Is batt (glass fiber) insulation OK to use when it is so susceptible to moisture degradation?


Is batt (glass fiber) insulation OK to use when it is so susceptible to moisture degradation?

Asked by Greg Lockridge

We are a closed cell spray applied polyurethane sub-contractor trying to bid on a LEED silver job (uiversity building) that specifies only glass fiber blanket insulation.

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Elizabeth DiSalvo's picture

Hi Greg,

Our company has done a couple of LEED Certified Homes and have been building homes that could have been LEED certified for over 10 years.

It is crazy to me that people still spec glass fiber insulation. That is my personal view.

I know fiberglass batt can be used to reasonable effect if installed in an extremely meticulous way and air sealing is meticulously addressed and then be an 'ok' solution but those levels of meticulousness rarely happen.

  • Especially on a large building like a university building.
  • I was also the on-site architect/ supervisor for a 55,000 SF school building in NYC for 3 years. I know first hand how impossible it is to monitor all of the guys working on the site all of the time. It ain't gonna happen.

Air infiltration

You have a few issues here.

First of all a LEED certified building needs to have a pretty great building envelope to achieve the basic requirements of LEED for air infiltration, insulation levels and quality control and maintenance.

The envelope should be designed in the first place to avoid all chances of 'moisture degradation'. Proper layering of exterior materials, flashing, caulking, moisture and or air barriers should be installed to avoid moisture infiltration on the exterior most levels.

Inside the walls themselves you can mitigate moisture issues from condensation and moisture transfer by controlling the temperatures within the wall and by air sealing.

Closed cell foam

The beauty of something like closed cell foam is that is kills both of those birds with one stone.

  • It has a great R value per inch that precludes condensation from happening within the wall assembly and it goes a long way toward 'killing' air infiltration.
  • (Moisture travels on air via vapor- so where you have air infiltration you are more likely to have moisture issues.)
  • There are always some small cracks with the foam install where there is pulling away from other materials but this is not the norm with a really good installation of closed cell foam.

Foam may not get every bit of air infiltration but the odds go way up of getting most. (and you are never going to get them all).

Caulking and taping

The hard part about batt insulation is that it is often installed poorly (like 95% of the time) and it does not really do anything to deter air infiltration- even when installed well.

  • So if you install batt and want to control moisture then you are going to have to take other measures to control air infiltration.
  • (You should do this with foam as well- belt and suspenders- but it is not AS necessary).
  • So we are talking about a lot of caulking and taping joints and meticulously following every crack between materials and around every penetration.
  • It is just a LOT harder to do with batt.

3.5 ACH vs. 1.0 ACH

As an example, we just finished a house in which the clients did not want any foam on site for worries over VOC's and what foam production does to the environment. (Both valid concerns and I supported them.)

  • They chose to use cotton batt insulation (which by its nature installs more 'fully' than fiberglass batt).
  • The contractors bent over backwards to air seal that house. It really took them a ton of time and difficulty.
  • We did a blower door test and got a mediocre score of about 3.5 ACH 50p.
  • Six months before, we completed a house of comparable envelope design that we blew both closed and open cell foam into and it achieved ad 1.0 ACH 50p blower door result.
  • VERY MUCH better! And very easy for the contractor.

Meeting the minimum requirements

The client needs to be very careful if they are going for LEED certification that they meet the minimum requirements for air infiltration and energy use for the building.

  • They may simply not be able to achieve the minimums required by LEED using fiberglass batt.
  • They may be able to... but it will have to be really thoroughly thought out ahead of time and then a construction plan has to be meticulously adhered to. Tough to do on a large job site.

Good luck!