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Spraying foam over fiberglass in attic Post New Topic | Post Reply

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Lynn Mether
Posted: Jan 19, 2011 08:17 PM
Spraying foam over fiberglass in attic
First of all long time reader first time poster-
Now I'm not suggesting anyone spraying foam over fiberglass in an attic but how is doing that any different from flash and crap in the walls(not a proponet of that either) just wondering.

I have over 10 years of experience specifying and installing open and closed cell spray foam. I've sold my business but I'm still selling for the new owners and consulting on large and custom specific jobs. 

I've expanded my knowledge into t

Posted: Jan 19, 2011 08:46 PM
It's not much different! You stil have a vapor barrier on the outside of the wall (ceiling) system! You still allow the air carrying the moisture with it through the fiberglass and coming in contact with the foam. So, it depends on how much you install. If you only install an inch of Closed cell or 2" of open cell, then you'll likely miss a couple of spots and have bypasses right around your insulation. So, if anything I would recommend 4" of CC and 6" of OC to make sure you have enough insulation and R-value to prevent condensation.
Hope this helps!
Lynn Mether
Posted: Jan 19, 2011 09:16 PM
Thanks for the quick reply. That is kind of what I was thinking.
On a side note has anyone ever seen an attic with fiberglass installed good? I have not. Usually instead of cutting the fiberglass to lenght the just overlap it and have huge gaps in it. There is no way like Jim said the get a perfectly even coating of foam.
Posted: Jan 19, 2011 11:29 PM
Are you talking about spraying on top of the fiberglass on the floor, or spraying underneath the fiberglass that's between the roof rafters?
Lynn Mether
Posted: Jan 19, 2011 11:33 PM
on the floor of the attic is what I was thinking of. I haven't ever personally been in an attic with the fiberglass stapled to the roof truss.
Ivan Pauliuchenka
Posted: Jan 20, 2011 05:43 AM
What you think about installing blanket type of ROXUL at 2" or so (they have 1"-3") and then spray foam on top of that, like 1 1/2 or so. Can bring the total bill 10%-20% down. Just a way of taking job from flash`n`bat guys.
Posted: Jan 20, 2011 12:58 PM
Fiberglass is not considered a suitable substrate for sprayfoam. It would be better to install the foam first then install the fiberglass over it. I did this to my attic in Virginia, removed 14 inches of blown in fiberglass then installd 3-4 inches of sprayfoam and covered it with a fire rated cellulose as an igntion barrier.
Posted: Jan 20, 2011 03:32 PM
...blowin fiberglass is an ignition barrier as well...mineral fiber..they leave it exposed in attics all over the place..lol,,

as long as the cc app is less than 1 1/2" over the bulk of the system you still have have permiable membrane,,,

re flash n batt: round here these morons are sellin "itty bitty skim coats" 1/4 -1/2" (maybe they aint morons,,hmmm)followed with a bibs or batt...
well you get a pretty decent air barrier,,a couple three r,,,and the same potential for condensation in that wall system whether the foam was there or not,,,you just aint got enuff..
that california dude hit it on the head,,,

n the problem is,,repeat afterme,i shot the foamdude,,is that after a year or two of high energy bills and peeps squeelin about there foam home not workin when they aint got no foam home they just think they do,,,and the filterglass folks resell their foamrigs on ebay,,and i end up on a curb writing burma friggin shave signs!!

......uh,,,so what are we talkin about here,,,
rofl,,,yeah,,foam over filterglass,,dont do it..
philip mullins
Posted: Jan 31, 2011 08:38 PM
in a rental house right now. gonna remove the fiberglass and "blow in" some chopped up oc shavings, then butter it with a little oc to seal it in there. ill let you guys know how it turns out.
steven argus
Posted: Feb 01, 2011 09:07 AM
Good luck, troutkiller. We tried it a few years ago. What a PITA. Keep everyone posted.
John Shockney
Posted: Feb 01, 2011 10:47 AM
I have tried it too, the problem is the loose foam blows around when trying to spray it.

What does work is using the pour adaptor for the P-II gun and about 6-8 inches of ¼ ID tubing on the gun this way it comes out of the tube as a froth and you can flow it over and around the scrap foam without blowing it all over.

Still a PITA

John Shockney
Posted: Feb 01, 2011 10:58 AM
I have also sprayed over existing fiberglass when it was very difficult or impractical to do, what I have found is the liquid foam flows into the fiberglass and acts like the resin in fiberglass boat construction forming something like fiberglass-reinforced foam.

It is pretty tough when you spray 2# over fiberglass


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