Insulating A Sloped Ceiling
Condition: Living under the roof in a 1-1/2 storey home with sloped ceilings at top floor. Previous owner blew celluose insulation in attic. Works well at flat sections of ceiling but has not settled evenly between rafters at the sloped sections (has all fallen to the bottom of sloped ceiling).
Problem: Ice-damning half way up the roof because of thermal bridging (freeze thaw cycle).
Solution: Rip drywall from inside at sloped sections and spray closed cell urethane foam in rafter bays. Rafters are 2×8 and we will fill with foam to the underside of roof deck. Will not leave any room for ventilation (a la SIP panel).
Any comments or better ideas?
Replies
To prevent ice daming, you must keep the underside of the roof deck cold. The only way to do this is to ensure adequate air movement from the soffets to the roof vents. in your sloped portion place more vents in all rafter spaces, stapled to underside of roof deck prior to spraying in the foam insuation.
Have you checked to ensure your soffets are clear and able breath properly?
He doesnt need vents if he foams the whole cavity. The warm inside of the house will never make it to the underside of the roof deck to cause ice daming.
PG-
your right with the foam. Another way to keep your roof cold is to insulate directly against it (seems obvious)!
This is the preferred method and heavily recommended when using spray foam insulation. Refer to Corbond.com or equivalent.
Stu
I'm a fan of foam, burt in your case, with the cellulose already in place, i might just finish the job and pak it dense with more cells. That would save ruining the rest of the house to accomplish the task.
Welcome to the
Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!
GMack's right. If you can keep the under-side of your roof deck the same temp as the ambient air, you'll never have an ice dam and your attic/living space under it will, theoretically, never exceed ambient air temps in the summer. The trick is how to do this.
Experimenting on 2 rehabs and 1 new construction, here's my approach.
After ripping off drywall and exposing rafters, rip some 3/4" whatever about 1" wide. Tack these up where the roof deck meets the rafter, on each side of rafter. Then cut rigid insulation the width between rafters (14.5" or 22.5") and push up to your rippings (stops) and tack in place. This will create an 'air-chute' under the roof deck. Then staple up batts against this and refinish attic/second floor ceiling. The under-deck ventillation needs to be able to travel freely from soffit to ridge vent.
The only potential problem with this approach is deep snow...which may completely cover the ridge vent thus stopping the movement of outside cold air up the 'air chute'. There are a couple of possible solutions to this. First, use an externally baffled ridge vent, Air movement around this vent will help prevent small amounts of snow from covering it over. Second, if possible, put in gable vents up at the gable apex (highest point) so that ourside air can circulate in the area under the ridge and above the collar ties.
BruceM
Thanks to all for the food for thought. I reside in Ottawa where temperatures range from -30 C in the winter to +30 C in the summer. We have used SIP roof panels for some of the homes we have built and these panels are designed without any ventilation and I think they work well. I liked the idea of blowing more celluose insulation and packing the bays between the rafters. However, I do not think that celluose will a) seal as well as the urethane (e.g. air leaks) and b) will not be a vapour barrier like the urethane foam.
I'm not familiar with SIP panels. Have you got a web site for more info.
GMack,
There are a couple of SIP (Structural Insulated Panel) suppliers we have used in the past - Plastifab (http://www.plastifab.com) which is based in Calgary, Alberta, Thermapan and Energreen. We used SIP roof panels on some cottage we built for someone in Quebec. We used re-claimed red and white pine that was salvaged from the bottom of the Ottawa River and built large exposed trusses with SIP panels that sat on top of them.
Like I said below I'm not familiar with these panels, but be careful here. I assume you have knee walls.There will be a cavity behind there as well as a cavity above the horizontal portion of the upstairs ceiling. Heat loss through the ceilings will condense in these cavities and be trapped without proper ventilation. If you close off the ablity to updraft from the soffets to the ridge effectivly removing the moisture, your trading one problem for another. You may be ok for the first couple years but eventually your ceilings under these cavities will mildew and rot as well as the insulation above them.This foam insulation sounds good, but may not be for all situations. Be careful and think this one through, weighing all contingecies before making your final choice.
thanks. you are right about the cavity behind the knee wall. i guess one way to address that problem is to bring it into the conditioned space of the house and insulate between the rafters over this section too. that adds to the cost but it may address the problem you outlined. the only attic space left is the one at the top which can be cross ventialted with hatches in both gable ends and ventilated with the ridge vent. i guess another solution to the problem you outlined is that i could add in some plastic vents at the section behind the knee wall. that sounds like a lot of work tearing off shingles etc.
I just built a 1 1/2 storey workshop myself. Its 36x40 ft clear span, with roof truss top chords of 2x8. My upper floor is my office and lumber storage. I had the same situation you have albeit mine was new construction. I used "Duravent" between the trusses, stapled to the underside of the roof deck and made sure they extended 12" past both the knee wall and the intended insulation level in the horizontal ceiling. I then friction fitted roxul 21.5 between the trusses and then covered the whole works with John Mansfiel with the foil facing the interior then drywalled. This gave me close to R 30. I have no problems at all, in fact the snow has to blow off the roof because there isn't any melting at all. My roof pitch is 6 1/2 in 12. Since your going to have everything open anyway no matter how you do this, why not take a proven method you know will work and avoid all the other agravation with extra vents and such. I'm just up the road (401) from you in Frankford. Our weather is identical.
Kind forum fellows,
May I just insert an idea here?
I live in a timber framed, 200 year old house the construction of which does not allow ventillation under the eaves. There is a 12 inch by 24 inch timber running the full length of the eaves, disguised as soffit and fascia. We have suffered from the deliterious effects of ice damming and water infiltration, not to speak of enormous icicling for all the years we have been here.
I hasten to add, we have done everything we could think of to alleviate the problem without success.
This winter, though, there is no icing, no infiltration, and NO SWEAT!
I would think the solution might be especially useful in lattitudes even more northerly than ours (NW Pennsylvania). Last July we installed a 'cold roof", as it is called by log house builders. Our approach was to employ the manufactured panels called "Cool Vent". They consist of a layer of extruded polystyrene insulation, spacer blocks to provide an air gap, and the nailing surface for the shingle roof that is applied on top of the panels.
A special drip edge covers the 4 inch thickness, but allows for air to enter at the eaves and good ridge venting allows the air to escape through the top. In effect, the shingle roof is always at or very close to the temperature of the outside air.
We spent about double the price of a good shingle roof, but having an end to the problem was well worth the expense.
Check with your local roofing distributor for it and experienced contractors if you need one.
Best regards,
Don