Yet another venting question...

NM_Shooter · July 13, 2009, 03:01:50 PM

So I am building a 14X20 and am planning an 8:12 pitch. Trying to figure out the best way to vent the roof.

I'm considering doing bird blocking with open tails, as I hate building soffits / fascia. Not entirely ruled out though.

But here is my dilemma... I am hoping to have the cabin dried in by the end of this season. I won't have the insulation on and ceiling in until next year. I need to start living in this for elk season this year though. If I put in bird blocking, I can at least tape over that to stop drafts for now. Probably will be using a ridge vent... not sure how to seal that up temporarily. Maybe the foil AC tape from the inside?

Questions:

1) What size holes for the bird blocking? Just buy a hole-saw and get after it, or can it be bought pre-drilled?

2) Am I creating more work by not building soffits? Now that I am thinking about it, I wonder if the bird vents might be problematic if I use hardi board planking for siding.

3) Saw a posting where some are not venting at all. Looks like you have to seal well with a moisture barrier if you do this. Tempting, but sounds scary.

Arrggh.

Comments?

Don_P · July 13, 2009, 03:53:53 PM

14x20... I'd be tempted to do exposed beam rafters, t&g, layers of foam, 2x sleepers, ply and shingles or metal, a built up or "wrap and strap" roof

July 13, 2009, 04:44:18 PM

Frank I am a little partial to soffit as it gives the front a more finished look but hey it's your house. So with that in mind you can buy the round push-in vents that could be installed in the bird blocking. They are approximately 2" diameter and have a screened back to prevent insects. Just buy a carbide hole saw and go at it. I would install at LEAST one per bay. They could also be used in the soffit or a rectangle like I used on my cabin. As I had always been told that you need 1 sq ft of vent to serve 100 sq ft of ceiling area.

MountainDon · July 13, 2009, 05:00:52 PM

How well does the ceiling need to be insulated, considering you are not going to have to meet the energy codes up there, right?

I'm thinking that if you can install foam sheets in the rafter bays tight enough and finish sealing them with Great Stuff or other liquid foam that may be the way to go. After the insulation is in the bays and properly sealed there will be no water vapor getting up there to condense on the underside of the roof sheathing. Then skin the inside with something simple, cheap and easy.

That way you could simply install 2x bird blocking before the roof is sheathed. Seal those well before insulating. Do not use a ridge vent.

NM_Shooter · July 13, 2009, 07:33:41 PM

Thanks..

I think the biggest issue may be heat in the summer time, and I am also hoping that venting helps keep the cabin cool. Don, I may try your idea of using the predrilled hardi vent and attaching that directly to the tails... no fancy soffits. Maybe I should try and at least insulate the ceiling this year, even if I don't get it covered. I don't mind looking at vapor barrier

July 13, 2009, 07:35:33 PM

Yep, we had brown wall and ceiling paper for a while last fall.

Just cut the tails to the right length if you go that way. The Hardie cement board soffit comes 12 and 16" wide (x 12 ft) and 24" x 8 ft. in the ventilated variety.

Don_P · July 13, 2009, 08:11:17 PM

Just a clarification since its come up a couple of times, 1:150 is basic minimum roof venting. It can be reduced to 1:300 if a vapor barrier is used on the warm in winter side or is balanced soffit and ridge venting is used. (they use much better Greek in the codebook, R806.2)

"The Westford House" on the Building Science website uses an unvented foam and batt combination in the roof IIRC.

NM_Shooter · July 13, 2009, 08:26:24 PM

I'm going to guess at what 1:150 means...

I suspect that means that for every 150 square feet of surface area, I need 1 square foot of vent space in, and 1 square foot out.

Can that be right? if I have an enclosed area of 10' by 14.5", that is 140 sq inches total. Divided by 150, that gives me 11.6 in^2 of vent opening? Sounds like a lot for each section.

My plan was to cut a slot at the ridge, and cover it with screen and place a metal ridge. How do you keep blown snow out of there while also allowing for venting?

Thanks.

Redoverfarm · July 13, 2009, 09:08:38 PM

Well I was at least close. In fact it is better than the 1:150 so there shouldn't be any problem with ventilation. Thanks Don_P for the clarification.

Frank normally the ridge has approximately 6" of lap to each side. It would take alot of snow to make it up grade to the cavity to cause any real damage. The only time I think it might be crucial would be that if someone installed a metal roof less than 3/12 and the snow accumulated and an ice dam would cause the water to back up.

Not real sure about the area of your build but in mine I used hardware cloth 1/4" grid over the screen. I was worried about squirrels and flying squirrels penetrating via the opening under the vent. Don't forget the ends of the vent as well. The bats love this area. Before I got mine finished they had crawled 20-30' from the ends in that area.

Frank are you sure of your calculations. The area most affected is from the outside wall to the ridge. 14-1/2". I would think it would be feet. I can only guess you meant 14' 1/2" given you had a total of 140 (maybe Sq ft?) Maybe we are talking about two different areas. If you are talking about each individual bay (rafter to rafter) then there wouldn't be 11.6" X 2". That is another reason I like sofit with a 3" X 12" grate every 3'-4' or so. Not the greatest in math so I might have missed something. If I did just overlook me as my wife does sometimes.

July 13, 2009, 09:40:01 PM

The ridge vent on our house in the 'burbs (metal roof) has strips under the edges of the ridge cap that look like very coarse 3M sanding pads, or maybe a plastic Brillo pad. They were adhesive faced and stuck to the roof metal just before the cap was installed. It's supposed to keep out the larger bugs and help block snow infiltration.

The way I read section 806 is that the total free ventilation area is to be no less than 1 sq ft of vent per each 150 sq ft of the area to be ventilated. That would be split between in and out.

By free I believe they mean the area of the vent cut out less the screen, etc material that would block airflow. Most vents have the free area stamped on them. At least the ones I've bought do.

NM_Shooter · July 13, 2009, 10:20:19 PM

Yes, I clearly made a mistake in that post!

Well, I was thinking of placing roof joists on 16" centers, so between them there would be 14.5 inches width. I SWAG'd the length of the rafter from wall to ridge at 10'. Let me calculate....Ok... closer to 8.5' length. So the surface area would be ~1480 sq inches for 16 OC, or ~2450 for 24" OC