Best method to insulate ceiling?

[KPSquared]

I am trying to figure out how to insulate the roof/ceiling in my garage. Here's a pic to start things off:

That is my attic/upstairs in the garage. It's going to be for storage and possible a spare room down the road. We would like to keep it heated although possibly cold storage for now.

My question is on a way to get the best R value possible out of a bad situation. You can see the angles on the trusses in those pics. It's only a 2x6 top chord so the most I'm going to be able to get is R-20 up there. I was thinking of using plastic soffit baffles stapled to the roof sheeting (to allow air to move from the soffits up to the roof vents through that narrow space) and then just put pink R-20 and poly in and call it a day.

Does anyone have a better idea? Or a way to get a higher R-value up there? I looked into spray foam. It's maybe an option but you'd have to spray really thick to get the R-value up and it gets EXPENSIVE in a hurry.

If I don't keep the place heated or livably warm full time, this isn't a big deal but I don't want to just piss heat outside when it's cold out.

 

[rsa]

Create an unvented attic. Insulate above the roof deck with rigid polyisocyanurate to at least the minimum R-value required to prevent condensation. Add air-permeable insulation insulation underneath the roof deck between the trusses.

Example detail from greenbuildingadvisor below. You could also use a nailbase like Hunter Panels Cool-Vent II (vented) or H-Shield NB (unvented).

Note: venting may be required over the rigid polyiso if you're in a high snow-load climate.

If that doesn't fit your budget, you can loosely fit and seal rigid polyiso between the top chords in either a vented or unvented application. It's fussy work, but an option for DIY'ers. Adding one to two inches of rigid polyiso over the bottom of the top chords will give you additional insulation and address thermal bridging. I wouldn't do it exactly this way, but to give you an example (click on the picture for the source article):



greenbuildingadvisor.com and buildingscience.com are good resources for additional details.

Recycled and factory-seconds rigid polyiso is available. Example: Insulation Depot or craigslist.

 

[digdug18]

That's too much work, I'd create an unvented attic using spray foam myself, the price will be about the same when you figure in the time your saving by NOT doing all the work above.

I'd put the plywood up on your roof, with snow and ice shield if in a region that gets snow, then whatever you want for the shingle/roofing material. And just have a contractor spray the underside to an R-30 or so, the more the better. Might as well do the walls as well in my mind, while they are there.

 

[ForceFed70]

Ha, Looks exactly like my garage.

For now I haven't insulated the ceiling of the attic room. I just insulated between the lower joists (R44).

My plan is to get spray foam when the time comes. Yes, it will be thick, but you only need the 2 x 4' wide (or so) strips sprayed. The peak area you can use regular batt insulation. I plan to staple those baffles to the roof where it will be sprayed so that there is still airflow as I have a ridge vent system.

Also, I plan to insulate the verticle walls of the room instead of trying to insulate the entire roof section.

 

[ForceFed70]

Create an unvented attic.

Personally, I don't like the idea of heating the attic space via the garage. I'd rather have 2 different zones if you will.

Also, you cannot use asphalt shingles with this method. Metal or clay tile only. Unless this is solved by using that "coolvent" product you mentioned?

 

[BadgerBoilerMN]

Nice picture...but did the guy that drew it install the space shuttle system in his own home...I think not. I like the idea of sandwiching EPS between finished roof and ceiling rafters but in my own home, a hot roof with rafters covered works great. (Asphalt shingle here in Minnesota.

Naturally any insulation spec. is first dependent on climate (outdoor design temperature) and building use (indoor design temperature).

In this case, the best balance seems to be a couple of inches of 2# foam under the deck with un-faced fiberglass finished to an R27 or so. With a thing joist space foam is going to be in the solution, but cut and paste will not give you air-tight construction in the long term.

Infiltration is the real challenge for heat loss and condensation issues. Foam is expensive because it works!

 

[KPSquared]

In this case, the best balance seems to be a couple of inches of 2# foam under the deck with un-faced fiberglass finished to an R27 or so. With a thing joist space foam is going to be in the solution, but cut and paste will not give you air-tight construction in the long term.

I'm not sure I have any idea what you are talking about. . . What is 2# foam? What is the "deck"? What is an "un-faced fibreglass finish"? And what the hell does "With a thing joist space foam is going to be in the solution, but cut and paste will not give you air-tight construction in the long term." mean? Mostly the "a thing joist space foam is going to be in the solution" part. . .

I'm just a layman. . .

 

[rsa]

I'm not sure I have any idea what you are talking about. . . What is 2# foam? What is the "deck"? What is an "un-faced fibreglass finish"? And what the hell does "With a thing joist space foam is going to be in the solution, but cut and paste will not give you air-tight construction in the long term." mean? Mostly the "a thing joist space foam is going to be in the solution" part. . .

I'm just a layman. . .

He's talking about "flash and fill". You spray two pound closed cell urethane foam against the underside of the roof deck to an appropriate thickness. That takes care of cold air leaking in and warm air leaking out, gives you a moisture barrier, and gives you some R-6 per inch insulation. Then you continue adding inexpensive air permeable insulation like rock wool, cellulose or unfaced fiberglass batts (unfaced—without kraft paper or another vapor retarder) that has a lesser R-value. The reason some people like it is that it can give you the benefits of using full-depth spray foam at a lesser cost—if you have enough depth that the "fill" insulation allows you to meet your total R-value insulation goal for the assembly.

What's the appropriate thickness? It varies with your location, interior relative humidity and temperature, total R-value of wall, etc., but the reason it varies is you don't want the roof assembly's to allow interior condensation. I see recommendations that 2" should be the minimum—it's not easy spraying closed cell spray foam to a uniform thickness.

I assume by "cut and paste" BadgerBoilerMN was referring to cutting and fitting rigid foam between the trusses. The reason "cut and paste" can work is that you cut the rigid polyiso sheets smaller than the space between the trusses so that you leave a small gap that can be filled with spray foam, creating a full moisture and air barrier so that moist air doesn't hit the cold underside of the roof deck and condense.

 

[rsa]

My question is on a way to get the best R value possible out of a bad situation. You can see the angles on the trusses in those pics. It's only a 2x6 top chord so the most I'm going to be able to get is R-20 up there. I was thinking of using plastic soffit baffles stapled to the roof sheeting (to allow air to move from the soffits up to the roof vents through that narrow space) and then just put pink R-20 and poly in and call it a day.

If you go with continuous soffit to ridge venting, what about furring-out or sistering the angled bonus-room portion of trusses' upper chords to give you more depth for insulation? You'll lose a little interior headroom, but it seems like a reasonable tradeoff. IIWM, I'd check with the truss manufacturer before proceeding.

Another option inside the bonus room is to attach rigid foil-faced polyiso to the underside of the angled portion of the trusses' upper chord. Hold it in place with 1x. That will also give you the air gap you need to create an effective radiant barrier from foil-face.

A couple of good reads by Dr. Joseph Lstiburek, Rules for Venting Roofs and A Crash Course in Roof Venting. If you're not site-building the vent channel, Lstiburek recommends AccuVent. Both site-built channels and AccuVent need to be sealed on the edges with spray foam.

 

[toyotadriver]

This would do it

http://www.danperkinsroof.com/1108_JCL_Perkins_A.pdf

 

[KPSquared]

Thanks for all the advice guys! I'll hopefully have my best option figured out by the fall. . . at least before winter sets in, although at this point, I don't even have a plan for how I'm going to heat the upstairs so it really doesn't matter. . .

 

[mpire]

Does any of this apply to the reverse if I am trying to keep the hot out and the cool in?