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Caution! Girl working
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Hello :) I have approx R-19 (old) fiberglass insulation with that paper-type vapor barrier in my attic and I'd like to get the R-factor up to about 45 and install a catwalk/flooring. Joists are more than 12" so there's plenty of space.

I know absolutely nothing about insulation so if this is a crazy idea please stop me lol! Can I cut Polyisocyanurate to fit tightly in between the joists and push it down so it stays in place, flush with the tops of the joists? This would leave an air gap between the current insulation and the Polyisocyanurate. I didn't know if an air gap was a good idea or not, or if the polyisocyanurate is recommended for use in an attic?

Also, if it has foil on one side, I assume it faces towards the living space?

The quotes I've gotten are too expensive and I have to do it myself so I was going for the most bang for my buck, which seems to be the Polyisocyanurate. Clean and high R-factor per inch, but only if it's OK for attics. Thanks!!!!
 

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I preface my response with I AM NO EXPERT.

Do not add too much insulation to where you have to 'push it down'. Insulation works by stopping air flow and limiting convective heat transfer. When you push down the insulation, you make it more dense and increase conductive heat transfer. So, by adding more insulation and packing it down, you're actually decreasing the effect of your insulation. I would recommend just adding blown in insulation and filling in the cavities between the joists.

As far as the foil backing, the foil prevents radiant heat (such as the heat you feel from a campfire). The foil works best when placed away from the living space, especially in hot environments. This prevents the heat from even getting to the insulation and further limits heat transfer.

I don't know much about polyisocyanurate but I did a quick google search on it and found a claim saying that it may not be worth the extra cost. Buyer beware!
 

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Caution! Girl working
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Discussion Starter · #3 ·
Thank you very much for the info. I've been doing research and discovered that disturbing the old insulation is a very bad idea, so I'm going to have to hire this job out. Hopefully, the contractor will be able to make a good recommendation. THANK YOU!
 

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You do not want to introduce another vapor barrier to the attic. The simplest thing to do would be to put unfaced insulation on top of what you have.
You want to keep the insulation from blocking the soffit areas so the venting system stays intact.
While you're up there, I'd look at the venting to see if it's enough for the area.
Most houses are under ventilated.
 

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Id recommend hiring someone to do the project unless youre experienced in that kind of stuff.

Have someone suck up all the old insulation. Once completely cleaned out, have them foam seal all the cracks (except above the canned lighting). Then have them blow in borate cellulose insulation.

As for pushing down the insulation...no..not a good idea as you want it fluffy, not compressed. For the most part, once you get a couple of feet of blown insulation up there, youre not going to be going back up there again. So any changes you want to make to the electrical, ceiling fans, etc, its best to do it before vs after.
 

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I recommend you removing your faced insulation nad having about 20" of Johns Manville white fiberlass blown in. Attic insulation does not need a vapor barrier.
 
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