Helpful? 0

Hip roof that intrudes into living space: "hip ceiling"?

I'm in Massachusetts (zone 5A), and have a 1960 garrison colonial with a 6:12 hip roof. I'm looking to do a lot of insulation/venting/roof work, and am about to post a detailed question. But, before I do, I need some terminology.

Instead of having the edge of the roof sitting on the junction between second floor interior walls and ceilings, the roof is mounted lower, with the rafters defining part of the second floor ceiling. So, at all four edges of the second floor ceiling there's a sloped section of ceiling, about 30" deep and 16.5" high. This makes access to that area between the rafters challenging, especially in the corners between the jack rafters, which will guide my choice of insulation.

Here's the term I need: what do you call my type of roof, or that area of my roof? "Hip ceiling"? "Lowered hip roof"? "A pain in the rear"?

And a follow-up: what other information would make my eventual question easier to answer?

Thanks,
Dan

Asked by Daniel Griscom
Posted Thu, 06/19/2014 - 15:36

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9 Answers

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1.
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I'd call it a partial cathedral ceiling under a hipped roof, and a pain in the rear (but not as painful as 2nd floor cathedral ceilings with kneewalls and mini-attic spaces.)

It's hard to properly vent hipped roofs without full attics in snow-country- the corner rafter bays that meet in the cathedralized ceiling area have no attic to vent into, and ridge venting along the hip is better than nothing, but less than ideal.

A photograph of both the interior ceilings & exterior of the roof (or similar images pointed to on the web) can be useful. Pictures of how it's framed can also clarify a lot.

Answered by Dana Dorsett
Posted Thu, 06/19/2014 - 16:35

2.
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Dan,
Something like this?

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Hipped roof.jpg
Answered by Martin Holladay, GBA Advisor
Posted Fri, 06/20/2014 - 04:58

3.
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Or this?

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Hipped roof 2.png
Answered by Martin Holladay, GBA Advisor
Posted Fri, 06/20/2014 - 06:15

4.
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Thanks for the pictures, Martin. The first picture, showing the slanted ceiling edge meeting a flat ceiling, is what I have. It goes around the entire perimeter of the house. The slope isn't as high as in the picture; measured from the inside, the slanted section rises 16" with about 30" of run. Note the triangular spaces blocked off in the corner; insulating those will be entertaining.

Answered by Daniel Griscom
Posted Fri, 06/20/2014 - 09:08

5.
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The first picture is framed with a kneewall and mini-attic- is that what you have going on as well?

How deep are your rafters? (2 x 6?)

Answered by Dana Dorsett
Posted Fri, 06/20/2014 - 14:26
Edited Fri, 06/20/2014 - 14:27.

6.
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Well, I don't know if I'd call it a "kneewall", as it's about 5'6" tall. A "shoulderwall"? And, there's nothing behind it except the great outdoors; it has windows whose tops stop just below the soffit.

But, I would say I have a "mini-attic". Headroom is about 4 to 5'. I think the rafters are 2x6; the wall studs are 2x4, or perhaps even less.

I'll take some pictures and upload them tonight.

Answered by Daniel Griscom
Posted Fri, 06/20/2014 - 14:36
Edited Fri, 06/20/2014 - 14:38.

7.
Helpful? 0

To be considered a kneewall there has to be a triangular cross sectioned attic space behind the wall (not the great outdoors), bounded by the floor, the roof, and the kneewall. That's what appears to be going on in the first picture.

If the rafters are 2x6 you can't get anything like code-minimum R-value insulation in that space and still have depth for roof deck venting. About the best you'd be able to do would be to slip R15 rock wool into the sloped ceiling with a ventilation gap above, which would likely become an ice-damming issue at such low R.

Given that the venting is mostly blocked in those areas anyway, you're better off insulating the sloped-ceiling sections with 2" of closed cell foam sprayed on to the underside of the roof deck (about R12-R13, at about $2-2.25 per square foot), and filling the remaining 3.5" with carefully trimmed R15 rock wool batts (or compressed R19 cheapo-fiberglass), which is an adequate R-ratio of foam/fiber for dew-point control in a zone 5 climate. As long as you have at least 40% of the R-value to the exterior of the batts you don't need an interior side vapor retarder, so it only takes 2" of R6/inch foam in an 5.5" deep cavity to accommodate 3.5" of any fiber insulation.

At 2" the closed cell foam gives between 0.5-0.7 perms of drying capacity for the roof deck. The center-cavity R would then be R25-R28, which is about as good as you are going to do there. When it's time to re-roof you'd be able to add rigid foam above the roof deck without creating further issues, which would take the ice-damming issue down even further by thermally-breaking the rafters. If you can afford the head-banging space, adding up to 2" of unfaced EPS between the sloped-ceiling gypsum and the batts/rafters would be OK., and would more than double the R-value at the rafters. EPS is sufficiently vapor open that the assembly could still dry, but vapor tight enough to not create wintertime moisture issues at the fiber insulation layer despite having only R12-R13 on the exterior (and thus violating the 40% rule by a bit.)

