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Thread: roof ventilation
06-15-2011, 11:32 AM #1
Hello out there in “Buildingland”
I am new here with sole thanks to the heroic patience of Brian Hannigan who led me through what appeared to me to be another universe. So here I am, full of confusion and desperately looking for help in details regarding completing the unfinished house I bought.
I have this question.
The trusses (I believe it is called a 4/12 pitch) sit atop the top plate (I am losing my memory a bit so I may have that name wrong.) Atop the top board of the walls.. The top member of the truss goes out and ends on a fascia board. There is no soffit enclosure. There is a space between the top plate of the wall and the underside of the roof. The space measures less than an inch. Maybe it is 3/4 inch. I should go out there right now and measure it but it would be difficult at this moment.
Now at each end of the house (house measures 20x48 ft) up right under the roof peak there is a metal ventilator (again, I should go there and measure it and again it would be difficult today. It is very hot here (but dry) and I am in less than great physical condition and feeling rather poorly today, and in order to measure that ventilator I would need a ladder that needs to be brought out there and I just cannot take on the task today. As I was saying – metal ventilator and it looks to be about 16x24.
I have been advised to leave those open 3/4 inch high spaces at the top of the wall as they are. (for ventilation) But I do not see how they would work because I have been told the insulation will block those spaces anyway. It does get hot here, max temp range this area from about 20F to 105F, But it is frequently very windy in my particular location.
I researched bird stopping but there is not enough vertical room for even one-inch diameter holes. As I see it, a soffit if I were to build one would not do any good because of the problem of insulation plugging up all those ¾ inch spaces.
The problem is that there is too little clearance between the top plate and the underside of the roof. If the clearance was enough for a vertical blocking board then holes could be cut into it.
I saw a post on this site with the formula for ventilation, but got kind of lost in it. It appears that the existing end wall ventilators are just a tad small. If I made them be over 2.6 square feet would that be sufficient considering the prevailing winds, or given the fact that it is not always windy would that not work? Would I have to somehow create more ventilation? Would more ventilators at the end walls do any good? Would it be possible for example to cut in a horizontal row of ventilators at each end wall at a height just above the insulation?Perhaps ventilators about 2 inches high and maybe a foot wide and enough of them to bring the total ventilation at each end of the house up to maybe 4 square feet ?
I hope I am not asking too long of a question.
Last edited by richard karl; 06-15-2011 at 01:59 PM. Reason: I think I made a mistake and until I can go out there and look more closely I do not want to cause people to spend any time
06-15-2011, 07:17 PM #2
Re: roof ventilation
I am new so I do not yet know where to put this so I will put it here.
I am very sorry for my recent and very first post "roof ventilation." I made a really stupid mistake and I am glad that so far no one has replied to my post. I will assume that the stupidity of it was apparent to some inspectors and that is why there were no replies.
It had been a while since I was at my unfinished house and I wrongly remembered the details.
I forgot that there is vertical blocking between each truss! It is of sufficient height to allow for 2" or 3" diameter vent holes.
The 3/4" gap I spoke of is the gap between the top edge of the vertical blocking and the bottom surface of the roof! I completely forgot that the blocking is there and that the gap is above the blocking so I had stated instead that the gap was between the top plate and the bottom surface of the roof.
So now I am thinking that rather than putting holes in each vertical block, I will simply put hardware cloth in the gap above the blocking and then insert some kind of corrugated material to make paths for air though the insulation and into the attic.
In partial defense of my colossal blunder I am 77 and suffering a bit of memory and cognitive problems. I must be aware of this in the future before I cause problems for others.
06-22-2011, 03:47 AM #3
Re: roof ventilation
Probably the most important thing that most homeowners are unaware of is that it is essential to maintain the attic air temperature to match the outside temperature. To accomplish this it is important to make certain that there is sufficient air flow coming in through the soffit vents and exhausting through either a ridge vent or hat vents.
To address your problem specifically, depending on the actual square footage, the 3/4 inch gap between the block and the bottom of the roof decking may be enough as long as they are not blocked by insulation. The formula you are referring to is 1-in-300; meaning, for every 300 sq. ft. of attic space there should be 1 cubic foot of ventilation - distributed evenly throughout that area. Here is one source for a Roof Ventilation Calculator that may help in your calculations. Also, the 1 cubic foot should = 1 cubic foot of intake (via the soffit) and 1 cubic foot of exhaust (through the ridge or hat vent(s)).
I hope this helps?
06-22-2011, 10:16 AM #4
Re: roof ventilation
Thanks Doug for the link. Boy, it's so clear in the calculator you linked me to! I am going to work through it later today to see how I stand at the moment. I may actually have the ventilation I need as you have suggested. The calculator looks simple.
I've been sidetracked for a couple of days with trying to get my 70 VW Beetle back on the road. All of a sudden it started acting up and I am afraid to go anywhere with it in this heat. Anyone know a good source for automotive advice? Not many mechanics around here and none I can afford.