Our roof is 1/3 hipped slate, and 2/3 is flat-sloped with a conventional roofing material, coated white
Woody S.
| Posted in General Questions on
Our roof is 1/3 hipped slate, and 2/3 is flat-sloped with a conventional roofing material, coated white.
The underside of the entire roof was coated with spray foam a few years ago. The house was built in 1930, and the attic joists have a layer of rock-wool in them.
We’d like to get additional R-value, but don’t want to screw up the expensive foam coating. Thanks for your time & attention.
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Replies
Woody,
A few questions:
1. Where are you located? (Or what is your climate zone?)
2. What type of spray foam was used -- open-cell spray foam or closed-cell spray foam?
3. What is the thickness of the spray foam layer?
4. Was the spray foam installed directly against the old skip sheathing and the underside of the roofing slates?
5. Where is the rock wall insulation: on the attic floor, or between the rafters (up against the cured spray foam)?
6. How thick is the rock wool insulation?
We're in St. Louis, Missouri.
The foam appears to be closed cell foam.
The foam is about 4 - 5 inches thick.
It was installed directly on the underside of the roof.
The rock wool is between the joists on the floor of the attic.
It's between 3 and 4 inches thick, about half the depth of the joist.
The hipped portion of the roof, where there is room to stand and storage space, has plywood covering the joists & rock wool. In order to heat the 2nd floor ( all the radiators were cracked & there was no other way ) we installed a high efficiency gas furnace up in the hipped portion of the attic. ( it releases little heat into the space )
Woody,
It's kind of a shame that spray foam was installed on the interior side of the sheathing on your slate roof. It will make future roofing repairs difficult, since the spray foam glues the slates into a monolithic mass. It also makes it impossible for the slates to dry on the back side -- which is unfortunate but not fatal.
Once the decision was made to install spray foam on the underside of the roof sheathing, those sloped roof assemblies now constitute the thermal envelope of your home. In your climate zone (Climate Zone 4), building codes require a minimum of R-38 for roof insulation. The spray foam layer is rated at about R-29 or R-30, so you are correct that it would be a good idea to beef up your insulation layer.
I hope that the gas furnace that is located in your attic is a sealed-combustion furnace. Otherwise, you have a serious safety problem, since your sealed attic won't provide enough combustion air for safe operation of the furnace. (Any furnace or water heater located in a sealed attic needs to be a sealed-combustion appliance.)
Your spray foam layer is thick enough that you can safely install air-permeable insulation (fiberglass, cellulose, or mineral wool) directly against the cured spray foam. You will need to come up with a method of attaching the insulation and holding it in place, however. Those details will depend on what you have for rafters, and your preferred insulation.
The guy who does our slate repairs, a 2nd generation specialist, didn't have a problem removing and replacing damaged slate after a hail storm 2 years ago ... and that was well after the foam had been installed.
The gas furnace is indeed a sealed unit. It brings in fresh air from outside, and exhausts the same way.
So you're saying that the only proper way to get more R value is to install insulation over the foam, against the roof, and not below, in the joists?
What's the problem? Would too much heat build up in the attic during the Summer?
If you are heating with natural gas, I can't see you saving very much by just adding attic insulation since you already have.
Woody,
That's good news about your slate repairs. And I'm glad that your furnace is sealed-combustion. So those worries are laid to rest.
Building codes don't recognize the practice of putting half of your insulation along the roof slope, and half of your insulation on the attic floor. I'm not saying that it doesn't happen, or can't work -- it's just not a code-recognized approach.
From a code perspective, you really need R-38 insulation at your roof line.
But, leaving code issues aside, the main problem with installing more insulation on your attic floor is that you lose some of the benefit that comes from bringing your furnace inside your home's thermal envelope. Ideally, you want that heat thrown off of the furnace and ducts to be indoors, not outdoors. The more insulation that you place on your attic floor, the more you are condemning your furnace to spend the rest of its life outdoors.
Not certain we understand the bit about "condemning your furnace to spend the rest of its life outdoors".
Not sure how we'd lose anything ... aren't All furnaces inside a homes thermal envelope? ( whether well insulated or not )
The furnace, like all closed systems, puts out very little heat into the attic space. Most of the heat is blown down into the 2nd floor living areas, where we'd like to keep it. That's why we wanted to put insulation in the joists, on top of the 2nd floor ceiling.
We come back to the question of whether Summer heat build-up due to floor insulation would harm the foam.
We forgot to mention that the skylight that lets in sunlight & heat is about 24 x 48 inches.
Would anything change if we installed an AC condenser unit onto that furnace/blower/duct system?
We have the fittings already installed to connect to an outdoor A/C unit if we decide to do Central Air.
Right now we're making do with window units.
Woody,
Your plan will work, but it isn't optimal. That said, the difference in energy use that we are talking about isn't much.
Unfortunately, furnaces are often installed outside of a home's thermal envelope -- in an unconditioned attic, an unconditioned crawl space, or a garage. This is not desirable, since the furnace and nearby ductwork give off a lot of heat. If the furnace is outdoors, the heat is wasted.
Your situation is not an example of the worst-case scenario. Your furnace is in an in-between space -- not really indoors, and not really outdoors.
Whether you realize it or not, your furnace and the nearby ductwork are releasing heat. Ideally, the insulation at your roofline would be thick -- to keep the heat in. Ideally, there would be no insulation on your attic floor -- so that the excess heat given off by your furnace and ducts would be able to flow through the attic floor to your living space below.