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Mike_SF 12-07-2013 01:53 PM

Vapor barrier/insulation in unfinished garage space.
So my house was built in the 50s in typical San Francisco fashion, where gas was cheap no need to insulate anything. Fast forward to now, and I want to insulate the garage space. The house is built in a very typical SF fashion where there's a garage on the first floor and the living space is essentially on the 2nd floor. Now the living room which is right over the garage gets a wee bit cold so I want to insulate the garage.

Luckily I can access under the floor without too much issue from the garage. 2x10 beams holding everything up. However there's also a bunch of conduit for electrical that's attached under, so any chance of properly finishing the garage with drywall is probably non-existent (unless I rewire the house with NM cable and drill through the beams... not happening any time soon).

Now I got nice fat R30 fiberglass rolls I can cut to fit, however one part that is posing a problem is the vapor barrier. I basically have to cut out a piece of plastic the size of each opening, and tape it and the entire perimeter to get a good seal. This is extremely time consuming. I want to know if there are any other ways to get a vapor barrier in there that is easier, other than hitting it all with foam (which is out of my price range). Any sort of vapor barrier paint? I know I could use paper backed fiberglass but I don't see how that would really be a vapor barrier since I couldn't seal the edges.

cleveman 12-07-2013 09:19 PM

Put a couple of coats of paint on it with an 1 1/2" brush. First coat black for contrast, second coat white for contrast and so we can see any mold developing.

Gary in WA 12-09-2013 12:20 AM

Welcome to the forums, Mike_SF!

Are the floor joists above the garage exposed now?


Mike_SF 12-09-2013 07:31 PM

Gary: Yes they are exposed. There's 2x10s, in some places quadrupled up (double car garage over big empty void), and you can see the subfloor of the living space (1" T&G boards).

I only put plastic over one of the open areas (on the subfloor) approximately 15" x 63" area and that took more time that I wished, so thought I'd see if there was an easier way before I went too much further.

Windows on Wash 12-10-2013 07:04 AM

A true vapor barrier will not be required between the spaces but my concerns are more health and safety. Without drywall and a finite and tight envelope layer, there is going to be communication between the spaces and the likelihood of fumes from the garage getting into the living space as well as the potential for fire with the lack of a complete firewall rated assembly.

Can you post up a picture?

Gary in WA 12-10-2013 01:19 PM

My reason for asking is for fire-safety; Your State code may be similar, check with local AHJ.

Here is the best (other than cc SPF) way to get warm floors above the garage, requires drywall covering --as per code requirement anyway; Fig.7;

If in SF;

Your Zone 3 doesn't require a vapor barrier, especially poly, remove it;

Air seal the drywall, especially around the drywall cut-outs for the over-head door, light, garage door opener mounting hardware, etc.;

If worried, the asphalt paper-faced fiberglass could be used as it varies in permeability with moisture- edges are not required to be stapled for effectiveness;


Mike_SF 12-10-2013 06:46 PM

Thanks for the help guys!

Health: While I'm not terribly worried about the fumes aspect, as I never "warm up" a car in the garage when the garage door is open much less closed, in fact I have a Prius and I start it up and back out so that it doesn't even kick the engine on until I leave, I do realize that codes are not really there for what I necessarily do but more for someone in the future who might buy this house from me... etc. That said the garage door does have ventilation grates on it that counts for something right? :)

Safety: Even if I somehow managed to get in fire-rated gypsum up, that still won't completely block off the garage area from the living space. It's kind of complicated to how the downstairs works, but the parking garage immediately transitions into a workbench area, where the water heater and furnace are, that can go to a "utility room" via a door (25'x15' utility room) or directly upstairs to the living area which does have a door to close off from the bottom but it's hardly ever closed. I know the codes now probably won't allow an unfinished garage area directly below living space however due to the multitudes of conduit, gas pipes, water pipes, etc that ain't gonna happen anytime soon, in the future maybe, but for now I'm more concerned about blocking some of the cold from going up to the living space above. In the future drywall will go up, and I'll also put in a set of double doors to physically separate the parking part of the garage and the work area, but for now insulation is my concern.

Gary: You'll have to excuse me while I take some time to read through all those links you posted and try to decipher a lot of it. But I shouldn't have the poly layer between the subfloor and insulation? If so I'll happily rip that out (ok maybe not happily since I still got an ache in my back from the bending I did on a ladder to get that done). But regular asphalt paper backed fiberglass is as good as I need? Well color me stupid, I always thought the tabs really had to be stapled to the joists/studs when installing. I'll happily buy some of that and just chug through cramming it into the voids. Drywall will have to come later though.

Here are some images.
Here's the first one I did which was the front of the house, partially above the kitchen area with a bit in the living room. I stapled up the wood slats because I thought I would need to in order to prevent the insulation from falling back, little did I know it has no issue staying up.

This is a typical void, wood 1" T&G as subfloor, wooden floor on top of that in the living space (wife won't go carpet).

These next few pictures show the mess of piping and what not that crisscross the garage making putting up drywall an act of lunacy. Maybe if I rewire everything with NM wire in cavities drywalling might be easier.

Gary in WA 12-10-2013 10:52 PM

No poly. Think about furring down the ceiling for drywall install ease, you appear to have some room over the garage door opener. HVAC rooms usually require 5'8" or Type X drywall on ceiling also, please don't become a statistic.


Windows on Wash 12-11-2013 07:07 AM


Furr down the ceiling so you have a nice easy and flat surface to drywall too.

If you aren't worried about loosing a bit of height, an 1" of rigid foam would do well here to uncouple the framing although the furring will do that as well.

Be sure to check local code and get the fire barrier and drywall seaming done tightly.

SeniorSitizen 12-11-2013 09:02 AM

DO NOT staple the fiberglass insulation tabs between the stud bays even though FHB says it's ok just to appease the dry wall people. Staple the tabs/ flaps to the 2x4 edges like they were meant to be installed so the insulation isn't compressed.

Gary in WA 12-11-2013 05:48 PM

Good point by Fairview.

From your post 7; " Well color me stupid, I always thought the tabs really had to be stapled to the joists/studs when installing. I'll happily buy some of that and just chug through cramming it into the voids."------------------- Your warm in winter side = facing paper should be next to floor; "A need for heating is the dominant design
concern, but the climate is mild enough that
energy consumption is relatively low." From;,d.cGU&cad=rja

Warm floors;


Mike_SF 12-11-2013 07:30 PM

Thanks for the responses guys, saved me a lot of headache... and neckache and backache.

Hmmm never thought of furring down the ceiling. Might have to give it some though, although it might be just as remove the conduit and run new wire.

HVAC room is not a room, the entire opened area is what has the HVAC. Not trying to be a statistic simply going by what was done. Originally it was a huge small car sized gravity furnace, that got replaced by forced air unit and the inspector didn't say a thing about needing to drywall. Basically I would have to build a room around it to enclose it, would drywall on the ceiling be useful at all if the sides are open?

And yeah I know paper side towards the warm. I wasn't implying to flip it and staple it in upside down.

Gary in WA 12-11-2013 08:41 PM

You would really contort, then... lol. The ceiling drywall works with the wall drywall to stop a fire... I just wanted you to know the gravity of the situation. The electrical J-boxes will require leaving exposed (cannot conceal in ceiling) or lowered (possibly- all of it?). You have a lot to think about now.... been there done that, as have most of the others here, keep at it!


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