Roof replacement concerns; weird airflow; increasing R-value
This is my first post and its long.. I attached an image of what I think the current setup of our house is. Until the actual roof is opened I won’t know for sure. I’m also new to all this and the correct terms to use. Zone 6.
95% of the roof is cathedral ceiling with a small attic in a corner. The inside ceiling is drywall and TG. There seems to be a layer of clear plastic behind the drywall. 80% sure there is drywall behind the TG boards.
I think that the attic air flow is from soffits in the front, these soffits are in an open area with no baffles. Then there is a ridge vent at the top with MDF on top of the rafters leaving a 1 inch air gap to the north side of no where..
I think that on the north side of the house the air is open to a gap that runs down the exterior wall behind the siding.
One reason I suspect this is because we had a lot of mouse activity, then I put copper mesh stuffed into the bottom of this exterior wall gap from the outside and the mouse activity in the attic stopped.
A bathroom exhaust van also seems to lead to this exterior air space! (powder room) There is no exterior vent for it, it is just being run between two floor beams between the first and second floor to the exterior wall. We heated the bathroom and used a thermal camera to check this.
In the front of the house there are also a few vents from the roof/ soffit area that open into the house. These are currently covered with a wood panel and are not air sealed.
We need to get a new roof . The plan is to remove all existing roofing, MDF, and insulation. I want to gut the current one to get rid of all existing pink insulation because of mouse crap in it. The MDF is also not in good shape based on what I saw in the small attic and we have one area that is dipped down.
There is also a chance we have rafters that might be damaged from water and/or rot. If these are damaged how would they be repaired? (haven’t looked into this yet)
The back bottom of the roof has a heated ice dam cable. Ice does build up in the back corner that gets minimal sun.
What would be the best way to re-insulate this? I’ve read the “insulate a cathedral ceiling articles” I could find on here but we seem to have a special air flow situation.
Ideally should this down the wall air flow be closed off?
How do the open vents to the house play into air sealing? I don’t know the initial purpose of these except maybe airflow for the wood stoves
How concerned should we be with the existing plastic layer?
There are two wood stoves in the house with pipe chimneys. (the winter temperature is not constant since occupation is not 100%, it varies from 55-80+deg at the inner ceiling depending on heat source used) The pipe chimneys might have to be extended if we add roof height.
I would like to use mineral wool between the rafters. I rather not use foam spray if I don’t need to, just a personal preference. I also would like to use a water and ice membrane on the entire north face.
The new roof is likely going to be shingles, we thought about a metal roof but just aren’t sure.
Most articles say to add foam board to the roof sheathing top. If we are removing everything existing is there a way to add height to the rafters and add more mineral wool instead?
How would thermal bridging be addressed?
Any input would be appreciated! Thanks for reading to the end.
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Replies
How deep are the rafters?
The orientation of the major pitch faces north?
What type of siding?
In zone 6 you need at least 50% of the R to be above the roof deck if it's going to be an unvented roof. More is better. For north facing pitches it's imprudent to cheat that number. In zone 6 R49 is also code minimum (but more is often still economic.) Hitting R49 in an vented roof with mineral wool takes ~12" of insulation + 1" of air gap, which is well above the depth of 2 x12 rafter.
The range of "reasonable" solutions depends a bit on your existing rafter depth, which isn't yet specified.
AMZ,
We need more information.
1. Many of your sentences talk about the "front" of the house. Does the front face north or south?
2. You refer to MDF, but your sketch shows no MDF. Where is this MDF?
3. You refer to an attic, but your sketch doesn't show an attic. Can you draw us a sketch of the section of your house that includes an attic, so we know what you are talking about?
AMZ,
You don't want an open vent from your interior space into the soffit area. That vent should be permanently sealed.
Here is a link to an article that describes all of the different ways that you can insulate this type of roof: How to Build an Insulated Cathedral Ceiling.
The house inspection says we have 10 " rafters. Sorry I forgot to add that.
The siding is T1-11 boards which we eventually will replace or recover.
The front I am calling is the front of the house which is the south side. The roof is sloped down the north side. Its one large rectangle plane.
