Insulating cathedral ceiling without a ridge vent

[graminizer]

Insulating a truss ceiling without a ridge vent

What is the best way to insulate the ceiling in my garage? I have 2 roof vents, soffit vents that run full length on each side, and I do not have a ridge vent. I want to keep the trusses clear for storage and only insulate up by the roof. What are my options?

 

[BlackTalon]

The soffit vents are the intakes. If there is no ridge vent, are there gable vents for exhaust? Best to sort the underdeck ventilation before proceeding, unless you are thinking about applying spray foam insulation to the underside of the sheathing and eliminating the eave vents.

Also you indicate there is a cathedral ceiling but you then say you want to keep the rafters clear for storage. So the roof framing system is rafters and not trusses? And if rafters, how will you store anything there since they are sloped?

A photo or two of the underside of the roof framing system would be helpful.

 

[graminizer]

This is the peak...the roof vents are close to the ridge.....I have 2 of these.

 

[LB-1911]

To clarify - You have trusses not rafters.

My 24' x 24' shop.....Hope you guys like it!
http://www.garagejournal.com/forum/showthread.php?p=4307194#post4307194

Cathedral ceiling shown below

 

[graminizer]

Sorry guys....I meant trusses!

What is the best way to insulate and still keep the storage in the trusses?

 

[sourdough]

It's not a cathedral ceiling, just standard trusses.

IMHO, plastic vapor barrier under the trusses, sheetrock under the plastic, 2" x 6" mostly full-length spacers on top of the truss lowers to accommodate at least an R-30 unfaced batt ceiling between the trusses above that. Then OSB (or if you have the bucks, plywood) on top of all that if you want to crawl around and store things up there.

You live in about the same close-to-marine environment that I do. It gets wet in the fall through spring. Lots of rain and, therefore, moisture infiltration. We get about 60" of rain here during that time.

My advice: don't store anything up there that can corrode or rot.

I would not do it.

 

[graminizer]

IMHO, plastic vapor barrier under the trusses, sheetrock under the plastic, 2" x 6" mostly full-length spacers on top of the truss lowers to accommodate at least an R-30 unfaced batt ceiling between the trusses above that. Then OSB (or if you have the bucks, plywood) on top of all that if you want to crawl around and store things up there

I want to keep the bottom of the trusses clear for storage and insulate up by the roof only.

 

[Denwood]

Spray foam the roof. Make sure they use closed cell, no ventilation required. See my thread link in my signature. I just restructured my roof (stick frame, not truss but same idea) to eliminate 16ft of truss ties, so can now lift a vehicle 10ft instead of 8ft. This way you would keep the lower truss members free.

 

[sourdough]

I want to keep the bottom of the trusses clear for storage and insulate up by the roof only.

Sorry, I don't understand. I would think you would want the top of bottom chord of the trusses (plus insulation and platform) for storage. Why would you insulate the underside of the roof? You have gable-end vents, soffit vent, and no ridge vents, correct?

If so, you have a cold attic with ventilation. Adding insulation to the roof will have no effect and is wasting money.

Maybe I don't understand your purpose.

 

[bczygan]

Block any gable end vents.

Spray foam as described above.



Or:

Close gable end vents.

Add continuous ridge vent.

Use propa-vents at every rafter space at the eaves.

Remove the roof vents and close the openings and patch the shingles.

Install batt insulation, maintaining minimum 1" airspace between insulation and underside of roof deck, continuous from eave vents to ridge vent.

If additional insulation value is needed, install rigid insulation panels across underside of truss top chords. Tape joints.

Do you want to do the work? Spray foam is a pro job.

Is insulation value important? Using just batts and leaving the airspace won't give you much.

Adding the rigid foam panels will add R value, but a lot of cost.

Price both ways and consider the trouble involved.

Make your decision.

As much as I like DIY, foam seems the logical solution.

 

[Denwood]

That's pretty much why I went with the foam. It also adds (according to industry tests) as much as 70% increased racking resistance in the structure.

To do the ceiling on my 16x24, it was $1600. Well worth it as the ceiling is now air sealed, and insulted in one shot to r20. I only heat the shop when using it, otherwise would have used a thermal reflective furred airspace method over the spray foam. We used this technique for my business Reno and it is proving shockingly efficient.

If you browse my garage thread you can see how bad the previous owner's fiberglass + poly at the ceiling was doing. I pulled all the r20 glass and observed moisture/air leakage issues that would have led to rot, particularly at the ridge.

 

[6768rogues]

I agree that spray foam is the way to go.
If your trusses were not designed for storage, the bottom chord will probably be designed for 10 psf load. That will hold ceiling material and insulation. Without the load of a ceiling, be careful to only store light things and nothing with a concentrated load.

 

[bczygan]

Oh, forgot........as others have mentioned above, trusses aren't typically designed to take much additional load on their bottom chord. Check with your truss provider to find out what the design load limits are for yours. You may be limited to only very light storage in that space. To minimize the load you might want to consider a metal ceiling or even just open framework, rather than drywall.

Store heavier things near walls, and lighter and bulkier things in the middle of the span of the trusses

 

[Nickmm]

So a very similar question came up for me today. I'm not very well versed at this sort of thing, and have a fairly large 30' wide garage I'd like to insulate this season. I'm currently doing r30/r38 faced batts on all the walls, but the roof is going to be a bit of a snag. I have the vented sofits on my two longest sides, but no vents on the roof currently, the place naturally feels like a sauna in the summer. We have fairly mild summers ranging in the 70s-80s, rare 90 degree days, so my main focus is heating in the winter.

What sort of options do I have with this building, after looking at the photos? Ideally, I would like to fully batt the whole place, as It can be done over time, as I've been doing so far. I can get a wall done when I have time and move on. On the ventilation side, I'm a bit stuck, as I could seal it off and have good heat retention in the winter, but would be worried about temps in the storage upstairs in the summer.

Thanks for the help, glad this subject popped up.