DIY Home Improvement, Remodeling & Repair Forum

DIY Home Improvement, Remodeling & Repair Forum (
-   Insulation and Radiant Barriers (
-   -   Screen Porch to 4-Season Room Build (

milwaukeehaze 10-09-2012 02:54 PM

Screen Porch to 4-Season Room Build
2 Attachment(s)
I have a question on roof insulation, but let me first give a very detailed and accurate description of my project first as it is necessary to answer the question. I had an existing screened in porch attached to the back of my garage, on a six inch thick concrete slab that I am currently converting into a 4-season room. I basically used the existing 4x4 treated uprights and framed out the lower walls and installed windows above on those 3 outside walls. I am in Wisconsin and want to use the room in the Winter at at least 60 degrees F with outside temps averaging in the 20’s in the Winter. Here are the details and a few pictures are attached for reference.

•15’ x 15’ area (one wall up against the back of the garage (wall is insulated on opposite side)
•7’ high ceiling
•The three walls all have 2-3 ft. walls I framed out below the windows, 2x4 studs, fiberglass faced insulation, yet to be dry-walled
•The windows & patio door on the three walls are all double-pane, insulated glass and vinyl clad
•The floor is concrete now, but I am adding a moisture barrier, insulated layer and then indoor/outdoor type carpet
•The heat source will be a natural gas, no vent 20K BTU wall heater

Now my question… The roof is simply a sloping (3-4” drop in 15 ft.) corrugated steel roof framed with 2x6’s on 24-inch centers in a grid pattern. The roof does over hang at the end of the room by about 2 feet with a 4” deep soffit. I know the ceiling being insulated is crucial because the metal roof will just conduct the heat right out of the room. What would be recommended for insulating the roof? Would I use faced or unfaced insulation? I ask this because I need to know if I will have moisture issues, and would the moisture be able to be vented out of the room through the roof (maybe through the corrugated channels in the steel roof out through the soffit (vented)? Or, should I use a moisture barrier insulation for the ceiling and put in a ceiling vent/fan? Any advice will help on the moisture/insulation/ventilation issue I may run into. Thanks!!!


nealtw 10-09-2012 06:10 PM

Just like a house you want air to enter the roof structure at the soffets and exit at the top where it meets the wall. You do not want moisture from with in the structure to get into the roof.
You said the roof was built with a grid patern, if the 2x6 blocks completely stop air flow under the steel roofing you will have to go to a spray foam. If you can get air flow next to the steel then you can use bat insulation leaving a little space for venting. We use poly sheeting stapled up instead of backed insulation for a better vapour barrier.

milwaukeehaze 10-10-2012 07:55 AM

2 Attachment(s)
Thanks for the reply Neal. I attached some photos of the inside of the ceiling/roof. The channels do run out into the overhang (soffit), so can you look at these photos and confirm your recommendation on the insulation? Thanks!!


nealtw 10-10-2012 04:43 PM

If this had been built for insulation it would have had 2x4s on the flat on top of the rafters instead of the solid blocking. That would have allowed airflow above the insulation. So I think you are back to spray foam. The other problem I see is the soffet vents to the main roof will now be closed off, so you might want a vented drip edge on the main roof above this room. And while I am at it, you will have problems with ground level to high, you should have 6 to 8" of concrete open to the air on the outside.
A little hard to do when all you have is a slab. I would dig a 1' by 1' ditch around the building and fill that with stone and make sure it can drain somewhere and add a gutter to that roof.
I see you have tyvac on the inside by the window, After insulating you want to use poly for a vapour barrier.
The roof should have felt under it, have a look at this site.

notmrjohn 10-16-2012 11:25 AM

You do have some venting thru the channels above 2X's but I don't think it is enough and where does air go once it reaches top of porch roof? I don't think you have enough slope over porch to rely on natural convection in winter, especially with metal roof.

Spray foam directly to metal, no air space above foam, and seal channel opeings to outside at eave. As neal sed "You do not want moisture from with in the structure to get into the roof." You certainly don't want warm moist air from inside to contact cold metal. There would be a lot of condensation from the unvented heater.
I don't think you would want to consider batts or other loose type insulation at all. Visit any thread about insulating cathedral ceilings, which is basically what you have, and see the arguments concerning batts and foam and airspaces and where they should be.

At risk of starting another argument, you may want to consider an air space of 1/2 to 1" between bottom of vapor barrier and top of ceiling by adding strips to bottoms of rafters so air can pass under blocking. Small inlets at bottoms and outlets at tops of each bay, inside porch room. Warm moist air would circulate through preventing condensation from dead air between vapor barrier and ceiling. But again I don't know if there is enough slope for that to be very effective.

