# Question on insulating existing drywalled walls with blown-in



## Holbs (Nov 4, 2012)

Santa was kind to me in regards to employment bonus. Tomorrow, going to insulate garage door with 1.5" R-Max rigid insulation with foil. Next, I might actually tackle blown in insulation for the walls. But first, a question: I have 2×4 spacing between outside structure and inside drywall. I have seen many video's of DIY blown in and they always start mid height for the hole to stick hose down inside and then up. Yet, many folks say blown in settles after a time leaving a void uptop. What if I cut 4" all along the top of my 9' high by 20' length walls, blow in the insulation, leave it uncovered for 6 months or a year, and then re-apply by hand more blown to top it off in before drywalling and finishing?
I would imagine, much easier to mud, tape, and finish a 20' long line of drywall than 20 or 30 4" circle drywall holes.


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## splatman (Jul 27, 2014)

Removing a bit of drywall from the top of the walls is just how I would do it. Holes at the midpoint do not make sense. It would seem the insulation would settle before the wall cavities are filled, preventing the cavities to be filled completely.

Cut the drywall even with the top plate (leaving a bit of drywall along the ceiling) and maybe 3" below, giving you a 3" gap. Widen the gap if it's too narrow. After filling, screw on 1×6 (or wider) boards all along the top, as molding, to cover the gap. Unscrew the molding 6 months or a year later to check for settling and add insulation, if needed. The strip of drywall at the top will allow the molding to sit plumb. Paint or varnish the molding before installation, so no working on a ladder with a paintbrush or roller.

Bonus: If you ever need to install new electric circuits, you would only need to remove the molding and drill the studs for the wires. Run the wires along the top of the wall, then drop to the switch or outlet location(s).


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## JBrow (Nov 18, 2015)

Holbs,

I see potential issues with the top only slot plan. The first is potential fire blocking somewhere within the wall cavity. If present, a top slot would allow only the upper portion of the bay to be filled with insulation. Fire blocking would prevent the lower portion of the bay from receiving any insulation.

The second problem could be damming of the blown-in insulation creating a barrier within the wall cavity and thus leave voids. This problem would probably show up most where electric wiring and receptacle boxes are found in the bay.

If no blocking exists and you can figure out how to ensure the entire wall cavity fills with insulation, I think your idea is a good one. However, I like splatman's idea of covering the wide slot with something that is removable. If you go with the wood trim idea, I would back up the wood trim with drywall attached to the wood trim in an effort to preserve the fire rating of the wall and keep the trim board setting on the surface of the wall rather than inset. Splatman's idea would save the work of repairing the drywall and allow future access to the wall cavity.


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## Holbs (Nov 4, 2012)

Jbrow… I ran my own electrical through out my shop. I know for certain, no firebreaks / horizontal bracings installed. All electrical lines and boxes are 52" high though. 
Blown in insulation can be dammed up? They are so little and soft  But something to keep in mind.


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## DirtyMike (Dec 6, 2015)

That is how I would go about it, I suppose you are considering cellulose?


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## Holbs (Nov 4, 2012)

I'll have to research the different material to blow in. If I remember right from my past inquiries, the stuff at HD or Lowes has flame retardant. But I currently forget which has the better R value and the details.


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## splatman (Jul 27, 2014)

To clear up any misunderstandings, here's a cross-section of the top slot with trim idea, utilizing JBrow's drywall backer suggestion:









One thing I forgot to mention, is what to do below windows. The solution: Cut a slot below the window sill, insulate, then cover the slot with a similar piece of trim, and call it a window apron.

For those of you considering this and have walls with blocking: Cut another slot right below the blocking, insulate, and cover with trim, just like at the top, except bevel the top of the trim so it doubles as a French cleat. Make sure it is firmly screwed to the wall studs. Or leave the top unbeveled and call it a chair rail.


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## rwe2156 (May 7, 2014)

Talk to an insulation company. You may be surprised at the cost difference, especially if you do the prep and repair. Plus they may give you better options what to use.


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