Retrofit insulation of flat roof

ROOFO

What is the best way to preserve heat and not capture moisture for flat roofed school building that has double cord wood roof trusses two foot deep. 3 inches of loose insulation between the bottom truss cords and an added 3.3 inches of rigid insulation and .60 edpm on top of the 1 x 6 t&g roof sheathing

is it best to add turbine vents at the high points to ventilate the 19" space between the two layers of insulation or to seal the space so that it becomes an insulated cavity??

RoofPro

Its hard to give a definitive answer from the information you provided – but I’m confident you do not want to install vents through the roof. The most common ridged insulation used under EPDM is Polyisocyanurate and 3.3” will provide an R value of over 20. If the insulation was installed in two layers (say one layer of 1.8” and one layer of 1.5”) and the joints are staggered you should have very little heat loss. Putting vents in the roof would create heat loss.
The 3” of loose fill between the trusses will provide about ½ the R value of the Polyisocyanurate (depending on the type of insulation and the spacing of the trusses.)
Creating an air barrier/ vapor retarder on the under side of the roof deck may be a good idea.

Grumpy

I'm with roof pro about the vapor barrier on the inside, but check on local codes about that.

However I might also consider venting of a specific kind... one way breather vents. These would be installed only through the membrane without cutting into the insulation, and I would only install these if condensation were an inssue under the epdm.

Installing a one way breather, not cut through the insulation, shouldn't effect any heat loss.

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-Grumpy
Chicago Gutters Chicago Roofing

Tatnic

If its a school, they could benefit from air to air heat exchangers to remove a good portion of the moisture before it enters the roof cavity. If you're still concerned, then closed cell foam will give your insulation and a vapor barrier all in one, but you'd want to blow it right onto the ceiling deck (ie replace the loose insulation with the foam), then ventilate above that. You wouldn't need any poly on the roof deck in that case since you have all sorts of room to achieve your R-value with the foam. You can also blow the foam over the lower chords and help to reduce the thermal bridging somewhat. Foam may be more expensive but you can't easily get a vapor barrier any other way.

AaronB.

Let's please refer to the application of our beloved SPF as spraying instead of blowing. :-)

Ansel

What are you trying to accomplish .... What is the problem the school is experiencing?????

It is not common to ventilated a flat roof, let alone ventilate it below the layer of insulation .....

As for one-way breather vents and venting between the trusses with turbines - neither would I recommend .... For starters, one way breather vents rarely vent more than a few square feet around them .... As for the turbine vents - where is the incoming air coming from - inside the building or through the soffit / fascia and if thru soffit or fascia how long is the run you are trying to vent?

So what is the problem or what are you trying to accomplish would be my starting questions .....

Grumpy

Forgive me if I am wrong, but I beleive that one way breather vents are rated at one vent for every 1000 square feet on many low slope lay over applications. Now we are not talking about a lay over application, however by that logic I would have to assume that the brather vent is capable of handling 10 squares of roofing.

Key word on assume, since we all know what happens when someone assumes.


Also keep in mind I too never suggested venting past the insulation. That'd be a waste of money to insulate and then ventilate. My suggestion was to vent the space between the insulation and roof system asuming it's some kind of mechanically attached or ballasted system.

__________________
-Grumpy
Chicago Gutters Chicago Roofing

Ansel

Rated 1 vent per 10 squares - yes they are but in real life I have never seen anything more than a dry area a few feet around the vent ... We actually installed a few in our early years and tore them out a few years later - the results were the same no matter who installed ..... most of the times there is a "dew point" issue with most overlay systems and the vents do little or nothing or may even help contribute to the issue ....

As for venting of a flat roof my questions in my last post still stands - for what reason and what is the problem ..... why do they want to vent this area and how effective will the vents be ... what is the problem???

Grumpy

Well since the original thread is well over a year old and the original poster never came back even once, I don't think that you will ever have that answer Ansel.

__________________
-Grumpy
Chicago Gutters Chicago Roofing

AaronB.

I agree with Grumpy