garage insulation

[redpowerd]

need to get insulation for the ceiling and im wondering what to use. its 36'by28' and is trussed at 4 foot centers. i thought about celotex but dont know how thick to get
thanks

[Murf]

If you have the budget for it, you cant beat sprayed foam insulation.

When you consider it is a one shot deal, no sheeting or vapour barrier is required and the job is finished in a hour instead of days, it's not that much more expensive than anything else.

Labour is a huge part of any job, so there is a big cost savings if you are going to have it done by others.

[karl]

Yea, what Murf said, and the d**n mice and squirrels don't make tunnels and homes in ti as readily too.
If you go that route make sure you get "closed cell polyurethane" as it doesn't act like a sponge like open cell does.

[redpowerd]

ah, i went the cheap route and bought the thinnest stuff they had. had to get it up and get it done. makes a big difference even though its r-6
the woodstove i have in there could heat a large barn. plus theres air space with the trusses.
thanks for the help

[SasquatchMan]

If you're heating with a wood stove, you hardly need any insulation at all, in my experience.  All you really need is some kind of air barrier to keep the warm air in the shop.  Lack of warm air is never the problem!  If I run out of wood, then there's an excuse to fire up the chainsaw and get out to the forestry reserve for some firewood!  It's all good.

[Murf]

There is one thing you do have to watch for though if you're not going to put in much, or any, insulation.


If there is much snow sitting on the roof and you heat and cool the roof it will quickly develop an ice dam.

Aside from the water problems you could also quickly get to the point where the load on the roof exceeds it's strength.

That will usually ruin even the best day.   :D

[Paschale]

I've got to insulate my garage too.  I was thinking of putting in the traditional figerglass rolls up there in the ceiling.  I've heard that you need to make sure there's a pocket of air, so someone suggested to me that I attach one-by's to all the rafter studs, then attach the insulation to that.  I'm hoping if I do this, it will keep things fairly toasty, and prevent ice dams.  Any thoughts?

Dan

[SasquatchMan]

Paschale, it IS important to have an air channel under the roof boards.  The idea is that if you have warm or no air there, you'll find that you have all sorts of condensation problems, which lead to rot at worst, and lifting shingles at best, plus the aforementioned ice damming.
Two methods are handy - one is just to strap off the rafters and insulate over the strapping with either batts or that foam sheeting (celfort, or styrofoam sm...).  The other way to do it correctly is to utilize some kind of air channel, by way of buying either a plastic or cardboard insert that fits between the rafters, and you place batting against these.

All I've done in my shop is poly off the rafters.  Very little ice damming, and no condensation problems.  I also redid the roof last year, and put full felt all the way up, and put that ice membrane around the last 3 feet.  Bulletproof.

[Paschale]

Thanks for the reply, SasquatchMan.  When you say, "strap off the rafters," what exactly does that mean?  You mention using Poly in your own roof--by strapping do you mean attaching poly to the rafters, and then attaching the insulation to that.

I have heard of those plastic inserts, but had forgotten about them until now, so thanks!  

As to the idea somebody posed of adding one-by's to the rafters, is this something I should avoid?  It sounds like this might not provide me with enough condensation prevention.

Thanks!

Dan

[SasquatchMan]

The idea of strapping and adding one-by material is the same, I think.  All you do is run 1x3 or whatever you've got perpendicular to the rafters.  Screw them on every 16 or 24 inches.  This provides "horizontal" wood to peg insulation against, using, say 3" spikes to pin batts in place, (or to provide support for the foam board).   I would poly off the whole thing BEFORE putting the insulation on, to make sure the rafter space stays clear.  I've had to do this in a local house where a contractor filled the rafter space with batt insulation and poly'd it, and now there are condensation problems in the form of horrible brownish drips...   It's difficult to retroactively fix this, but reasonably easy to do it right the first time.
This is all with respect to insulating right against the roof rafters.  It's MUCH easier just to board off the ceiling flat, and fill the ceiling with insulating material, leaving the attic cold.

In general, a layer of poly goes where you think cold air meets warm material.  it's kind of a tough call as to which side of the insulation this is, but I ordinarily just put the poly where it will do the most good.  On rafters, the poly goes on the rafters, between the rafters and the insulation, on a flat ceiling the poly goes between the drywall and the ceiling joists...

My roof is literally just poly stapled to the rafters, providing a cold air space from the soffits to the peak.  

[Kirk_Allen]

I would appreciate some info on insulating our attic.

The attic is a full size livable space that just needs finished.  The problem is how to insulate it.  The rafters are exposed 2"x4" oak trusses.  

I would like to drywall as much of the attic as possible but not sure how to do the ceiling.  My concern is moisture or condensation which creates a whole list of problems.

Whats the best way to inuslate the exposed ceiling without boxing out all that space?

Thanks
Kirk

[Furby]

Kirk,
If it was me and I only had 2x4" rafters, I'd sister up a 2x6" min. and then have the spray foam installed. You can get away with a smaller amount of depth, with spray foam, but will have to shell out some $$$.

[Buzz-sawyer]

One thing to remember is that you gotta have ventilation under those shingles to dissipate summer heat, I once used rigid foam to leave a 1" air flow then installed the insulation, commercial products are made for this purpose also. ;)

[ARKANSAWYER]

Kirk,
  Foam board with the foil on the face will reflect alot of heat back.  So on the inside it will reflect it back inside and the other it will reflect back to the roof.  If you put it on the bottom side of the rafters and make sure they can vent both top and bottom then you will solve alot of your problem.  I have but two or three layers of this and it will give a fair R value.  
ARKANSAWYER

[Paschale]

Hey Kirk,

This month's issue of This Old House has a huge article on insulation--discusses the pros and cons of the various types, ranging from the sprayed in poly all the way to borate treated shredded denim jeans.   :  Anyway, they discuss the various types, and how best to use them, and where to use them.  Plus, they talk about concerns for different parts of the country.  If you swing by a local bookstore and have a look, I think you'd find it helpful.

[Kirk_Allen]

Thanks for all the info guys.  Being that its 2x4 trusses it looks like I am going to have a tough time getting any decent R value while still leaving a space for air.  Furby, you may be right on the 2x6.   As far as strenght goes, the 2x4 are rough cut Oak and they are a real 2"x4" board.


I will try to track down the This Old House mag and see what they have to say.

Thanks l

[Buzz-sawyer]

Kirk
I think your best shot is to either sister some 2x10-12 or  run some 2x stock at a right ange , on top of the current rafters. youll need a ridge vent or end gable vent for the hot air to drain out too.

[SasquatchMan]

You can also put Styrofoam sm or some other sheet foam up across the rafters, and board over that with drywall for a cathedral ceiling... that stuff is easier than many other techniques, and your airspace is whatever size the rafters are.  I think the R value on the sheet foam is functionally higher than the r on other types of insulation.  It seems warmer to me, maybe just because there's no air flow through it at all...

[SwampDonkey]

Improper insulation in the attic (up stair ceiling) can lead to condensation problems, especially if its not ventilated properlly. My grandfathers old farm house had condensation problems because all the heat was escaping into the attic and condensing on the cold roof. My uncle had to insulate, put in end vents and also one of those roof vents that twirl in the wind. My auxillary attic (above the bedrooms) has 8 inches of wood fibric insulation and its well vented. My main attic is insulated on the floor from the garage underneath, no heat in there though.

cheers