the condition spray foam of any kind is designed for.

In BPI training they use an example where you have a 10'x10' drywall wall which is completely sealed and it only allows 1 pint of moisture to pass through this wall. Now if you drill a half inch hole in the middle, it will increase the moisture transfer by 90 pints! That's with only half inch whole! So, what this shows is air transfer is the primary mode of moisture transfer at over 98.99% and that only leaves 1.11% of moisture transfer that can occur through diffusion (passing through solid material).

So, when your talking about open cell spray foam, it is an air barrier which means it blocks 98.99% of all moisture transfer. So, that means that only 1.11% can transfer through the material. Yes, if the roof leaks it will allow mass water to come through the insulation material but it will dry out slowly without harm. I've seen this happen and heard about it happening from customer after customer. I've also heard comments such as, "I'm glad I didn't use closed cell foam because it would have ruined my roof". I can't say that would be true but I have to say that I've never had a problem like has been described in this article in over 10 years of business and primarily using only open cell foam on the underside of roofs. So, am I a fluke or is there something else going on here?

By: Allison Bailes

Allison Bailes — Sat, 07 Mar 2015 02:36:00 +0000

Skye: See
Skye: See what I wrote in the article above under the heading "Moisture from above." Joe Lstiburek studied the possibility of moisture diffusing through the shingles on a house in Houston and found it wasn't the source. He did essentially the same experiment you suggest but it was on one house, not two. See the photo above.

He now thinks it's interior-generated moisture that finds its way into the attic and then ends up at the ridge by what he calls the Ping-Pong Effect. I saw something that fits with this in a spray foam attic in Atlanta last summer.

David B.: I don't think it's as easy as you suggest to control humidity in a sealed attic or crawl space by simply addressing the source. In new construction it's easier to get close to doing that without mechanical assistance, but once a home is built, it gets much harder.

By: Richard Beyer

Richard Beyer — Sat, 07 Mar 2015 01:36:49 +0000

My brief 2 cents on David's
My brief 2 cents on David's question to Skye.

Let's not confuse open cell -vs- closed cell foam. Open cell foam is just that, open cell. Chemical manufacturer's claim if a roof leaks, water will migrate through the foam structure therefore not trapping the water.
Common sense tell's me if this is valid marketing then what stops water vapor from moving through the foam under pressure? Stopping air versus water vapor are two different things. Water vapor blows through concrete and tile grout while air does not. If I place a grout sponge into a bucket of water it floats. If I force it into the water it will absorb water. If I hold it out of the water the water drains from the sponge until the water weight finds a balancing point. If I string the sponge up it will still be wet the following day even though the sponge was designed to not hold water.

Seems to me the process of water vapor movement through pressure differences in a home are not far apart. Hydrostatic pressure and capillary action come to mind. The sponge will release the water over time but the OSB maintains the water because it's blocked from drying out by the sponge attached below it not allowing air movement along the panel which normally dries forces the drying out of wet wood.

Just my humble opinion. ;)

By: Skye Dunning

Skye Dunning — Sat, 07 Mar 2015 01:35:59 +0000

Air transport (yeah it's
Air transport (yeah it's "sealed" but of course there is still some leakage) and vapor diffusion. Afternoon rains and overnight dew load the shingles up with moisture which comes through. Especially when the sun gets on it. Super high vapor pressure there.

It would be interesting to monitor two identical houses with identical leakage, tested at the attic hatch, one with a typical shingle install & the other with a class 1 vapor retarder under the shingles.

By: David Butler

David Butler — Sat, 07 Mar 2015 00:14:37 +0000

@Sky: if the attic is
@Sky: if the attic is properly sealed and insulated, where's the moisture coming from?

By: Skye Dunning

Skye Dunning — Fri, 06 Mar 2015 23:37:45 +0000

I'll chime in here. We are
I'll chime in here. We are on the coast in southeastern NC. Very humid summer conditions. We have lot's of experience with sealed attics.

Mechanical systems can absolutely be implicated in sealed attic failures. We have lots of moisture migrating through the assembly to the attic. That moisture then dries to the inside of the house. The rate of that process is somewhat dependent on how dry the house is of course. If you have a system that is maintaining rh levels in the 60's (or above!) instead of the mid to low 50's in the house, you're going to have even more problems than normal in the attic.

I say more problems than normal because these attics, even when sealed and insulated correctly and a house with good rh levels, can have rh levels running in the 70's & spiking into the 80's. You can imagine what happens when the sealing is not what it should be.

We found this out by monitoring rh levels in sealed attics, just like we do in sealed crawls, starting back in 2005 or so. After seeing a number of these attics and coming to the conclusion that this was "normal" we started recommending drying potential be installed (usually via a dampered 6" collar on the supply trunk line) and requiring rh monitoring.

This is another case where you can't listen to advice from well meaning people around the country when you're working in the hot/humid climate (sorry David). We commonly have problems not seen in other areas of the country.

By: Richard Beyer

Richard Beyer — Fri, 06 Mar 2015 08:28:36 +0000

David,

David,

I do not disagree with your comments, but the following should be added.. HVAC equipment can be pointed to if it’s over sized. This could be the case in the Florida home. Poor air exchange and exhaust ventilation may also be the culprit however, it’s unlikely ventilation unless the family takes very long hot showers, cook a lot of seafood or boil pounds of spaghetti a week or have many occupants and lets no forget about the high humidity in the southern states. Unfortunately, a tight home needs constant Rh monitoring which most consumers are not made aware of until something fails. In my opinion, the residential code (IRC) you refer to has not caught up to the vast use and troubles of SPF. There’s certainly no shortage of energy promoters protecting it’s reputation. Allison posted a blog article some time back showing an installation of a dehumidifier in an attic space. I tried to located it to post here and was unsuccessful finding it. I also enjoyed reading Allisons blog about over size AC units shown here….

http://energyvanguard.com/blog-building-science-HERS-BPI/bid/76640/My-Big-Fat-Oversized-Air-Conditioner 

and this article

“Interaction of Unvented Attics With Living Space in Three Northeast Homes”

http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/unvented-attics-livingspace-northeast.pdf 

This illustrates proven building science is not in while most of these discussions are a theory of probabilities.