Insulation in the floor structure above the garage?

[JakeKohl]

I'm new to the forum and have hours ahead of me just reading about other people's projects - great site.

I've been working on a 2 story 24x36 garage that is attached to a pre-existing 12x19' workshop. The 2nd story will be office space for my existing home business. Both up and downstairs will be heated and cooled but the garage will spend most of it's time at very minimal temperature settings and there will be times where I have the doors wide open for extended periods (fumes from composites). It has taken be about three times longer than planned to get to the point I'm at now - but I'm pretty happy about things.

In short, I'm at the point of insulating the garage. I'm hiring a contractor to do it and have already selected the outfit and am going to go with blown in wet/trimmed cellulose in the walls and loose blown in fill in the attic(s). All the contractors I interviewed had different opinions regarding insulating the floor structure between the 1st and 2nd floor. One didn't want to insulate it (and was defiant about it), one didn't know what to do but suggested spray foam after I brought it up (saw the $ potential), and the 3rd (the one I selected) was knowledgeable and recommended continuing with the loose fill cellulose by stapling up a mesh screen, cutting slits, and blowing it in to mostly fill the cavity before the drywall goes in.

I'm most interested in venting - I've lived through poor ventilation in an attic above a house before (moisture) and want to avoid anything approaching a mold issue. Is it OK to blow in the cellulose and then box in the entire floor structure with no ventilation?

Jake
www.teamseacats.com

 

[matouse3]

It will not be OK from what I have read, as you are not providing a moisture or vapor barrier with the cellulose. Here is my thread (as I was just discussing this topic), and I think there are some really good links in there that you should read.

http://garagejournal.com/forum/showthread.php?t=139203

 

[JakeKohl]

It will not be OK from what I have read, as you are not providing a moisture or vapor barrier with the cellulose. Here is my thread (as I was just discussing this topic), and I think there are some really good links in there that you should read.

http://garagejournal.com/forum/showthread.php?t=139203

That thread appears to be discussing the attic / roof trusses....and I agree, the roof needs to be vented. A spray foam contractor I interviewed advocated spraying the roof decking/rafters and leaving the attic area sealed and unvented. I'm not a fan of that!

I'm talking here about the floor structure between the first and second stories. I will have a temperature differential between the two - what's the best plan with regards to insulation / moisture?

 

[matouse3]

The thread wasn't the exact topic you are discussing, I know, but the links provided in the thread are what I was directing you toward. I think you can find your answers there or at least some advice.

Do you have access to the joists for spray foam? If so, I would say that that is your best options. Not cheapest, but best. I did the ceiling of my second story (old farm house) to separate from the attic space and it worked great. No moisture issues and the insulation is air tight.

As far as your roof venting comment, I would read those articles, they have made me a believer once you understand what the true purpose of venting is.

Good luck.

 

[jbs]

I am in a similar situation. My upstairs is heated (not sure about cooling yet). The downstairs is occasionally heated (during use). I insulated with unfaced R30 fiberglass between the floor joists. When I do the wood floors upstairs, I will first put down builders' felt to act as a vapor retarder.

My thinking is that the upstairs is living space and will have the most moisture (just from people since there is no kitchen/bathroom up there). The downstairs will not have significant moisture. When my heater (electric) is on downstairs, it will actually lower the relative humidity down there. In addition, I used drywall downstairs, which will serve as an air barrier (ADA). So I never anticipate warm, moist air moving from downstairs, through the ceiling, and condensing on the bottom of the vapor barrier above. That is the only scenario that I see where moisture could be a problem.

Just my 2 cents. Obviously, even if I were done with it all, it would be sealed up, and not accessible, so I couldn't tell you if my logic is correct.

 

[rsa]

Disclaimer: see my sig

Is it OK to blow in the cellulose and then box in the entire floor structure with no ventilation?

Yes.

While he's in there, why not fully dense pack it instead of "mostly filling" it?

Look through some threads on Green Building Advisor. A google search something like garage ceiling dense pack site:greenbuildingadvisor.com will pull up some valuable threads. Loosen or tighten the search terms depending on the results you get. Note the comments about air sealing. You'll see recommendations about installing rigid polyiso to the joists in addition to the dense pack cellulose. It would seem mandatory in a cold climate with living space above, but it cuts down on garage ceiling clearance and adds to the price. But if you need the R-value and thermal break, I guess it would be OK in South Carolina.

You might find How to Install Cellulose Insulation interesting to read before the contractor starts.