Using rigid foam to upgrade roof insulation in MN
My 1930’s 2-story Tudor style home in Minneapolis needs a insulation upgrade. Problems include a challenging roof design, with 3 knee-wall attics and multiple sloped ceilings in the upstairs (finished off in the 1960’s). Although areas that are accessible are insulated well, the majority of roof area is not. Over the years I’ve sealed/caulked with abandon, installed new storms over the original double hung windows (which are in overall good condition) and had a blower door test recently done which found the house to be within guidelines for air leakage. My thinking is, I believe the roof deck and to a lesser extent the total building envelope is causing a bad convective loop which is accelerating heat loss, causing ice dams and making the house feel cold. Since it’s time for a new roof, I’m proposing using a appropriate thickness of rigid foam panels (such as ISO) over the entire roof deck couples with 1×3 strapping and covered with 1/2” CDX plywood. I reference Fred Lugano’s article “Upgrading a foam insulated roof” in Fine Homebuilding 115 for technique. Then shingle normally over that. This should help with thermal bridging of the roof deck. Since I would be air-sealing the roof totally, in my climate, do I need to ventilate it further or not? Short of tearing out lots of drywall and re-insulating I think this may offer the best solution to my house’s problem. Any thoughts?
Replies
Insulating over the roof deck
is often a great strategy. Make sure you take into account what it will do to your fascia boards--they'll get taller.
Check this product out. Might save you some time/money.
http://www.hpanels.com/2009/pages/pdfs/Lit_Prod_Color/Cool-Vent.pdf
You'll likely have to remove the crown and install new - either larger or moved up the fascia.
hunter won't save you money
they are not outragious, but my distributor would charge me shipping on top, plus buying in full pallets, where regular foam was free shipping
adding a shadow board lowers the apparent thickness of the facia
make sure you don't have venting under the foam, or it is all for naught
hunter won't save you money
Maybe not if you're just looking at material cost. But if you look at the big picture and figure the saved labor, it's a different story. It's an in stock item here.
Good points on the other items, though.
venting under foam
The correct way to vent or not to vent the roof is where I'm most confused in this application of rigid foam. As indicated, the house has knee-wall attics, (currently vented with roof deck vents) and a couple of no-mans land valley areas also vented. Are you saying these areas should not be vented? I gather the new roof decking (above the foam) should be vented from fascia to ridge. A local roofer says the sloped ceilings still need to be vented. Nothings simple!
Thanks.
You could probably do it either way, but it would make the most sense to bring everything below the roof into the "envelope".
But for certain don't put vent channels directly under the foam in the areas where the ceiling is against the rafters, either above the sheathing or below it (eg, such as regular eave-to-peak vent channels used for vaulted ceilings). Venting below the foam would negate about half of its insulating value.
Did you ever find a contractor in the area that was familiar with what you want to do? I am in south Mpls, and want to do the same thing. I have found only one or two that I really think understand the issues here...