Cutting insulation

yummy mummy

I am about to cut my blanket insulation that was previously installed in my basement. (It has a vapor barrier on it)

Would I be able to cut the insulation to the size that I need and leave the vapor barrier that is attached to it? I have tried to pull the insulation apart from the vapor barrier, and some of it is sticking. I would make it a lot easier to cut.

It would also help me to staple it to the studs.

I also plan to put a vapor barrier throughout in the normal fashion.
(So in essence, the insulation that would be between the studs would have double vapor barrier over it.

Can I do this, (double vapor barrier, or is this a no no?)

I am not sure if I am explaining this correctly.

HarryHarley

I would not use a double vapor barrier as described. Water vapor will condense between and cause mold problems

majakdragon

As stated, do not use a double vapor barrier. For cutting batt insulation, I have found it easier to cut by using a straight-edge (piece of thin wood or metal) and compress the insulation as thin as possible and then cut with a sharp utility knife.

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KUIPORNG

The best is to buy new precutted insulation to avoid all these hassel..... I tried to use old insulation before.... it is a real messy job compare with using the new materials... old insulation can be used at odd places where it is not 16 inches... and you should have plenty of those... save the standard slots for the new materials so that at least you can do some of the job at a easier fashion...

yummy mummy

thanks for the advice.

I am going to try to cut it, and if I get really annoyed, then I will purchase new one, as you stated Kui****g.

Bonus

Cut a slot in a piece of scrap plywood, then compress the insulation with the plywood (stand on it) then cut through the slot. This gives you a nice smooth, square, straight cut if you're careful with your placement.

yummy mummy

Thanks Bonus for the advice.

I have done exactly what you have told me, and I do get a nice smooth cut.
The only problem is that afterwards I have to peel back the vapor barrier that is there, and the task become quite messy. But I will do a little everyday, so that I don't eventually give up.

AtlanticWBConst.

You can also use a scrap piece of plywood to cut on (protects the knife blade). Then take a scrap piece of 2x4, with the insulation: paper face up, use the 2x4 to compress the insulation flat. Then use a sharp utility knife to cut the paper and go thru the insulation at the same time. Comes out nice and straight. use the same technique to cut long lengths from the roll or batts.
You can hold the 2x4 or even a 2x6 down with your 'off-cutting' hand and your knee...

The 2x4 compresses the insulation for easy cutting and also acts as a straight edge for straight cuts.
That's how we do insulation on our jobs..

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troubleseeker

I would definately not do the double vapor barrier, it is just asking for trouble. Compress with any type of straight edge for cutting. Many of the pro insulaters use a 8 to 10 inch kitchen knife for cutting. The long blade goes through the depth of the batt easily, and it can be readily sharpened.(fiberglass will dull the edge pretty quickly)

yummy mummy

Thanks for the advice troubleseeker.

I will definitely not use double vapor barrier.

mighty anvil

A vapor barrier material that is not sealed to the studs covered by a continuous vapor barrier on the warm side of the insulation will not cause a problem.

AtlanticWBConst.

Huh??? ....Could you re-phrase that ???

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yummy mummy

huh.....from me too.

I don't quite understand.
But nevertheless I will not be doing double vapor barrier.

mighty anvil

If the paper or foil facing on the insulation is not stapled to the studs it will not form an effective vapor barrier. When you cut the insulation cut the excess facing off or leave the edges folded back. Even if the loose facing could retard vapor movement, no vapor would condense on the warm side of the insulation so it's not really a problem.

yummy mummy

The insullation that I have already has a vapor barrier on it (plastic stuff).
If I cut it I wanted to leave the plastic vapor barrier intact as opposed to detaching the insulation from the vapor barrier.

There is no paper face on the insulation.
(Only insulation and plastic vapor barrier.)