Need help for metal roof insulation

[magic_garage]

Hi All:

I'm in Hawaii and if you're not familiar with our weather, the temperature is mostly in the 70 - 80's majority of the year with winter going into 60's on the low side. Humidity is pretty high about 60 - 70's for most of the year with winters going down to 40-50's.

Ok so here's my situation. We had our home renovated about 18 months ago and the general contractor (GC) we had wasn't that good so if some of the designs/decisions doesn't sound very good, you know why. At the time of the renovation, the GC got a roofer to inspect our asphalt 3-tab roof, and they deemed the roof to be ok, so I listened to them and we didn't replace the roof, instead they coated the roof with Gaco silicone roof coating. The roof is 19 years old. We've had a bunch of small leaks throughout the year and the GC has been coming back to add more coating to try to seal them. I haven't been able to totally put the GC at fault since we had termite tenting and photo voltaic panels installed as well. Anyway, so I've decided that I want to get the roof re-done and I'm really leaning towards a standing seam roof. The questions I have is what I should do as far as having the roofers install a radiant barrier or not, or rigid foam w/foil insulation or not.

As mentioned, the current roof is asphalt shingles on 1x8 wood planks as a base. About half of the roof is over a cathedral ceiling, exposed ridge beam for the living room and the remainder of the house has an attic. The roof for both sections actually have independent ridge lines so there's a transition between roofs. In the attic space, our GC installed the foil with bubble radiant barrier attached to the rafters in the attic. We also have a ridge vent across the entire ridge of both sections of the roof that was previously done in the past. We also have two solar fans in the attic section of the house/roof.

The living room gets considerably hotter than the rest of the house thats under the attic. So given my setup, would a radiant barrier on top be really beneficial? The ridge vent on the cathedral ceiling definitely will get capped but what about the attic section?

One metal roof contractor offered a rigid foam (white) with foil on one side as an insulating layer but he was going to install purlins only on the cathedral ceiling section fearing that he will miss the rafter and the nails/screws will be exposed inside. But on the attic section, he'll lay the foam layer on the underlayment then metal roof on top. The question for this is, does the rigid foam type of insulation need an air gap or will it work sandwiched between layers? The sample the roof contractor showed me had the foil attached to the foam so it got me thinking this is should be part of a radiant barrier?

Please help. Any other suggestions/comments are highly welcome.


Thanks!

 

[aptdweller]

I can't say that I am familiar with common practice in Hawaii, but you would be best to forget about radiant barriers. They are only useful in a small number of cases, and insulating a roof isn't commonly one of them.

Next, you need to decide whether the attic is part of the conditioned space or not. Around me, they almost never are, so the roof would be well vented. This is very helpful for a long shingle life. What would be done is a vapour barrier over the rafters and batt insulation over that.

If you want the attic as part of the conditioned space, your options are more limited. Closed cell spray foam is simplest, but likely the most expensive. Alternatively, you could do rigid insulation, but you are relying heavily on your contactor to do a good job of it. I can't say that I have been impressed by what I have seen in the past.

 

[bandit067]

I live in the south and we have pretty hot and humid summers. I have reroofed all my houses, shops, sheds in metal. I would agree, forget about adding a radiant barrier. I have two houses reroofed in metal over shingles. I have had them both ways they do it here in the south. Some go straight on top of the shingles and some installers prefer scotching the roof with 1 x 4s first. They say leaving that air gap aid in not transferring heat into the shingles therefore into the attic. I cannot say one method works better than the other, but I can say both metal roofs have made my houses cheaper to cool. It seems the asphalt shingles transfer heat in to the attic all nite whereas the metal seem to cool off shortly after dark, or rain.
Now if you want insulation barrier, I would look at spray foam on the inside of the attic......but that's another thread........JMHO

 

[magic_garage]

aptdweller,

Thanks for the response. I lost you on your 2nd paragraph. I need to clarify a few things. The attic is NOT part of the conditioned space. And the cathedral/open ceiling living room has stained cedar planks so I can't cover the ceiling up. This is why I was thinking of insulating on the roof vs. inside.

I like how you mention "relying heavily on your contractor to do a good job", because here cost to do anything is very expensive so if a contractor really is good, they will take more than your arm and legs. With that said, I'd rather go with a less contractor dependent (to do a good job) design even if that means giving up more R value.

So would the rigid foam be of any benefit for me? Any other thoughts?

Thanks

 

[magic_garage]

I live in the south and we have pretty hot and humid summers. I have reroofed all my houses, shops, sheds in metal. I would agree, forget about adding a radiant barrier. I have two houses reroofed in metal over shingles. I have had them both ways they do it here in the south. Some go straight on top of the shingles and some installers prefer scotching the roof with 1 x 4s first. They say leaving that air gap aid in not transferring heat into the shingles therefore into the attic. I cannot say one method works better than the other, but I can say both metal roofs have made my houses cheaper to cool. It seems the asphalt shingles transfer heat in to the attic all nite whereas the metal seem to cool off shortly after dark, or rain.
Now if you want insulation barrier, I would look at spray foam on the inside of the attic......but that's another thread........JMHO

bandit067,

Thanks for sharing your experience with metal roofs. So you would just add insulation on the inside instead of trying to insulate on the roof side? My problem is, in our living room which is a vaulted ceiling with stained cedar planks, I don't want to cover that up, but I have a lot more options with the attic.


Thanks!

 

[aptdweller]

I was being a bit general there, because I wasn't sure what the plan was for the attic. If it isn't conditioned, I would go with the traditional vapour barrier and batt insulation. It is the cheapest and fastest method.

For your cathedral ceiling, exterior foam is the way to go. I would use 2 1" layers of foam, overlapped so that there are no full depth seams. Your roofer will want to do 1 2" layer, so you will have to determine whether it is worth the extra cost.

The best practice for mounting metal roof over insulation is to do strapping over the foam so that there is an air gap. Again, I'm not sure what your builder will want to do or whether this could create a bug problem for you.

I would recommend that you check out Buildingscience.com, as they have though through all of these cases. Check out RR-404 and RR-108 for more talk about unvented roofs that you will ever want to read.

Edit: Also, the correct foam to us is XPS, which is usually pink or blue. The white stuff is Styrofoam, which is significantly less durable.

 

[Rookie2]

FYI, there is a forum somewhere on here that talks about metal roofing color having a lot to do with attic temperature. I'm sure white reflects radiant heat the best.(I'm in the process of getting quotes for my house also). As a note, when I was insulating my 40x60 metal building I tried a layer of kitchen foil between the trusses and about a foot above the ceiling (4'x4' suspended tile). Using an infrared temperature gun ,It dropped the i/s temperature of the ceiling tile about 2 degrees. It proved to be too much work but if I were to do it over I would def. use some type of reflective paper or foil under the furring strips but not touching the metal.