Insulating a garage attic without venting?

[JimVonBaden]

A few photos to show what I have, and where it is goging:

Basic, no ridge vent, soffit vents or gable vents.

Style of rafters and openings on sides.

Current state of build.

Now, before you ask, I didn't have any control over the build, it was an incentive to buy the house, which did not include venting and the sheet rock done with no insulation.

The intent is to insulate only the ceiling above the sheet rock. I will be adding gable vents to both ends to vent the "attic" space, and I will be using it for lite storage. I'll add plywood or OSB over the center section, and a drop-down stair.

I can blow in insulation, add bats with or without backing, or even use foam sheet, I prefer not to use expanding foam because I can't do it myself.

My main concern is that any significant insulation will physically touch both the sheet rock and the OSB roof pannels. I am concerned about heat and moisture issues. Is this a valid concern, and if so, what would be the best approach to avoiding issues?

Thanks,

Jim

 

[innkeeper]

What is going to happen the garage? A vapor barrier would be great, and I would use blown insulation!!!! And don't forget to VENT...

 

[racerbob4]

Its not hard nor expensive to cut a ridge vent in your garage. Buy one bundle of matching shingles and the ridge vent aluminum and take off the ridge shinges. Set your saw to slightly more than the depth of the plywood and cut crefully. Remove the plywood and install the ridge vent. If in doubt about how to do this or the extent you need to cut ask the techs that sell the vent which will be at the roofing store. I like the ridge vents that have shingles glued on top of them. The roofing guys will also recommend a mastic for that purpose.

 

[JimVonBaden]

Its not hard nor expensive to cut a ridge vent in your garage. Buy one bundle of matching shingles and the ridge vent aluminum and take off the ridge shinges. Set your saw to slightly more than the depth of the plywood and cut crefully. Remove the plywood and install the ridge vent. If in doubt about how to do this or the extent you need to cut ask the techs that sell the vent which will be at the roofing store. I like the ridge vents that have shingles glued on top of them. The roofing guys will also recommend a mastic for that purpose.

That is an option I had not considered. I am certainly capable of doing that. Should I also add the gable venting to get crossventing going?

What about insulation?

Innkeeper,

If I use blown-in insulation, where do I put the vapor barrier, bleow or above it? Is there an issue with it touching the OSB Roof sheeting?

I will be using the shop for a lot of motorcycle and some auto maintenance, as well as shooting video for my company. It will get used a lot. Plus my car and bike will be stored in it.

Jim

 

[innkeeper]

If there is not a lot of moisture going to be present other then normal, I'd would go with out..

 

[rsa]

http://www.greenbuildingadvisor.com/blogs/dept/building-science/lstiburek-s-rules-venting-roofs

http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting

Read the parts about not using vapor barriers.

When you get around to insulating the wall, think about dense-pack, blown-in cellulose or an unbonded, virgin fiberglass like Johns Manville Spider or Knauf Jet Steam.

EDIT: It's a small, accessory building, not a home. You might-not/probably-won't be punished for not following building science recommendations. Disclaimer: I'm not a building professional.

 

[JimVonBaden]

http://www.greenbuildingadvisor.com/blogs/dept/building-science/lstiburek-s-rules-venting-roofs

http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting

Read the parts about not using vapor barriers.

When you get around to insulating the wall, think about dense-pack, blown-in cellulose or an unbonded, virgin fiberglass like Johns Manville Spider or Knauf Jet Steam.

EDIT: It's a small, accessory building, not a home. You might/probably won't be punished for not following building science recommendations. Disclaimer: I'm not a building professional.

Those are great links, though probably more applicable to a house.


The big issue, as I see it, is that insulation will be in direct contact with the roof OSB. I have no soffit vents, and can't add them. So, will the insulation touching the OSB present an issue, and will gable vents with a ridge vent be proper venting to prevent issues? I didn't see a section on vapor barriers. Where would the vaper barrior go, or do I need one?

Thanks, I just want to do it as good as possible, and only once, plus keep cost reasonable.


