Garage Ceiling and Insulation - Rigid Foam Board Install with Pictures

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Old 01-01-15, 04:05 PM
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Garage Ceiling and Insulation - Rigid Foam Board Install with Pictures

I searched high and low for ideas on the best way to finish my garage so it would retain heat.

It took a lot of time to come up with what I did, so I wanted to give back to the community and show what I did.

My garage was already insulated and sheet rocked on the three walls. That left new garage doors (mine were original 1960s) and the ceiling.

The ceiling was exposed rafters/sheathing. My original plan was to simply stuff insulation between the rafters and call it good. That is until I read just how bad of an idea that is. There needs to be an air gap between the insulation and sheathing so condensation doesn't build up and cause mold and wet insulation.

The general consensus is do it right, install sheet rock and insulate on top of that. There are other methods too, but this foam board was the best for me. It made the most sense, and gave the most similar appearance to finished ceiling.

Sheet rock is heavy. Installing it on the ceiling has got to be one of the worst jobs I can think of. Taping and mudding is no joy either. The cost to have it done was in the thousands.

After a ton of searching I came up with rigid foam board and blow in insulation on top of that. I opted for 2" thick 4x8' sheets. Total cost was just under 700 dollars for rigid board, tape, and 3" screws with fender washers.

The install was easy for one person to complete. The board does not sag at all and has an R value of 13.1. I've read you can also use as low as 1" board without sagging problems.

The brand I used was R-Max Thermasheath. Got it at the Home Depot- here's a link - Rmax Thermasheath 3 - 2 in. x 4 ft. x 8 ft. R-13.1 Polyiso Rigid Foam Insulation Board-613010 - The Home Depot

After installing and taping the R-Max board I managed to find blow in insulation ( the nice white cellulose type) for $3.00 / 55 gallon bag. I picked up 30 bags. All thanks to Craig's List.

My garage holds heat better than any other room in my house.

I also added storage and an access door. I couldn't be happier and my wallet thanks me.

I hope this helps someone else down the road.
 
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Old 01-01-15, 04:08 PM
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Old 01-01-15, 04:11 PM
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Old 01-01-15, 04:14 PM
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I hate to burst your bubble but most building codes require a minimum of a one hour fire resistance wall and ceiling covering in a residential garage if the garage is part of the house. You may need to cover all of that foam insulation with fire code (5/8 inch) plasterboard to meet those code requirements.
 
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Old 01-01-15, 04:36 PM
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That foam board is fire resistant.... **Edit** I stand corrected.

http://www.rmax.com/files/7314/1832/...et_09-2014.pdf

It is specified that a layer of gypsum board be placed over all interior applications. Oh well...... I live on the edge...

Specs:

R-Matte Plus-3/Thermasheath-3 is rigid foam plastic thermal insulation board composed of environmentally sound, closed cell, polyisocyanurate foam bonded to a durable white-matte (non-glare) aluminum facer and a reflective reinforced aluminum facer. This product is suitable for use in wall sheathing applications in new residential, commercial and agricultural buildings and for thermal retrofit construction to existing buildings. This Energy Star qualified Rmatte Plus-3/Thermasheath-3 provides R13.1 with dimensions of 2 in. x 4 ft. x 8 ft. This product is designed for exterior and interior, non-structural uses.

Rmax Polyiso sheathing offers the GREATEST R-value per inch currently available in a rigid foam board.
R-Matte plus-3/Thermasheath-3 is applied to the exterior or interior face of wood or metal studs to cover all studs, sills, plates and header constructions, thereby providing insulation over areas not normally covered by insulation products
Superior resistance to moisture and vapor in above grade applications.
Polyiso ridgid foam board offers superior fire protection properties compared to any other ridgid foam board insulation.
Energy Star qualified to meet or exceed federal guidelines for energy efficiency for year-round energy and money savings.
When taped and flashed, the Rmax Polyiso board greatly reduces air infiltration and prevents thermal bridging caused by heat and cold tranmission through studs.
Lightweight and easily cut to size
 
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Old 11-03-15, 03:28 PM
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Insulating garage ceiling

I have a three bay garage with an apartment over it. The floors of the apartment are cold in the winter. I just removed 1 inch of styrofoam board that went across the floor joists and 6 inches of fiberglass insulation that went between the floor joints. There are quite a few heating ducts running along the joists which complicates my decision to go with expensive spray foam since the ducts are flush with the bottom of the 2 X 10's so there's not much room for insulation when I strap the ceiling and install gyprock.
I was thinking of using 2 or 3" PolyIsocyanurate sheets which have a reflective surface on it.
This would mean, there would be a continuous insulated surface which would have the same R value even over the duct work and I wouldn't have to work around bracing, wires and plumbing. There would alot of air between the joists. Is this a good or bad idea and why?
 
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Old 11-03-15, 05:44 PM
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The polyisocyanurate (and other rigid insulations) as a continuous insulated surface is an excellent idea and is actually used fairly commonly in commercial construction, especially in retrofit applications. HOWEVER, it must be covered with a thermal barrier such as sheetrock as stated in the R-Max literature. The reason for this is that when it burns, it creates large amounts of smoke and toxic gasses. I wouldn't take that lightly.

So you have to be thinking about the thickness of the insulation if installing sheetrock over it. I actually used 1" polyiso over rafters in addition to batt insulation in a house I built for myself in the late '70's. I went over the polyiso with 5/8" sheetrock and used 2 1/2" screws to get the penetration into the rafters that I wanted. It took some getting used to, driving those long screws, but it was doable.
 
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Old 11-03-15, 06:30 PM
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Do you see an issue with having so much free space between the rafters? The thermal envelope would be 10 inches away from the bottom of the floor with only air between the rafters. A number of articles that talk about using fiberglass batts state that it's critical that the batt touches the upper surface (bottom of the floor upstairs) otherwise it's not that effective.
I'd prefer to not fill that void in the joists since there are a lot of wires, ducts and bracing that's in the way and also thinking that any air leaking from the hot air ducts will do so above the iso panels.
 
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Old 11-03-15, 07:30 PM
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I have never heard that the batts should touch the floor above, but I've been retired for almost 5 years now; maybe research has changed things. If you have an air space with no air movement, the air space actually adds slightly to the R-value. In fact, when I used to do the calculations to show conformance to the Minnesota Energy Code, even an air film with moving air had a very small R-value. But I have to say, if it were me, I'd get some insulation in that joist space, even when it's a PIA with all the cutting and fitting if you use batts. I think you're going to find you will be limited as to how much thickness you can get with rigid insulation and still attach sheetrock directly with screws; that means less R-value.

If you can find metal Z-furring channels, they will allow you to use much thicker rigid insulation. They look like this: http://www.steelconsystems.com/sites.../Z-Furring.pdf You screw the channels to the joists at 24" o.c., push 24" wide pieces of insulation between the channels, then screw your 5/8" sheetrock to the channels with standard 1 1/4" screws.
 
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Old 11-04-15, 03:25 AM
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On a side note... you need to install safety cables on your garage door extension springs
 
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Old 11-04-15, 09:55 AM
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Thanks for your input Bruce.

After talking with a local hardware store owner, my plan (at the moment) is to screw 2X3's on edge across the floor joists and fill the space between them with 1-1/2" of Styrofoam followed by another 2" of Styrofoam (in the other direction), and 5/8" gyprock. The store owner suggested that I put a vapour barrier up first, which I question.
 
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Old 11-04-15, 10:13 AM
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The vapor retarder goes on the warm side of the insulation. Assuming the garage is not heated, the store owner is correct.
 
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