The MDF looks like its resting on the rafters from the top down. The air gap above this. I can't see what is on top of the mdf spacing it from the deck sheathing.
The attic area is small, 6' by 10' or so. This attic area is over a bathroom area only.
Any idea what the vents from the soffit would have been originally for?
I'll assume those are milled 2x10 rafters, which are 9.25" deep, not 10". The most you can get in there and still have a vent space with rock wool or high density fiberglass batts would be R30, which is well shy of code minimum.
If you dense pack cellulose into the 2x10s it would yield ~ R34, which means it would take at least R35 above the roof deck to have reasonable dew point control at the roof deck. Installing a self-healing peel'n'stick and a secondary set or 2x10s above you could get there stacking R15 rock wool (3.5" thick) and R23 rock wool (5.5" thick) which would leave 1.25" of vent space, and have comfortable dew-point margin with a total of R38 rock wool to the R34-ish cellulose.
It sounds like they used MDF for the exterior side air barrier for the insulation on the old low density batts(?). If installed snugly without gaps rock wool doesn't absolutely need an exterior side air barrier, but you may want to install something vapor permeable such as corrugated cardboard insulation baffles side-stapled to the new rafters to reliably preserve the vent space, taping the seams between adjacent baffles for better air tightness. MDF would be too thick to guarantee the minimum 1" unless you compressed the batts. If you go the more rigid board route, half-inch asphalted fiberboard sheathing would be more moisture tolerant than some grades of MDF, and is still sufficiently vapor permeable. If it's asphalted on one side only, the asphalt side faces the vent channel.
With a 3/4" nailer deck atop the add-on rafters for the roofing above that's going to add 10" to the total roof thickness, which is pretty significant. The details of how to fasten the add-on rafters to the structural roof would have to be figured out and reviewed by the inspectors.
"The siding is T1-11 boards..."
"...air is open to a gap that runs down the exterior wall behind the siding"
T1-11 is usually the structural sheathing, not just siding. The description sounds as if the T1-11 is somehow mounted with gap between it and some other sheathing?
Thanks Dana
I ripped a piece of the existing MDF off and the side to the roof sheathing is black. I assume this may be a asphalted layer.
Adding 10 inches to the roof seems a bit crazy just to meet new codes :( It seems like the only other option would be to go lower inside which would be even a larger construction mess.
I really don't know what is behind the T1-11. I think its just a plywood of sorts. I need to investigate better.
If there is this open air gap from the exterior wall to the roof does this need to be closed and then just have the roof vent to soffits on the rear north slope?
AMZ,
The description is a little confusing -- but it sounds as if the MDF is a ventilation baffle used to create a ventilation channel. Is there roof sheathing above the 1-inch ventilation channel? If so, what type of roof sheathing -- OSB, plywood, or boards?
Sorry to be confusing, I can only see the roof from the small attic space until its open from above. From what I can tell the MDF is being used as a ventilation channel but I can't tell how its attached to the rafters. There is roof sheathing above this that looks like plywood from the bottom. I made a small hole to look up. I plan to make a larger hole tomorrow to try to see more.
Whatever the existing roof sheathing is will be replaced completely.
AMZ,
The situation isn't as complicated as you make it sound. I think you have a ventilated roof assembly.
1. Open up your soffit to figure out what's going on. Seal off the mouse highway (the wall connection).
2. Install soffit vents.
3. Insulate between the rafters with the insulation of your choice.
Alternatively, seal up all vents and install rigid foam above the existing roof sheathing. It's your choice.
Correcting deficiencies in the roof venting doesn't fix the R- value issue. AMZ expressed an interest in avoiding foam.
Rock wool R32s are 8" deep, which is sufficient for continuing with a vented 2x10 roof with thinner baffles/chutes. That's about as good as it's going to get without above the roof deck insulation. If going with foam above the roof deck you'd need at least 7" of polyiso for dew point control on R32 rock wool snugged up to the roof deck or R34 cellulose. But since it's continuous, not thermally bridged by rafter 7" of polyiso would also meet code on a U-factor basis even without the cavity insulation.
A thinner solution would be R23 rock wool snugged up to the roof deck with 5" of polyiso above for dew point control. But if you don't have access to the roof deck from below that may be hard to implement.