View Full Version : re-roofing house, want to add insulation

02-10-2008, 12:23 PM
It looks like my 25 year shingles are going to crap out at 22. I built the house myself using info presented to me in 1984, some good, some not so good. I built with 2x6 rafters, sheathed with 1/2" ply. I can see a line that melts first when there is snow on the roof where the collar ties meet the rafters. If I get all the old shingles off, back to the sheathing is there a way to improve r-value without breaking the bank?

02-10-2008, 12:37 PM
You might get into the ice dam problems.

02-10-2008, 12:42 PM
Can you get the sheathing off easily and then re-use it? Or most of it? If so, I'd say pull it off, fur out the 2x6 rafters by at least 4", add cellulose or fiberglass, and then put the sheathing back on. If not, consider adding 2" of styrofoam to the inside. Remember to maintain the vapor barrier on the inside and airflow to carry away moisture on the outside of the insulation. That's why you can't just add styrofoam on the outside of the roof.

02-10-2008, 12:43 PM
I do get dams, I prevent them from raking off the bottom 4' of roof which seems to work.

Paul Girouard
02-10-2008, 01:07 PM
Icynene !


Spendy BUT it will do what you need done and does NOT require a air /venting space.

You would have to remove your sheathing and spray it in from above. You may have a problem finding a contractor in your area. That link may help with that.


02-10-2008, 01:19 PM
Injectable low expansion foam.

Ken Hutchins
02-10-2008, 01:38 PM
I do get dams, I prevent them from raking off the bottom 4' of roof which seems to work.

What even you do for insulation.
Metal roof is the way to go it elimates the ice dams.:)

02-10-2008, 02:17 PM
JMAC, Sorry to hear about your roof troubles. I to had to replace 25 year asphalt shingles early; after 18 years! Discovered the builder never put any paper over the ply sheathing and the south side of the building showed the worst wear. Also heard that asphalt product by a few companies produced a lot of crap in the late eighties. New shingles are guaranteed to last the duration only if you use 15 or 30 pound felt.

Had two six foot wide membranes attached horizontally before the new shingles went down. Guaranteed to prevent leaks even with ice dams.

As to R value, if you got an attic then would recommend new insulation between rafters.

Heard that metal was good. Many northern neighbors like Ken have it.

Good Luck


Ron Williamson
02-10-2008, 02:31 PM
Are the shingles damaged in specific places or generally bad all over?
Heat leakage/lack of ventilation won't do as much damage as trapped moisture.

02-10-2008, 06:43 PM
ice dams happen because the insulation on the cieling isn't stopping enough heat and moisture from escaping into the unheated roof space. biggest problem is usually improper vapor barrier and ventilation in older houses. just adding more insulation may not stop the vapor problem and adding more ventiltion won't stop the moisture if insulation is inadequet.....you need both.
vapor barier should be closest to inside space as possible, obviously under insulation.

there is a remedy, although somewhat labor intensive. i have done older houses by setting a new layer of heavy poly on the exhisting ceiling( inside the house) and re-rocking the entire ceiling. then adding more insulation and good vetilation( continuous ridge vent and enough soffit venting to match). another way vapor barrier can be done is to set a layer of 1" blue dow on the ceiling( inside) and rocking over it for a new ceiling. this method also increases insulation by the value of the rigid insulation installed. although it cost more than blown insulation in the attic, this method is very efficient....... the warmest house i ever built(current house) has 1 inch blue dow on the entire inside under the drywall. construction was otherwise conventional with 6 inch walls and normal batt insulation and 16 inches of blown in the attic. a 43k btu gas fireplace will heat the entire home .....it's -15 here, right now.

02-10-2008, 06:46 PM
Is it possible to fasten foam insulation panels under the ply between rafters?

02-10-2008, 06:53 PM
No! That way, foam acts as a moisture barrier on the wrong side of the bulk of the insulation. It'll make the insulation under it soggy.

02-10-2008, 07:12 PM

The joint between the collar tie and rafter is always a difficult area to insulate. The collar tie runs across the face of the rafter creating a 1.5" jog in the insulation and it's hard to keep warm air from getting thru this area and warming the plywood right here. Furthermore, with only 2x6 rafters, you probably have insulation hitting the underside of the plywood, which is also causing some melting. Did you use propervent to hold the insulation away from the plywood and to create an air passage so cold air can enter at the soffit vents and escape at the ridge vents?

Here's what I'd do: after you strip the roof, snap two parallel lines 18"-24" apart across the roof directly over the collar tie joint. (If you have a plywood joint in the right space, you only need to snap one line). Set the saw only as deep as the plywood, make the cut, and remove the plywood. Stuff the joint area from above as best you can with insulation, then buy some propervent and lay it in on top of the insulation so it can't touch the plywood. Put the plywood back and nail it down.