In the hobbit-attic above the flat section of roof you can install chutes to keep fiber insulation from contacting the roof deck, and add ~15" of open blown cellulose, which will settle to ~13.5" over a couple of decades, performing at R50-ish. Ventilate the mini-attic with ridge venting on the top ridge and on the hip-ridges down to the ceiling level (but not below, since that is now blocked with closed cell foam.) The most important aspect to keeping the attic above the flat ceiling dry enough is to air-seal the ceiling prior to insulating. I assume there is an access hatch? If it's an operable hatch, be sure that it has a good weather-stripping and latching mechanism to limit leaking there. The hatch itself can usually be insulated by gluing stacked up foil faced polyiso foam on it (at least 6-8" thick.)

Answered by Dana Dorsett
Posted Fri, 06/20/2014 - 16:43

8.
Helpful? 0

Wow. That reply just made my annual fee worthwhile. Thanks.

That said, I'm sure there's more for me to learn, so here is some more info. First, the pictures I promised:

  • Exterior view of the house; I'm mostly concerned with the main roof, as the garage roof is fairly clear
  • Interior view of attic, with current blown-in cellulose over rolled fiberglass.
  • Interior view of second floor room, showing bevelled ceiling

(BTW, it took me about six tries to get the pictures properly uploaded. Has anyone else notice how strange and awkward the board software is?)

Exterior.jpg AtticInterior.jpg SecondFloorInterior.jpg
Answered by Daniel Griscom
Posted Fri, 06/20/2014 - 20:50
Edited Fri, 06/20/2014 - 20:59.

9.
Helpful? 0

Now for some more information. The house footprint (excepting the garage) is 32' x 24'. There are two layers of shingles, and it's time to re-roof. The rafters and the second floor ceiling joists are indeed 2x6. The ridge is about 44" above the ceiling joists. Under the ceiling joists are 1x3s crosswise every 16" or so, and under them are plaster boards (these aren't drywall; IIR they're about 3/4" thick by 12" wide) covered in plaster; this means there's a 3/4" gap between the ceiling material and the joists. Insulation is about 3" of fiberglass covered with 10" of cellulose.

Since we replaced the windows a few years ago we've had a significant moisture problem in the winter, both in the attic and in the second floor, probably because under that fiberglass are significant gaps in the building envelope (e.g. I bet the interior wall cavities don't have tops to them). And the moisture has caused mold on the roof sheathing, as well as on a few spots on the sloped section of the second floor ceiling. We haven't had any problems with ice dams.

There are about eight small soffit vents, plus one power vent near the peak of the roof (the power vent is never used). The attic is accessed by a pull-down stair with an insulated box cover; I'd be OK with giving up on the stair if a simple hatch would be easier to seal and insulate.

Heat is gas-fired hot water baseboard, and we're going to be buying a high-efficiency direct-vent boiler in the near future. No A/C in the house (except for occasional use of a window unit); we use lots of fans. We're on a hill, so the roof gets a lot of sun in the summer, but we also get a good amount of wind.

I want to fix this attic so that I never have to fix it again (at least while I own the house). My plan has been as follows:

  • Remove all of the insulation
  • Patch all the holes in the attic floor so it becomes basically air-tight
  • Clean up the mold (if necessary; perhaps not if I'm going to spray foam on top of it)
  • Remove the power vent
  • Add a continuous soffit vent around the entire perimeter of the roof (or at least away from the corners where the air will go somewhere). That gives me 88 useful feet of vent, with about 5 square feet of area
  • Add ridge vents, both along the center ridge and the hips. Another 48 useful feet of vent, with about 5 square feet of area. Total of 10 square feet of area, evenly balanced.
  • Reroof, with a generous ice and water shield
  • Consider adding a whole-house fan, pulling air from the second floor hall and blowing it out all those vents
  • Insulate well, making sure to keep the venting free, and to insulate the corner spaces

So, some questions:

  • Does any of this new information change your recommendation?
  • Are you suggesting spraying 2" of foam only on the roof that is above the slanted ceiling? Those slots are (as you noted) 5 1/2" tall and about 40" deep; can installers do that with any accuracy? How do they spray into the sealed-off corner spaces?
  • It sounds like you're suggesting the slanted sections will have little or no space for venting from the soffit to the attic; am I right? Does this mean I don't really need as much soffit venting?
  • I'd love to replace some of my fans with a whole-house fan. Even if I ignore the soffit vents, I'll still have enough venting for a reasonable fan. Sound right?

Mondo thanks,
Dan

Answered by Daniel Griscom
Posted Fri, 06/20/2014 - 22:53
Edited Fri, 06/20/2014 - 22:55.

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