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JarekBialik 09-28-2010 05:42 PM

Cathedral insulation in garage loft - at an impass!
3 Attachment(s)
Hi all, I'm insulating a loft space above my detached garage. The bottom walls are complete, vapoured and taped. Now its time for the top.

I using a product called durovent for air space ventilation between the bottom of the roof and the insulation. I'm doing a cathedral stlye insulation with the insulation presssed to the roof with out an attic space se.

Along the bottoms the soffit runs the length of the garage, but at the top there are only two vent ports on either side, not a full length top vent.

So my question is how high do I go with the air vents. I've added some pictures to hopfully clarify this. But basically I'm not sure if i should take the air vent all the way to the apex of the garage as shown? Or should I stop short and make a small attic space where there would be totally free flow from one vent to the other. Or can I take each air vent run to the very top of the truss?
My only concern with that is would only the two trusses where the vent holes are cut be the only ones properly ventilated that way?

Like i say pls have a look at the pics and let me know, project is on hold till I find out what to do as I dont want to make a mistake and wreck my roof!


tinner666 09-28-2010 06:56 PM

Cut a slot for a ridge vent and vent the ridge. Don't cut the last section of each end.

tinner666 09-28-2010 06:57 PM

You can remove and close the holes form the two static vents.

JarekBialik 09-28-2010 07:37 PM

thanks for the response, I don't know If I can make a ridge vent, I think I'll have to stick with the two vents. I just don't trust mysef to do that job.

I'm guessing from the response that If i stick with these two static vents that I'll have to stop shy of the apex of the roof by about a foot with the durovents air chanels. As shown on the truss where the vent is cut out, and then make a little attic area where there would be a small but free triangle of air exchange.

I'll adjust the panels and take another picture so illustrate what I mean.

Also should I leave a few inches between panels, there in the picture i have the panels right against each other, but I read that I there should be some spacing.


JarekBialik 09-28-2010 07:47 PM

although with the small free air space I'll lose some of the head room in there, where there isn't much head room to begin with!

tinner666 09-28-2010 08:24 PM

I know of no reason to leave space between the panels. I could be wrong.

JarekBialik 09-28-2010 10:35 PM

3 Attachment(s)
I guess it comes down to two choices, I uploaded two more shots. First is the small dead air space, the foam zents go to the level of the cut out vent as the first picture shows, then make a small attic dead air space (exaggerated there for clarity) I would make the real bottom smaller but shown there for the idea.

or version 2 as the picture shows, all of the vents to the apex of the roof and the cutout vent under the right panel of the two installed. Hopefully there would be enough cross venting show how between the struts.

Also just wondering, am I really trying to create a flow through air system or is just a cavity open at the soffit end only sufficient? It seems air would flow in and out of the soffits as well?

Thanks for the help, as you can tell Im a beginner in insuation and construction.

Thanks agian

JarekBialik 09-28-2010 10:41 PM

Idea 1 (as shown by the first two pics) seems like overall a safer plan as there would be more airflow for sure, but I would loose head room in there, and thats really not ideal.

Idea 2 (shown in pic three) gives me maximum head room but I'm worried doesnt properly ventilate all of the lanes.

Anyways going to sleep, tomorrow is another day on the project!

thanks for any and all input,

Michael Thomas 09-29-2010 09:36 AM

1) It's pointless to supply soffit venting and insulation baffles in rafter cavities which are closed off at the ridge.

2) There may be some advantage if you are able to supply a path for cross-connect between the eave vents on opposite sides of the roof (as for example by means of a dropped ceiling below the ridge), though it would be better to install a continuous baffled type ridge vent, see for example the "Research Exposes Attic Ventilation Myth" page at:


JarekBialik 09-29-2010 11:26 AM

Thanks for the response.

I see that option2 is definately out, ie where I would separately insulate each truss channel. This is what I thought, as I releazed that they are closed off at the top ridge, but thanks for confirming.

I see also that all the pros advocate a ridge vent, this makes sense, I also wonder why my garage builder and others around here only use these cut-out vents, must be cheaper as I see all the garages in my neighbourhood have these.

I will try to implement a version of the false ceiling, I dont think I'll be able to create a ridge vent in time for winter here, so small false attic where there is cross connection between both roof vents it is.

Thanks for the help

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