Stupid roof insulation design (or lack of) – Help
I have been away and had a project done for me. There were many phone conversations with the contractor, but my focus was not on the project. Upon returning I realized that the insulation detail I wanted (4 in of polyiso on the roof) was not installed before the metal roof was attached) This was 100% my fault not the contractors. Please help me fix this. It was suggested that I leave the metal and create a vented roof instead by adding 2 by 4s perpendicular to the purlins to create a channel under the purlins for a vent from the soffit to the ridge above the cathedral ceiling under the roof.
However,
If the purlins were of sufficient depth (2×6 purlins) would venting each of the bays on the gable end suffice. I understand that there would probably be no chimney effect but if you had a 4/12 roof would there be much of one the traditional way?
If a vent were created under the purlins from the soffit to the ridge how big would it need to be and would a completely vapor open board like comfortboard 80 or similar be a good lower boundry for the vent or would something like a foil faced polyiso be better. In my mind the polyiso might create interior condensation issues but be more of a weather barrier than the comfortboard. While the comfortboard would allow the interior moisture to move to the outside, although maybe too well. Thanks
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I'm not so sure a vent channel UNDER the purlins is going to help as much as you'd think. The vent channels are supposed to be right up under the sheathing, and there is no way to do that with the assembly you have. I suppose vent channels under the purlins are better than nothing, but I'd be concerned about possible mold formation between the purlins if there is sufficient moisture getting up there. If you do go this route, I'd put a proper vapor barrier under the assembly, such as 1/2" polyiso with taped seams, or poly.
Spray foaming this and treating it as an unvented "hot roof" assembly is safest. If you use spray foam, you want to use closed cell spray foam here.
Bill
I appreciate your reply. I was actually trying to avoid spray foam for fire protection issues. What do you think about venting this horizontally through the gable ends? Or would I be better off just removing the metal and adding the polyiso on top of the roof?
The vent channels rely on convection airflow to work, so they need to "ramp up" from the eave to the ridge. I think with horizontal venting, you'd end up with stagnant air and the vent channels wouldn't be effective.
Putting the polyiso on the top is by far the superior assembly as far as insulating and roof longevity goes.
Bill
CKsmile,
As Bill said, any venting has to occur either directly under or over the roof sheathing to work. A vent channel below the purlins would be both ineffective, and negate all the usefulness of the insulation above. Unfortunately with what you have now your only option is an un-vented roof assembly. Given how shallow your purlins are, if you want to leave the metal roof on your only choices for cavity insulation are spray foam or foam board.
There is also a bit of hardware and the trusses in the way of a proper ceiling air barreir continuity. If you had some taller I-joists for purlins you could drill holes near the top to allow for air movement, this would let you vent the roof.
As is, the vented roof option as is has sailed. The only vented option is if you don't need the height, you could frame in a dropped ceiling and loose fill that.
Otherwise, spray foam is pretty much your best option right now. I would do a flash and bat install.
What type of metal roof?
Exposed fastener would be pretty easy to remove
Sometimes screwing around to fix a mistake is more bothersome than starting over
Deleted
Yeah, bigger peak cap, unless the fascia is vertical, then it doesn't matter. I hate vertical fascia but trusses all seem to be cut that way. In this case it would be a good thing
gusfhb,
I've had good success pulling off quite large sections of snap-lock panels too.
It would be interesting to see what a roofer would quote for the work. I got into doing my own metal roofs when I realized what a small percentage of the cost was materials.
That looks like ~5 pitch, is my math right that amounts to less than 3/8" for 4 inch?
Certainly not something to ignore, but not that difficult to hide.
Yup. In the weeds. I've deleted my post.
No no, it is not nothing, if you ignore it you might have an issue, but just not a big deal
Thanks to all that looked at this for me.
The metal roof is exposed fastener (that i would like to reattach anyway - the screws don't go into the purlins just the 7/16in osb)
The pitch is 4/12 so adding 3.5 to 4 inches of depth should not affect the coverage if I can get a slightly large ridge cap.
My plan at this point is to remove the roof and spot apply a sealer to the ice and water shield to cover the screw penetrations.
Install 3.5 in foil faced polyiso in 2 layers with offset and taped seams. Cover with 2x4 2 ft on center running up the roof attached with 7in screws into the purlins to leave an air gap between the metal and polyiso.
Attach 1x4 perpendicularly to the 2x4 at spacings that match the existing holes in the metal.
Also I would cover the vent openings with something like coravent.
Is 1.5 in of penetration into the purlin enough?
Also Is there a problem with the polyiso being the bottom layer of the vent or should that be covered with something. I like the foil as a radiant barrier, but maybe it is not stable enough.
What you are proposing will work. You don't need to top vent unless in heavy snow country (greater than 60lb of snow load). If the iso is foil faced you don't need anything on top. If paper faced, you need a layer of synthetic roofing underlayment over it.
Removing and re-installing the roof will cost you the same as a roof install as most of the cost is labor. There is nothing wrong with screws into the roof deck, that is proper install. At this point I would bite the bullet and spray foam+batt for the roof. If you really don't want spray foam, you can install one of the vinyl blanket roofing batt insulation bellow the purlins and add ridge vent. Won't be high performance but will work well enough for a shop.