A recent hail storm has damaged my shingles and I’m looking at a re-roofing job, so I’m thinking this would be a good time to add additional insulation to the 3-season porch.
The roof is a shed style and has two skylights (6/12 pitch). The interior ceiling is 5/4 X6 tg.cedar over 6″X10″ beams that are 48″ OC. I don’t want to change the interior so I’m adding 2″polystyrene on the roof side. The roof presently has 3/4″ foil faced poly between the ceiling and roof sheathing.
I have a couple of questions.
1.What is the best approach? I was thinking of ripping 2X4’s to 2″ and placing them 24 oc. between strips of insulation and covering with 1/2″ cdx. All placed over the old sheathing.
2. I have two skylights (Velux) 48″ square. Is this something that one person can remove and reinstall or maybe support from below. Raise and lower? Interior trim fix will be done later.
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You can save youself a lot of ripping and get a better job by using 2x3 on edge instead of ripped 2x4 for the crib. You want the crib to stand proud of the foamboard by a good half inch so you can keep the underside of the new roofdeck cold.
Lag screw the crib to the old roof deck with ¼" lags. Pre-drill and counter-bore the 2x3s to sink the head of the lagscrews. Use washers. Set the lags on 16" centers, and get them into the beams on every second piece of cribbing. (You're stuck screwing to the old roof sheathing between those beams.) A pneumatic impact gun will make this part of the job go a lot more quickly. If you don't have a compressor, consider renting one and the gun for the day; or rent a corded, heavy-duty electric impact driver. You'll wear out a lot of batteries trying to do it with a cordless impactor.
Now glue in your foamboard and then nail on the new sheathing. Cut or drill vent holes in the old roof sheathing at the eaves before you do that, and make sure the insulation panels don't block the clear airway to the soffit vents. Use a continuous ridge vent to vent the upper ends of the bays. (I hope you're not in serious snow country.)
One caveat: 2x3 is notoriously lousy framing lumber; the general run of 'economy grade' 2x3 is twisty and hooked and just a PITA to work with. Go to a real lumberyard (NOT HD!) and twist their arms into finding you some #1&2.
Because you're stuck with 2' spacing on the crib due to the width of the foamboards, use 5/8" sheathing unless the roof is very steep in which case you could get away with ½". Don't use OSB. It will sag.
Dinosaur
How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....
good suggestions. The pitch is 6/12 so I'll use 5'8" ply instead of 1/2"
I do have a corded Makita impact driver. was not sure if I would use landscaping screws or a framing nailer. I will use screws on the beamed areas since I have the extra material to tie into.
I am in snow country, but I'm not sure how I would vent this. The original insulation is sandwiched between the wood ceiling and roof sheathing. No venting was used. I could open the eve area and leave some clearance below the 5/8" ply, but the (ridge vent?) would be where my house siding to roof flacing would be. Does this work?
the (ridge vent?) would be where my house siding to roof flacing would be. Does this work?
Ooops. I was thinking gable roof, not shed. There may be some proprietary ridge-type vent strips you could adapt to a shed roof, but I've never seen one. Normally to vent a shed roof you use a couple of big vents in the end walls, but built the way I suggested originally, your crib will block sideways airflow from one 'rafter' bay to the next. So that won't work.
I think the solution is to lay 1x3 furring on top of the crib and nail the sheathing to that. That way you'll have 1¼" air space above the insulation and below the sheathing, and unimpeded air flow in all directions. With that, you can use standard louvered vents in the gable walls.
Don't worry about venting below the original roof deck; no snow can land on it so it doesn't have to be kept cold. But remember you will need to cut vent passages through the old roof deck at the eaves so the original soffit vents will be able to get cold air into the space below the new roof.
As far as attaching the crib members to the roof, use the lags even for those that land between beams. You won't find a framing nailer that shoots 4½" nails, which is what you'd need to nail through a 2x3 the long way. Even if you went nuts and hand-nailed with 4½" spikes, they won't hold that well in direct withdrawal if they're only into the sheathing. Go for the lag screws. Buy 'em in boxes of 100 or 500; it's a lot cheaper that way. Washers, too.
Oh, yeah: don't forget to flash the joint where the roof meets the house sidewall. You're going to have to pull a row or two of siding to do that properly. Get the flashing underneath the felt or housewrap, right onto the wall, and lay it over the top edge of the last row of shingles. Have your tin guy bend the stuff with a 'crush' fold along the roof edge and a small down angle so it makes a neat finishing detail where it meets the shingles. And don't nail it to the roof, only to the sidewall. Use a continuous bead of pitch to glue it down on top of the shingles.Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
venting - dino just described a method that will provide for a ccold roof, but you'd need to figure out how it will vent out at the top and how you will detail it to keep wasps and such from entering at the bottom.But you said at first that this is a three season room. If that is the way it is used, there is no need for ventilation unless there is a sauna or hot tub in there making humidityMy concern then goes to the foil faced that is there already. That is a open celled insulation board that is very permeable and will absorb moisture in a roof/cieling situiation like this. The proper insulation board is a closed cell board like extruded polystyrene
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The original insulation is my best quess. I did not build this. When removing the skylights I see that the poly insulation is 3/4" foil faced (both sides) This should work as a good vapor barrier. I plan on adding 2" This will be closed cell.
The venting I was talking about was to keep the roof free on ice. Sorry if this was confusing. Dino offered a good suggestion of cross bracing so it allows air flow to the roof from the two sides, eve vents will work since I have a ft. overhang on each side.
thanks again for the good suggestions