Insulating the Roof

[SaintJimbob]

Alright, crowd-source brain trust, here's the skinny:

-20x24 brick garage
-The roof and walls are all 2x4, stick built hip roof, with three 2x6 chords running the width of the garage.
-There are 4 2x4 posts coming from common rafters to the 2x6s below.
-No sheet sheathing, just 1x8-12 boards, then tar paper, then shingles
-There are some gaps between the boards, between 1/4 and 3/4 inches, you can see the tar paper above.
-Some of the boards and 2x4s have water stains, but the roof was just redone in '09, so they could be old.
-No sofits to the outside, but there is a short ridge vent at the top.

So... I need to insulate the garage, including the roof. Here are some of my initial thoughts:

-Due to possible (or even eventually probable) leaks, I might go with foam vs fiberglass?
-Should I caulk or expanding foam the gaps in the boards?
-After insulating, I need some lightweight covering over it all.
-I could just throw in a drop-ceiling, insulating over it, but then lose the ample loft storage.

Thanks for your input.

 

[Bondo]

-I could just throw in a drop-ceiling, insulating over it, but then lose the ample loft storage.

Ayuh,.... That would be yer Best bet,.... no need to heat cold storage,....

Ya need an air gap, 'tween the insulated surface, 'n the roof,....

 

[Chris705]

I vote you go with closed cell foam on the roof deck. Cover with fire resistant coating . Provides air tight sealing & will allow continued use of above ceiling tie area. No need to foam in gaps in roof sheathing. Other options are less expensive but you end up sacrificing other benefits you want. At least call in and discuss with a foam contractor.

 

[Chris705]

Most shingle manufacturers are ok with spay foam direct to deck because there is no longer baking of the underside of the shingle. Fiberglass or other insulation needs air space because the air gets superheated and cooks the shingle. Hence the need to vent attics and cathedral ceilings that use fiberglass or blown in insulation.

 

[DC73]

Spray foam would be a good choice (pricey but good). If you choose the drop ceiling route, could you frame out for a drop down attic stair and then deck above the insulation so you could keep your storage?

DC

 

[James-W]

If you choose the drop ceiling route, could you frame out for a drop down attic stair and then deck above the insulation so you could keep your storage?

DC

I think this is a good idea. It lets you insulate the garage while keeping the storage area intact.

 

[C2 Turbo]

I apologize to OP for asking a personal question members in his thread, but I think it's relevnt and will help others to understand insulation too.

Some times people talk about house being "too air tight". Can some one please explain what that means and how that can be prevented?

The reason I ask, 'coz we are building a house, and I have to decide as to how to insulate the walls/roof?

I am getting quotes on foam insulation vs 1" foam+fiberglass vs all spray insualtion?

Thanks

 

[DonPowers]

This may be overkill for your situation, but this is how I maintained an open ceiling.

Have an old log cabin with exposed log beams and pine ceiling.

Stripped all of the old roofing off to the pine boards and put down:
6 mil vapor barrier
4 inches of Polyisocyanurate insulation (4x8 sheets R-6.8 per inch)
Laid 2x3s on flat side and screwed to log purlins.
Put a 1x8 spruce deck over 2x3s
Tarpaper over the spruce and shingled.
Put in a vented soffit and ridge vent.

has held up for over 20 years with no leaks or issues. When I replace the roof, will strip shingles and install metal.

R-27 is less than desirable for insulation these days but it works, just keep chucking wood in the stove.

 

[SaintJimbob]

Thanks for all the great ideas, gang!

I think I'll split the difference; leave myself a couple feet above the joists for long board/stud storage and then drop ceiling w/ fiberglass batts above. So, with the shrinkage from the hip roof angles, I'd guess a 12x16ish grid. As for the roof slope from the wall to the drop ceiling, I'll probably do foam and OSB.

Thanks!

 

[DC73]

I apologize to OP for asking a personal question members in his thread, but I think it's relevnt and will help others to understand insulation too.

Some times people talk about house being "too air tight". Can some one please explain what that means and how that can be prevented?

The reason I ask, 'coz we are building a house, and I have to decide as to how to insulate the walls/roof?

I am getting quotes on foam insulation vs 1" foam+fiberglass vs all spray insualtion?

Thanks

Make your house as tight as you can afford. The thought behind a house being "too tight" is that you won't get fresh air brought in and the house air will become stale or even polluted. But it doesn't matter how tight you build it because that's easily overcome by installing a fresh air intake into the HVAC system. That way you can control how much air comes in and the quality of the air coming in is better than it would be if it has to pass through the various cracks and crevices of a leaky house. You can filter this incoming air when you control it yourself.

You can also install an HRV (heat recovery ventilation) system which basically exchanges air with the outside through a heat exchanger for better energy efficiency. An ERV is basically the same thing but also helps with humidity control. Search for house ventilation on BuildingScience.com and GreenBuildingAdvisors.com.

DC

 

[C2 Turbo]

DC,

Many thanks for the links especially BuildinScience.com is awesome.

 

[Denwood]

Spray foam the roof deck. I was on the fence on this but based on the unmelted snow on my shop roof (a very good sign), even 3.5 inches of foam with some air space is proving very efficient. The eaves are sealed shut, as this setup does not need to be ventilated. My build thread has quite a few thermal (FLIR) images of the ceiling before and after drywall.

Before/after..still lots to do.

One of the outside FLIR shots. Temps were in the -20C range.

 

[SaintJimbob]

With no soffit action outside, and the ridge vent above, is it cool to do the drop ceiling with insulation (kraft-faced fiberglass batts)over it?

 

[Denwood]

In attic roofs like this it is typical to install roof vents near the base of the enclosed area. These provide air intake in lieu of soffit vents for convective cooling. Otherwise summer temps in that area wil be very high.