Getting full roof replacement- other things to consider in cojunction?
Hi All,
Had some bad leaks recently so I am getting a full roof replacement.
It is a flat unvented roof system, which is basically un accessible with varying distance between joists and rafters of about 18″-6″ (not really sure of exact clearance on the low end, since I’ve never been in that area of the roof).
Since the roof will be open from the top it seems like a good of a time as any to do additional work that otherwise would be tricky or require taking of sheetrock.
Some first thoughts that come to mind:
Exhaust fan for the bathroom – Currently only a window.
Insulation- Right now there is only r13 foil faced batts so the upstairs is hot or cold depending on the season. With the roof open we can possibly do a loose fill, or remove the old batts and do under roof fluffy insulation with rigid foam on top.
Air sealing – general air sealing of the various metal clad electric wire. Also there is a big hole where the furnace flue exits out – I’ve air sealed the bottom side of it, but the top is still open.
For more info – Zone 4a, right outside NYC in NJ. Townhouse, so can’t change roof geometry.
Anything else I might be missing – what have other people done when they are getting roofs replaced?
Replies
A lot of this depends on where you are, and the orientation of the house.
moving to a sloped roof can make it easier to prevent future leaks. It also can enable rooftop solar, which may be another consideration.
This would allow also for a vented roof cavity, and enough insulation.
Provisions for enough insulation and making sure the new roof does not leak for a long time would seem primary considerations.
If you do not have air conditioning, routing ducts is one additional thing to consider.
Thanks for the input - adding some info the main post as well so others can see this as well:
zone 4a - suburb of NY in NJ. House faces approximately North East,
I can't change to a sloped roof (easily or at all possibly) since this is a townhouse unit.
But we do have AC already and all the ducts or in the envelope-ish some ducts run through the exterior walls, not ideal, but not the worst.
Agree though that preventing leaks is primary concern, and adding insulation would be a big benefit.
Just as an update to this - finally got the 2 quotes from the same company put together - 650 sq ft flat roof, interior townhouse:
quote 1: 10.6k - mod bit rolled roofing, no insulation
quote 2: 22k - TPO over 3" polyiso foam.
Was pretty shocked at the difference between the two, and even though I initially was in it for the foam insulation (and wanting to see what r19 batts in addition to that costs), the difference between the two is crazy high.
I would look for another estimate.
How much of the change is for the TPO vs the foam?
One approach to think about is adding rigid insulation about the roof sheathing. You can add as much R value as you want, and it stops thermal bridging through the rafters. Rock wool also has 3 or 4 flat roof products that are made with various skins for mopped, rolled, or adhered membranes. https://www.rockwool.com/north-america/products-and-applications/roof-insulation/flat-roof-insulation/#products
Thanks for the ideas there - forgot rockwool had some options here as well.
I think the guys I am working with do Polysio rigid board, but may have an option for rockwool as well.
My current best options seem to be:
Polysio on top (3 inches or so) fiberglass or rockwool batts underneath
Fill the entire space with cellulose
Rigid foam exterior and through a separate contractor (or possibly the same) flash and batt with spray foam/batt insulation after the roof is down.
1 or 2 are probably a lot cheaper then doing any kind of spray foam with finished ceiling removal.
One thing to think about with polyiso is that it actually perfoms worse as it gets colder outside. The testing is done at 75 degrees (I think),
but its performance drops a lot as it gets colder, opposite of what most insulation does. So you pay more for less. XPS and rock wool don't do that. Just Google "polyiso thermal performance at cold temperatures" for lots of science, like these. https://www.greenbuildingadvisor.com/article/cold-weather-performance-of-polyisocyanurate https://insulationcorp.com/where-are-your-r-values/
This one is from Owens Corning, so you know they wouldn't publish it if it made their XPS look bad, but it's pretty dramatic. http://www.owenscorning.com/NetworkShare/EIS/10019949-FOAMULAR-XPS-vs-Polyiso-Tech-Bulletin.pdf
You should go over to GreenBuilding Advisor and ask for guidance about this: "Insulation- Right now there is only r13 foil faced batts so the upstairs is hot or cold depending on the season. With the roof open we can possibly do a loose fill, or remove the old batts and do under roof fluffy insulation with rigid foam on top."
I think you will not want to do this, if I'm understanding you correctly (creating a decking/air gap between two independent layers of insulation). I paid for a membership and got excellent up to date advice (sometimes there is cross-posting between people over there and on here, but not much).
I actually did do a post there once I got bit more information.
Currently if there would be new insulation added with the TPO/polysio the batts on the ceiling would be taken out too to avoid issues with that.
If only replacing with new mod bit, the insulation would stay,.