Jim

 

[rsa]

Those are great links, though probably more applicable to a house. The big issue, as I see it, is that insulation will be in direct contact with the roof OSB. I have no soffit vents, and can't add them.

Cor-A-Vent has a product, IN-Vent, designed for your situation:

So, will the insulation touching the OSB present an issue, and will gable vents with a ridge vent be proper venting to prevent issues?

So a vented roof with only gable and ridge vents won't work—it'll only wash a tiny portion of the underside of the roof deck. Neither is insulation recommended to be installed against the roof deck in a vented roof. As you said, you may very well get away with it in an accessory building. I suspect that would be the case.

You could also go with an unvented roof. A DIY approach is to cut rigid insulation to width and install between the roof rafters with no vent space (make it a loose fit and shoot an expanding foam between the rigid foam and the rafters). It's labor intensive, but you have a small roof. Sometimes you get lucky and can score used rigid foam on from roof takeoff. Extra points if you run a layer under the rafters (use 1x3 strapping to hold the foam, provided an air gap for the foil (if present) to work as a radiant barrier and to attach drywall (to protect the foam from fire).

I didn't see a section on vapor barriers. Where would the vaper barrior go, or do I need one?

Oops, missed a link.

From Understanding Attic Ventilation (note, you're in Climate Zone 4):

Class I vapor retarders (i.e. vapor barriers – see sidebar) can be installed in vented attic assemblies in Climate Zones 6 or higher (see Map 1) but should be avoided in other climate zones as top side condensation can occur in summer months during air conditioning periods.

No interior attic assembly side vapor control is required or recommended in climate zones other than Climate Zones 6 or higher (see Map 1) for vented attic assemblies (note the distinction, this is not the case for unvented attic assemblies as will be discussed later). With vented attic assemblies moisture that diffuses into the attic space from the conditioned space is vented to the exterior by attic ventilation.

 

[JimVonBaden]

Thanks RSA, that gives me something to think about, and work with.

Jim

 

[jocool1585]

That InVent product is pretty neat. That's exactly what I've been needing for my garage.

 

[toyotadriver]

I would look into doing this:

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

 

[JimVonBaden]

I would look into doing this:

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

Cool idea, but it seems like an extreme solution considering the roof is less than a month old!

Jim

 

[geologist]

That first picture totally threw off my perception of how big your garage is. It made it look almost like a dog house until I used the other pictures to get a better perspective. Maybe it was an odd angle or something. That sheetrock is looking pretty nice.

 

[p_mori7]

I would cut out the gable ends to install gable end vents. You can buy plastic ones at the hardware store. Lay down your bats right over the sheetrock. Trim the end of the batts at an angle so that they are about 1" below the OSB. For the angled sheetrock, just make sure there is a 1" air space between the top of the batt and the OSB.

 

[JimVonBaden]

That first picture totally threw off my perception of how big your garage is. It made it look almost like a dog house until I used the other pictures to get a better perspective. Maybe it was an odd angle or something. That sheetrock is looking pretty nice.

Thanks! It isn't real large at only 20X20, but not tiny.

It should have the driveways paved and the sheetrock mudded now. I'm going over tomorrow to have a look.

Jim

 

[JimVonBaden]

I would cut out the gable ends to install gable end vents. You can buy plastic ones at the hardware store. Lay down your bats right over the sheetrock. Trim the end of the batts at an angle so that they are about 1" below the OSB. For the angled sheetrock, just make sure there is a 1" air space between the top of the batt and the OSB.

That is my fall back plan, unless someone has a reason to do otherwise. It is just a garage, so I am not as worried about perfection as I would be with a house. I am more concerned about being too hot in summer than too cold in winter. I can handle cold much better than heat.

Thanks,

Jim

 

[stsmytherie]

http://www.greenbuildingadvisor.com/blogs/dept/building-science/lstiburek-s-rules-venting-roofs

http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting

Read the parts about not using vapor barriers.

rsa, thanks for these links. I hadn't been able to really wrap my head around the venting issues and these totally unlocked it.

