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Old 10-31-2018, 11:40 PM   #16
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Re: Best practices for new insulation in 2x4 walls


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The foam has to be scraped away level with the boards after it expands and dries.
And that is one of the problems with foam, it has trouble getting into narrow cracks. The same putty knife attention to some caulking or Mastic pushes the material well into the cracks where it stays.

I've used mastic on ducts and it is very similar to the mastic used for flooring, how similar I don't know. Where ducts can be a challenge accessing all the way around I look at those walls and it looks easy. There would be no need to make it pretty, just push it into any gaps. Big gaps might need something more than just the mastic so some foam there could be useful

One of the misconceptions about using can foam is that is is providing some insulation value. The truth is, so little it would be difficult to measure. But the air sealing by whatever method is vary measurable.

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Old 10-31-2018, 11:50 PM   #17
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Re: Best practices for new insulation in 2x4 walls


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I did look into Rockwool, and I do think that might be a better option. I just need to find a place that sells it. I've never seen it at a big box store.
I just needed a little bit of insulation last week and picked up some Rockwool from Lowes. It was cheaper than fiberglass in small quantities.
The HO liked it so much they went out and bought some from Home Depot, but it was another brand and not as good as the Rockwool brand.


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I have heard that it will hold moisture and mold, so keeping water out is a must.
Keeping water out is a must no matter what you're insulating with! Fiberglass won't do any better than rockwool.
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Old 11-01-2018, 12:01 PM   #18
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Re: Best practices for new insulation in 2x4 walls


I can start trying mastic and foam in various places. Many of the gaps are plenty big enough for the foam can straw to fit in. I've never tried the mastic, but I'll get some and see if it would be a manageable job. If I spent some money, I could get the large spray foam kit from Lowe's which would probably be enough to do an inch or so on the 3 exterior walls I'm dealing with. It just might make R13 fiberglass too tight.

I looked into Rockwool, and apparently the other three Lowe's in my city carries it, just not the one closest to me. I'm leaning towards using that, although I do have plenty of electric and drywall blocking to contend with.
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Old 11-01-2018, 01:53 PM   #19
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Re: Best practices for new insulation in 2x4 walls


A fine tooth saw, bread knife, or some people use an electric knife to cut the mineral wool. Always make the width tight by 1/4" to 1/2", less for smaller gaps.

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Old 11-01-2018, 02:16 PM   #20
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Do I split Rockwool around electric the way I would fiberglass? Is it as sensitive to compression as fiberglass?
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Old 12-14-2018, 01:09 PM   #21
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Re: Best practices for new insulation in 2x4 walls


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Do I split Rockwool around electric the way I would fiberglass? Is it as sensitive to compression as fiberglass?
Yes, you split it on the edge for the studs if if the wires run down the joist and split it horizontally if cables running left to right. It is essentially compressed fibers so compression shouldnt be an issue. It cuts nicely and is easy to handle. wear glasses if installing it in overhead areas.
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Old 01-10-2019, 03:31 PM   #22
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Re: Best practices for new insulation in 2x4 walls


Creating an air seal in every stud bay, at every horizontal joint between the sheathing boards would be a tremendous amount of work -- and hard to get perfect; though, if you're the labor, that's a big consideration.

I'd just make sure you have a plan for the whole wall assembly, inside and out, before you button up he interior with drywall. You may be doing it in stages, as opposed to a complete inside/outside gut, but at least know you'll have access to both sides if not now, in the near future.

If the exterior air barrier options aren't in the budget when you re-side (taped zip sheathing, taped rigid foam, etc.) then you might want to spend some quality time with canned foam and a caulk gun.
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