Don't let water from neals ditch drain under slab, you may need to install a buried non-perforated drain pipe out to some lower area or to a dry well. And make sure rain gutter spout drains out away fro slab, maybe into same dry well.

You may want to line trench with very heavy plastic to keep water from going under slab. Then line trench with permeable cloth (French drain wrap) fill almost to top with coarse jagged drainage rock, fold cloth over trench width, cover with inch or two of aggregate to prevent aggregate from clogging with silt.

I had to install such a trench along wall of my house, I used plastic rain gutter flat back caulked and screwed against slab with top outer edge below ground level, heavy mill plastic under sod laying over outer edge of gutter. Luckily had enough slope to let it drain out of extended end. Had a little silt build up at first, heavier rains washed most out and once grass grew out silt problem ended.

milwaukeehaze 10-17-2012 05:36 PM

Thanks for the great advice! The ceiling drops 3" in 14' so let me know if your venting idea would work. Thanks!

nealtw 10-17-2012 06:22 PM

If you use spray foam you won't need a vapour bairrier. Inviting warm moist air behind the drywall, I don't think that would be a good idea. With any kind lowering of the temp. the moisture would drop out of the air and land on the drywall. I would be a good place for insects.

notmrjohn 10-18-2012 10:22 AM

"Inviting warm moist air behind the drywall" See here's where the argument starts, especially with cathedral ceilings and basements. The foam is a vapor barrier. You are going to have warm moist air between it and drywall. For long time folks thought we needed vapor barrier against back side of drywall. Moist warm air passes right through paint and drywall. But it passes both ways out of and back to room. As long as you have ample insulation beyond vapor barrier there is little if any condensation in air space. You are going to have at least 5" of closed cell foam, an R value of 30.

Recommended ceiling insulation Milwaukee is 49. Can you lower ceiling 3"?

You do not want air convection movement inside a wall , you get warm moist air at top of space, cooler at bottom, cooler air releases moisture, convection keeps air moveing in circle bringing more moisture to cooler area. In cathedral ceiling you don't want cool air pooling at bottom. You need either an internally vented (insect screened) airspace below insulation and vapor barrier allowing relativly rapid air flow, foam all the way to top surface of ceiling or as close as you can so there is no airspace, or additional vapor barrier against top surface of ceiling.

I do not think you have enough slope to get rapid enough air flow to prevent condensation. Fill entire bay with foam. Your climate, with metal roof, rafters and blocking may get cold enough to actually cause cold strips on ceiling. 3" thick strips of foam board insulation would give R of about 15 there and give more space for spray above.

Make sure to use real good vapor barrier on concrete. If you do trench around pad, you may want to insulate outside edges of it.

A vented stove will cut down on moisture in room.

Sorry to confuse with self conflicting advice, I was rambling and thinking out loud.
I think you'll be better off with no venting or air space above ceiling. Foam the entire cavity, deepen cavity if you can lose some ceiling height.

nealtw 10-18-2012 11:17 AM

OK I'll bit on that. Air will not travel thru drywall, moisture will. But moisture needs a reason and it is usually cold to warm, not warm to cold. So the area behind the drywall can be thought of as dead air.
Vapour barrier is a misnomer as nothing used as such is 100 % and paint of varrious types all have there own % number. So if that space isn't greatly warmer than the room and paint has some resistance to it we can expect the moisture level to stay the same there.
Then if you use open cell foam you do need a vapour barier also, that to would solve any problem with that air.

notmrjohn 10-18-2012 11:55 AM

"OK I'll bit on that." I done been bit and bit back.
I don't wanta get into it again. There are threads on another forum, possibly here that run into the hundreds of posts, this way and that way, links to genuine peer reviewed reports and tests, proving, disproving, air thru drywall, paint etc. Dead air convecting air. Permeable barrier, non permeable on and on. The OP and Original Question fall by wayside.

I will say this, if moisture can pass through something air can pass thru it. Gas is less dense than liquid.
"But moisture needs a reason and it is usually cold to warm, not warm to cold." I'm not sure what you mean by that. Transference of water from air mass to air mass does not create a problem. Moisture generally travels from wetter to dryer. Warm air holds more moisture than cold. But when warm moist air contacts cold air the water condenses, unless cold air is extremely dry. When warm moist air contacts cold surface it condenses faster. Warm moist air contacting cold winter air and on cold surfaces in attics is what causes condensation problems. Debate is, is it best to prevent that contact in first place or allow it and deal with condensation.

If haze doesn't mind, I don't mind a short arguement, but if it turns into links and reports I gotta look up, I'll try to find the threads where it already been hashed and re-hashed. Some got rather heated and just turned into rants and name calling.

I don't think haze is gonna get a perfect situation. I think the easiest and best that can be accomplished is to fill cavities with closed cell foam and cover it with drywall.

All times are GMT -6. The time now is 12:32 PM.