foam over fiberglass | Ask Mason Knowles - SPF Consultant | Spray Foam Technical Questions and Answers | SprayFoam Magazine Q&A Forums

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Posted: Jul 13, 2009 09:17 AM	foam over fiberglass Mason, Have a client who has several housing units that they want to install R-19 oc foam over existing R-11 fiberglass batt. These units have very low sloped roofs approx 1 pitch and these attic areas only have approx 2' of clearance from the roof deck to ceiling joists. I have expressed my concern in spraying directly to the batting but they want documentation showing the concerns of spraying over the existng batt. In a typical structure this would be no problem, however there budget numbers are tight and they really do not want to remove the insulation if not completely necesary. I have provided them with plenty of anecdotal but they want documentation. does any such article or testing exist. thanks
mason Posted: Jul 13, 2009 12:36 PM	You don't need documentation just common sense. Fiberglass doesn't have sufficient strength to be a good substrate and if you have a roof leak, the fiberglass would get soaked and the foam wouldn't allow it to get out. I am not aware of any testing or documentation, I haven't seen it in practice except in roofing situations, where the foam was sprayed over fiberglass roof insulation. The foam roof didn't hold up well, plenty of substrate blisters, but the fiberglass was wet also. Anybody have some anecdotal history?
Posted: Jul 14, 2009 06:45 PM	Thanks Mason, Being that the foam would be sprayed directly over the fiberglass I do not think we need to be concerned about substrate adhesion as this is not a sidewall or roof deck application. The foam would be applied over the fiberglass. My concerns were more from an applicators standpoint, it is much more difficult to spray to batt and achieve consist thicknesses,and not to mention the joys of spraying it with an air purge. Also I do not know what the condition is under the batting. I am very reluctant to apply and certainly would require a waiver however I was just looking for some documented info to help me show the importance of removal. Also I need to mention how little common sense people seem to have in regards to building science. when I recommend conditioned attics people look at me like I have 3 heads. We have made great progress but still have a long way to go. Thanks Again

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Posted: Jul 13, 2009 09:17 AM

foam over fiberglass

Mason,

Have a client who has several housing units that they want to install R-19 oc foam over existing R-11 fiberglass batt. These units have very low sloped roofs approx 1 pitch and these attic areas only have approx 2' of clearance from the roof deck to ceiling joists. I have expressed my concern in spraying directly to the batting but they want documentation showing the concerns of spraying over the existng batt. In a typical structure this would be no problem, however there budget numbers are tight and they really do not want to remove the insulation if not completely necesary. I have provided them with plenty of anecdotal but they want documentation. does any such article or testing exist.
thanks

mason

Posted: Jul 13, 2009 12:36 PM

You don't need documentation just common sense.

Fiberglass doesn't have sufficient strength to be a good substrate and if you have a roof leak, the fiberglass would get soaked and the foam wouldn't allow it to get out.

I am not aware of any testing or documentation, I haven't seen it in practice except in roofing situations, where the foam was sprayed over fiberglass roof insulation. The foam roof didn't hold up well, plenty of substrate blisters, but the fiberglass was wet also.

Anybody have some anecdotal history?

Posted: Jul 14, 2009 06:45 PM

Thanks Mason,

Being that the foam would be sprayed directly over the fiberglass I do not think we need to be concerned about substrate adhesion as this is not a sidewall or roof deck application. The foam would be applied over the fiberglass. My concerns were more from an applicators standpoint, it is much more difficult to spray to batt and achieve consist thicknesses,and not to mention the joys of spraying it with an air purge. Also I do not know what the condition is under the batting. I am very reluctant to apply and certainly would require a waiver however I was just looking for some documented info to help me show the importance of removal. Also I need to mention how little common sense people seem to have in regards to building science. when I recommend conditioned attics people look at me like I have 3 heads. We have made great progress but still have a long way to go.
Thanks Again

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