Best way to insulate attic in attached garage

[Tinkerer2]

The attic space over my garage is about 1000 sf. About 200 sf is floored with chip board. I'd like to keep that part floored. There are ceiling lights, electrical boxes and wiring under the floor. All floor joists are 2X6. I want an R-factor of at least R-38 for the non-floored area and was thinking about InsulSafe blown in insulation. However, I don't know what to do under the flooring. Not sure how far I can get the blower hose under the flooring due to the wires, boxes, etc. Therefore, there would also be an issue trying to push/pull a batt across that space under the floor. I know I will have to settle for what I can get under the floor but what might that be? Taking up the floor and putting 6 inch thick batts may not be an option because I think I would destroy all of the chip board trying to get the nails out.

What might my options be?

 

[pcmeiners]

Get you *** up there in cooler weather or in the early morning and drill 2.5-3" holes in the chip board (carbide hole hawg bit) every 6ft, in every rafter cavity, rent a insulation blower, pick your poison fiberglass or cellulose. Blow in the insulation until the machine bogs down, move on to the next hole. Yes it is not easy or worthwhile to try and remove the nails, perhaps the chip board was screwed down? Filling 2x6 cavities will produce an R value of approx R20. Cellulose is the best insulator with the main negative, if it get wet it can be an issue.

 

[Tinkerer2]

Get you *** up there in cooler weather or in the early morning and drill 2.5-3" holes in the chip board every 6ft, in every rafter cavity, rent a insulation blower, pick your poison fiberglass or cellulose. Blow in the insulation until the machine bogs down, move on to the next hole. Yes it is not easy or worthwhile to try and remove the nails, perhaps the chip board was screwed down?

Never thought about doing that. Is there an easy way to properly plug those holes? And do you have any idea how much would get packed in there when the machine bogs down? Idea on R factor?

 

[WisJim]

When I had a similar situation, I tore out the existing flooring and added another set of joists perpendicular to the existing ones, added 11 inches of insulation and then new flooring. Lots of work but worth it over the years.

 

[pcmeiners]

With the Hole Hawg bits, you keep the round plugs you drill out to plug the holes later, done it, used a little body filler to glue them back in the holes. Once t, he machine boggs down the insulation is pretty tight. You need to aim the nozzle in both directions. My present attic has some chip board, insulation below it, basically in an unusable space so I plan to shoot at least a foot of cellulose right over the chipboard.

Wisjim way is the best way but it is a lot more work.

 

[Gozo]

Me, I’d pull up the flooring and insulate between the joint and then put the boards back.
Is there going to be a flat ceiling, or pitched along with the roof? One thing I’ll pass along from experience; make sure you’ve got good roof ventilation. I’ve used premade baffles to enable air flow from the soffit vents to the ridge vent, and the the insulation goes over (well, under from the looking up at it point of view), then the ceiling material like drywall. Makes a big difference in roof life as well as moderating the temperature extremes.

 

[nadogail]

Never thought about doing that. Is there an easy way to properly plug those holes? And do you have any idea how much would get packed in there when the machine bogs down? Idea on R factor?

Save the plugs cut from the holes, after the insulation has been blown in work a piece of wood through the hole and fasten it with a screw through the floor that penetrates both ends of the piece you inserted. With lots of construction adhesive set the plug on the cross piece and screw it down. It will be stong and airtight.

 

[pcmeiners]

Makes a big difference in roof life as well as moderating the temperature extremes.

Agree 100%. Years back I read a number of studies stating it does not matter much if you have poor ventilation in an attic. Most of the studies were paid for by those in the roofing industry. My present home has the original roof from 1983, as a contractor I did many asphalt roofs and I know my present roof will easily last another 15-20 years more. Why, because I have exceptional ventilation, I have an open vent area of at least 120squft, a ridge vent and the soffit openings. If it is 95° outside my attic is only 2° more, NOT 30-55° more like the average attic.
As result I use very little AC, if not so spoiled could do without it.

 

[paredown]

One thing to watch for (depending on how old the build is)--if there are older can lights from below--some were made as "IC" for insulation contact, and others were not (non IC), and those require that you leave space around the can so you don't burn the house down.

Or swap out the cans with new LED disc lights or LED in modern cans and fire away.

Don't forget your eve ventilation as well is you are blowing...

 

[mm08822]

Pull up the flooring and add 2x4s onto the 2x6s. Now you should have about 9" space to add insulation.
Put each sheet back with only 6 screws. No need to nail the piss out of each sheet.

 

[Tinkerer2]

I found an insulation company and it sounds like they are going to it right. Baffles for the soffits. They are going to blow insulation in under the flooring. The floored space is about 8X17 and you can get on each side of the 17 ft length to blow in insulation. Only obstructions are some wiring for lights and 2 junction boxes (one for garage door opener). They are going to put up a plywood dam around the floored area to keep the insulation off the floor. Putting in R-38. Floor joist are 2X6 so can get 5.5 inches of insulation making that area R-12 - R-15 maybe. Not as good as it could be by removing the plywood, adding sisters to make the area deeper and reattaching the plywood but that is a lot of work.

Wondering whether it might be a good idea to lay 1.5 inch polyisocyanurate sheets over the plywood and then add 7/16 OSB over the top. This would make the R rating R21 - R-24. Price would be about $250. Is that a good gain for the cost?