Rafter vents not needed with Spray Foam?

[Gnfantic]

GM everyone, electrical is almost done so looking to insulate the garage. I know spray foam adds strength to the structure and does not absorb moisture (closed cell) so I am trying to go this route but it is about 35% more then open cell So my questions to you guys...…

Both companies I called for estimates stated to me the rafters do not need venting (I was going to install the baffles). Because no moisture will get in there due to the foam. But what about the Sun blasting on the roof? I thought HEAT was also a concern..... Comments???

2-) For the roof rafters, should I go for closed cell??

3-) These guys are charging roughly $3.50 a sq ft for closed cell at 3in thick. Should I look into do -yourself kits???





thanks!!

 

[rburke65]

Don’t know about your vents, but I’ve never heard anything good about the DIY kits.

 

[purplezr2]

GM everyone, electrical is almost done so looking to insulate the garage. I know spray foam adds strength to the structure and does not absorb moisture (closed cell) so I am trying to go this route but it is about 35% more then open cell So my questions to you guys...…

Both companies I called for estimates stated to me the rafters do not need venting (I was going to install the baffles). Because no moisture will get in there due to the foam. But what about the Sun blasting on the roof? I thought HEAT was also a concern..... Comments???

2-) For the roof rafters, should I go for closed cell??

3-) These guys are charging roughly $3.50 a sq ft for closed cell at 3in thick. Should I look into do -yourself kits???





thanks!!

Typically if you do the math it doesn't come out in your favor for the DIY kits, plus you have the mess to deal with. I paid 2.70 for 3" thick. They masked everything off and did clean up afterward. I was very happy.

 

[jmhjgh]

Purple: who did you use in Minnesota? Thanks

 

[Gnfantic]

Typically if you do the math it doesn't come out in your favor for the DIY kits, plus you have the mess to deal with. I paid 2.70 for 3" thick. They masked everything off and did clean up afterward. I was very happy.

$2.70 a sq for closed or open?

 

[purplezr2]

Purple: who did you use in Minnesota? Thanks

I used a guy out of Grafton ND, Insulation Place I believe was the name.

$2.70 a sq for closed or open?

Closed Cell.

Two other quotes I got were 2.85 and 2.80 a sqft for closed cell.

 

[yeldogt]

no vents ...

You will not regret the foam

 

[Stuart in MN]

Read up on 'hot roofs', where the roof deck is insulated and no vents are used. Check to see if they are permitted in your area.

 

[purplezr2]

OP, is I understand right, you are considering spraying the underside of the deck that the shingles? are attached to?

If so make sure they spray the rafter too. I know someone that spray only between the rafters and they have ice dam issues due to the rafter transmitting heat and melting the snow. This is a very northern climate.

 

[Gnfantic]

Can you guys help me out with this math?? I dont know what it costs a sq ft and I cant afford it!

Closed Cell - 2 lbs
GARAGE ROOF RAFTERS
1,120 SQF - 5.5" - R37.4 $5564

 

[Red 17]

If there is moisture in the air, and a pathway up, you will have moisture inside that building. Search for "attic mold". Some kind of vent is needed imo. ymmv. jmo.

 

[Jlbc212]

Back in the 60's and 70's "deck" houses were common form of construction in my area. They have cathedral style ceilings with a base layer of cedar, a center layer of closed cell foam insulation, and a top layer of plywood upon which the shingles are nailed or stapled. The asphalt roof shingles bake from the heat of the sun causing a shortened life of the shingles. For many the aesthetics of the natural finished cedar ceiling outweighs the expense of a shortened life span of the shingles. Unless there is some reason, like the need for the additional space up to the rafters, I'd avoid the use of spray foam insulation especially directly under the roof sheathing.

 

[kngelv]

No vents needed for closed cell.

James

 

[Justin James]

Can you guys help me out with this math?? I dont know what it costs a sq ft and I cant afford it!

Closed Cell - 2 lbs
GARAGE ROOF RAFTERS
1,120 SQF - 5.5" - R37.4 $5564

1120 sq. ft. x 5.5" thick = 6160 bd. ft. of foam.

1 bd. ft. of foam is 12" x 12" x 1"

$5564.00 divided by 6160 = .90c per bd. ft.

I paid .95 per bd. ft. this summer so all in all not a bad price. Also seeing more homes where they are spraying closed cell on the underside of the roof deck. Wish I had that option on our house. Around here it helps stop the truss lift issues so many homes have now. One of the biggest advantages that closed cell has, besides the insulating quality, is the air sealing.

 

[stm317]

The whole idea with spray foam is to completely air seal the building, so vents will negate a big advantage of the foam.