Now the scheming can begin in earnest...

 

[rsa]

rsa, thanks for these links. I hadn't been able to really wrap my head around the venting issues and these totally unlocked it.

Same happened with me. When I eventually install HVAC in my shed, I'll redo the insulation, take down the vapor barriers and pull the exhaut fan. I did everything wrong!

 

[rsa]

I have no soffit vents, and can't add them.

I came across two more products for people with little or no soffit: Pro Flow™ VENTED DRIP EDGE and SmartVent.

 

[TommyK]

Without some type of soffit vent a ridge vent isn't going to do much for you. The simplest solution although not necessarily the best solution, is to install 2 gable vents as has been suggested. Do not install both a ridge vent and gable vent. If you are willing to do a little more work either the drip edge vent or the kor a vent will work well with a ridge vent.

 

[JimVonBaden]

I came across two more products for people with little or no soffit: Pro Flow™ VENTED DRIP EDGE and SmartVent.

Interesting stuff. I'm not sure I can get that to work with mine, but it is intrigueing.

Jim

 

[Motofixxer]

With your building specs. The only real option I see is spray foam. Check into some bids, see what they quote you. It might not be that bad. It might be around $1000 or something. By the time you add all the extras and the time...it would be very close price wise.

The problems you have are no venting, so you have to install venting and you still have other issues to contend with like no vapor barrier. The spray foam would take care of all that for you. If you want to do it yourself, there are self contained kits to do that. Just a cardboard box that has 2 tanks, some hose and a nozzle. You just turn on the tanks and start spraying. Talk to your local stores or suppliers. You may want to check with the ones that service more of the contractors, than like Lowes or HD. Just get the kit and spray the underside of the sheathing in between the rafters, and your done. Then you still have storage space up there etc, clearance to run wiring or whatever you need without blown insulation everywhere.

 

[pattenp]

I may have missed it but I didn't see anyone else comment on this. You do not use gable vents in combination with a ridge vent. Use one or the other in combination with soffit vents.
I just saw that Tommyk made this point.

That is an option I had not considered. I am certainly capable of doing that. Should I also add the gable venting to get crossventing going?

What about insulation?

Innkeeper,

If I use blown-in insulation, where do I put the vapor barrier, bleow or above it? Is there an issue with it touching the OSB Roof sheeting?

I will be using the shop for a lot of motorcycle and some auto maintenance, as well as shooting video for my company. It will get used a lot. Plus my car and bike will be stored in it.

Jim

 

[KGarage75]

I'd go with a spray foam kit as mentioned above. I used Foam-It-Green's 602 sq foot (at 1" thick) closed cell version. www.sprayfoamdirect.com. I'd spray the roof deck at 1". You can then do the walls with fiberglass.

There's no sense in wasting money on foam for the walls as you're not going to be airtight unless you close up the overhead door opening and put in a regular exterior door.

 

[JimVonBaden]

Lacking wall insulation, I am not sure how much I will get out of ceiling insulation, but I am not thinking $1000+ is worth the cost for the benifit for spray in insulation. Am I wrong?

Jim

 

[Treeman]

Installing ridge or gable vents without any soffit vents will result in a negative pressure situation in the attic and air (hot/cold/or moist) will be drawn out of the work space.

Don't forget the advantages of cooling the attic space during the hot summer also, when deciding between vent/no vent system.

One crude way to get lower ventilation would be to drill (hole saw) the appropriate sized holes in the fascia just below the drip edge, between every joist space. Then cover the holes with a continuous soffit vent:

Here's yet another idea: fascia board vent http://www.gaf.com/Roofing/Resident...ra-FasciaFlow-Vent/Cobra-FasciaFlow-Vent.aspx

 

[Motofixxer]

Lacking wall insulation, I am not sure how much I will get out of ceiling insulation, but I am not thinking $100+ is worth the cost for the benifit for spray in insulation. Am I wrong?