When you re-roof, install a continuous ridge vent, and if you don't have one, install a continuous soffit vent.

Good luck.

02-10-2008, 07:42 PM
good advice from shoes ...........as said, proper vapor barrier and venting is ussually the problem with older homes.

biggest problem with adding insulation is that it will be compressed at this area and not as effective as being put on the ceiling as rigid foam would be.

02-10-2008, 09:05 PM
Things that worked-

passive solar salt-box design looks good and takes only around 2 cords of wood to heat

basic layout inside works well

hasn't fallen apart

Things that didn't work so well

2 x 6 rafters leading to already mentioned problems

didn't insulate under slab

I built a 2 x 4 floor system on top of slab and didn't insulate that either, have cold floors in Winter

Didn't know what a proper vent was, put in gable vents, no ridge vent.

What about pulling off the sheathing and running some framing lengthwise and insulating that to get rid of the rafter wood's conducting heat out.

Paul Girouard
02-10-2008, 09:38 PM
Anythings possible if you have a lot of money , check out the Icynene spray it's the only good answer , affortable, one you've gotten and it works very well .

02-10-2008, 11:33 PM
JMAC - don't feel bad about the propervent. I don't think it was even on the market back in the mid-80's. I built my saltbox house in So Maine about the same time you did, and I didn't use it either. I did go back a few years later and shoved pieces of strapping up between the top of the insulation and the underside of the sheathing. Push it up flat and then rotate it 90 degrees to create and air channel. The gable vents should work if they're big enough, but since you'll be re-roofing anyways, it'll never be easier to put in a ridge vent.

I have to re-roof this summer, too. I'm on my 23rd year with my shingles. I've thought long and hard about metal roofing. I used it on my 32x32 shop, and I like the way it looks and the fact that there are no horizontal joints going up the slope. It should never leak and ice dams aren't an issue. It has drawbacks, though. If someone were standing under the eaves when my shop roof decides to slide, they could be killed, so you must use fixtures along the edge to keep the snow from sliding off. It will still creep down from the upper portions of the roof, though. One side of my shop has an attached carport with a flatter (3/12) pitch than the main roof. This valley accumultes all the snow that slides down from the upper (8/12) pitch and creates an unnatural snow load in this one area. Also, I'd need to build a catwalk up to the chimney if I use metal so i can clean it twice a year. I can walk up the 6/12 shingled pitch, but a 6/12 pitch on metal is un-walkable.

Bill Perkins
02-11-2008, 10:32 PM
One proven method is this . Strip the roof down to the bare ply wood and nail 2by4 or 2by 3 nailers continuously around the perimeter of the roof and also across it's interior , 4 feet on center . 1 1/2 in. rigid foam in 4by 8 sheets is then fitted in between , ripping the foam down on the table saw by the width of the furring strips , so it fits snugly in between . A second 1/2 in. ply deck is added over this . If you choose to go with 3 in. insulation the furring strips are doubled , or 2by 4 is ripped to 3 in. and toenailed down .

The insulation is stopped at the exterior wall line . From there out to the eave short 2by pieces are installed 2 ft. O.C. to support the plywood .Use plywood rated for 24in. oc.

The cheapest add on is to nail 1by 4 around the perimeter only and glue 3/4 in. insulation to the ply wood deck . The shingles are applied directly over the rigid foam using the extra long nails intended for reroofing over existing layers of shingles . I've done this a couple of times and it makes a noticeable difference ; well worth doing .Get shingle nails long enough to run through the foam and the ply deck .They're out there if you look .

Ron Williamson
02-12-2008, 06:19 AM
If he has attic/soffit venting,the foam above it won't do anything to stop heat loss.If he doesn't have venting,the foam will trap heat and moisture in the insulation below it.

Bill Perkins
02-12-2008, 09:57 AM
He's doomed.

Paul Girouard
02-12-2008, 10:15 AM
He's doomed.

Not if he uses icycene, all other things add height to the roof distorting the roof looks , making the facia "extra wide" , cost bigger $$ , create other issues.


3.6 R-value per inch , no venting. Would give about R 20 . No change to the look of the building , no huge facia board , no "goofie " odd ridge in cross section.

Ron Williamson
02-12-2008, 12:39 PM
We foam all the tricky stuff that's hard to seal or vent,ie. sloped and cathedral ceilings,bulkheads/rimjoists,odd kneewall-into valley,stub rafters into dormer fronts.
Some guys will even foam the warm side of glass batts.