As for price, something close to $1/board ft is pretty common for closed cell foam. 5.5" is a lot of thickness. Because foam air seals so well, you can often get the same interior temps with a lower R value as you would with more of another insulation. So 3 inches of spray foam has an R value of R18ish, but because it air seals so well it might feel the same as having R34 fiberglass or cellulose insulation. The outer couple inches of foam do the most work. Once you get more than about 3" of spray foam, it really starts to become cost prohibitive for the limited returns of the extra thickness. You could probably reduce that to 3-3.5" and not be able to tell any difference in quality. That would probably take your price down around $3k.

 

[Gnfantic]

Thanks guys, I appreciate the help. I tell you one thing, when I go on the 10ft scaffold to do wiring I am shocked in the difference in temp from such little distance. My roof is black and I will have a/c and heat in this shop so I really want do this right. If I do vent it, the air will be coming outside up to the ridge. and then completely covered in foam, wont be any leaks.

 

[yeldogt]

Back in the 60's and 70's "deck" houses were common form of construction in my area. They have cathedral style ceilings with a base layer of cedar, a center layer of closed cell foam insulation, and a top layer of plywood upon which the shingles are nailed or stapled. The asphalt roof shingles bake from the heat of the sun causing a shortened life of the shingles. For many the aesthetics of the natural finished cedar ceiling outweighs the expense of a shortened life span of the shingles. Unless there is some reason, like the need for the additional space up to the rafters, I'd avoid the use of spray foam insulation especially directly under the roof sheathing.

Studies show that the temp difference is not that great -- and that's using a properly working vented roof. Almost no building out there has a properly vented roof. If you look at wats required -- you quickly understand that what is done is not really working. The real world difference is even lower.

Roofing material is also a lot better .... there is now 30 years of doing this in Florida. In the 60's it was 20 year asphalt.

 

[yeldogt]

If there is moisture in the air, and a pathway up, you will have moisture inside that building. Search for "attic mold". Some kind of vent is needed imo. ymmv. jmo.

Humidity -- needs to be controlled in any tight building. Attic mold is more often in cold climates with conventional construction where the ceiling is batts.

The only concern with foam -- it has to be think enough in a cold climate

 

[bigdav160]

Do you only plan to foam the roof and not the walls? If so I think you're defeating the purpose of foam which is the sealed envelope Justin mentions.

But with a sealed building there are other considerations mainly for air quality and combustion air for gas appliances.

 

[LX-Markham]

I ended up with a vaulted ceiling and no way of properly venting it. That was the reason I chose the more expensive route of closed cell spray foam: un-vented construction.

https://www.buildingscience.com/documents/digests/bsd-149-unvented-roof-assemblies-for-all-climates

 

[onlyn8v]

Thanks guys, I appreciate the help. I tell you one thing, when I go on the 10ft scaffold to do wiring I am shocked in the difference in temp from such little distance. My roof is black and I will have a/c and heat in this shop so I really want do this right. If I do vent it, the air will be coming outside up to the ridge. and then completely covered in foam, wont be any leaks.

Tell me about it! I did the siding in my 30x24 this week in port jeff and when I cut the ridge vents in you could really feel the heat. It was like Backdraft!

I got 2 quotes to do the spray foam insulation in my garage, and both were over 6000. I couldnt swing that so Im going the old fashioned route.

Where on LI are you?

 

[bigdav160]

LX, no fire barrier?

 

[Gnfantic]

Damn. I bet that garage is warm in the winter

 

[LX-Markham]

LX, no fire barrier?

Drywall.

 

[DC73]

You can do a mixture of closed cell and open cell (or other insulation) to save some bucks. However, you must have a proper thickness of closed cell foam to prevent moisture from condensing on the bottom of the roof. The proper thickness varies with climate. There are resources on the internet to help with this.

Here is one article to read: https://www.finehomebuilding.com/2015/11/11/insulating-unvented-roof-assemblies


Take a look at the chart at the end of the article. The column for air-impermeable insulation gives the minimum R-value you will need for closed cell foam. The next column gives the recommended total R-value you need.


DC

 

[APG12]

Random off topic question: what happens if you need to replace sheathing or even drywall after spray foam? Is it a nightmare?

 

[Stuart in MN]

Random off topic question: what happens if you need to replace sheathing or even drywall after spray foam? Is it a nightmare?

The foam will be glued to the sheathing, but how often does sheathing need to be replaced? It should last for the life of the structure.

 

[APG12]

The foam will be glued to the sheathing, but how often does sheathing need to be replaced? It should last for the life of the structure.

Just a curiosity question. I am about to start getting estimates for spray foaming my garage, but siding will be in the near future. I’m pretty sure my sheathing is in good condition, but just wandering what would happen it needed to be replaced for whatever reason.

 

[yeldogt]

I ended up with a vaulted ceiling and no way of properly venting it. That was the reason I chose the more expensive route of closed cell spray foam: un-vented construction.

https://www.buildingscience.com/documents/digests/bsd-149-unvented-roof-assemblies-for-all-climates

A beautiful sight !