Jim

The benefits of spray foam are many especially in your situation. The R value per inch is higher, so in your rafters you can get much higher R in that rafter space, in addition, you don't use venting with a foamed roof. It also takes care of your vapor barrier problem just for starters. You need additional time and materials for a more traditional vented attic and blown insululation, so the difference in cost is much less.

 

[nwav8tor]

Lacking wall insulation, I am not sure how much I will get out of ceiling insulation, but I am not thinking $1000+ is worth the cost for the benifit for spray in insulation. Am I wrong?

Jim

Jim,

I'm no expert, but these are my thoughts:

Are you going to have heat out in the garage? Without heat, I wouldn't worry too much about no vapor barrier...

Again, without heat, you Probably don't have to worry about venting the attic to prevent ice dams in cold weather. You would want to vent to reduce heat build up in the summer though. I'd go with a gable vent fan on the end and put an inlet vent on the opposite gable end just above the insulation level or even through the sheetrock opposite the fan so air would be drawn from the garage itself (just ensure the vent isn't blocked by the insulation).

If you insulate the attic, I'd also use blown-in insulation for the walls. It's a PITA having to cut holes into each stud bay, but it does work. In your case, the lower foot or so would be blocked from the insulation because of the blocking, but the rest of the walls would be insulated. I know Home Depot have blow-in insulation and with the purchase of 10 bags (i think thats the number) the use of the blower machine is free. You could get extra bags and use the same product to insulate the attic. Use a hole saw to cut the access holes in the walls and either cover them with trim or tape and mud the sheetrock plug back into place...

My .02 worth (and it might even be worth less),
Paul

 

[JimVonBaden]

Hi Paul,

Thanks for the advice. I am definitely leaning towards your advice. It makes the most sense for the money, and is simple to do.

Jim

 

[JimVonBaden]

OK, so it is now getting hot. The cold was fine, I like cold, and I took the edge off with a small 1500 watt oil filled heater. Worked great on the uninsulated garage.

Now that it is getting hot, however, I am sweltering, and the sheet rock joints are buckling in the sealing. Obviously the dark roof shingles and lack of venting is causing high heat build-up.

SOOOOOOooooo, I am going to relook at this, but I still think I am going to go with two vents on each gable, since I can't put one larger one in the center because of the framing.

Something like this:

Is it possible to OVER vent the attic space?

Then I am going to add bats between the rafters along the ceiling, making sure they do not touch the roof OSB.

Three questions:

* Is it possible to use these vents for my intended purpose without over venting?
* Will these vents keep water out when it is storming, and birds, squirles, etc.?
* Is R19 bat insulation OK, and should it be faced?

Please feel free to add comments.

Thanks,

Jim

 

[Falcon67]

I'd put cuts from R13 bats on the sides (looks like 2x6, so there's 2" of air gap to the deck) and R19 batts across the flat. Cut gable vents in the ends and call it good. If you were going to live in it, you'd probably want to pull the ceiling, vapor and insulate. If you use it like most of us and heat/cool it once in a while - really no reason to over engineer. If heat becomes a problem, buy a $80 t-stat controlled gable vent fan from Lowes and stick in one end. But I don't really see that happening. My last shop was 20x24 with an insulated ceiling and only gable vents at both ends. No issues with the roof deck or insulation over 12 years. The roof deck on that building would laser at 150~160F on our 100f+ afternoons.

 

[mpire]

I have a similar situation, but my garage is kind of tacked on to the front of my house, so I can't really add a ridge vent or a fan or anything to be honest. The soffits have vents built in already. I have no vapor barrier or anything really. Just bare sheet rock screwed to the trusses and that's it. Just trusses and plywood up top.

The plan is to empty out all the **** above the attic and then blow in some cellulose insulation. The garage is air conditioned now, so I am wondering if I need to attempt to crawl around in there and put in a vapor barrier or something like that. Can I get away with just blowing in the insulation?

Another thought was to put in a bathroom vent fan type unit in the attic and have it **** air out of the highest point and blow it out one of the soffit vents. Considering this is just an occasionally air conditioned space, is all this even worth it?