Venting the shed roof

[garywayne]

Hi again.

Earlier this month, or last, I enquired about insulating my shed roof. Part of the sequence of construction was to allow for airflow between the insulation and the roof.

I have chopped 2" off of the top of the walls, and installed bug proof mesh. To allow for airflow through the rafters I need to make way for the air through the noggins.

My question is. How big a hole do I need to make in the noggins to allow for sufficient airflow? (Please ignore the insulation in the picture. This picture was taken before the modifications where made).

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Thanks in advance.

 

[garywayne]

All I need to know, before I start chopping out great big chungks of wood, is, do I only need to make a couple of 1" holes, or, do I need to take out, say, a 6" or 8" x 1" piece?

Thanks again.

 

[Dibs-h]

What size are the noggins? Could you not place in the insulation a little lower down?

 

[garywayne]

The noggins are 2" x 4". The insulation takes up 2" of the 4". Which leaves 2" to vent.

The roof will be vented right through from one side of the shed to the other once I have the noggins vented.

 

[garywayne]

I find it hard to believe that no one can advise me, what with all the shed builds.

Thanks. :twisted:

 

[Benchwayze]

Now I can visualise, Gary...


To avoid weakening the noggins, I would think holes of no more than 1/3 the thickness/depth of the noggin, and not too many between each rafter. But that's just my guess.

In the end, they are just there to allow air to circulate.

Mike Garnham might well tell me I am completely out, but then he knows his stuff.

HTH

John

 

[Dibs-h]

Rather than weaken the noggins with holes - a few holes here and there may not achieve the kind of ventilation required. Most products I've seen make a reference to being equivalent to so many cm2. Based on that - a few holes here and there would provide next to nothing.

One option would be to place the insulation below the rafter - venting the entire space between the rafter (and the noggin). You may then have issues placing the insulation, etc. and then the final covering. It would admittedly be much easier using an solid insulation board like Kingspan, etc. under the rafters.

HIH.

 

[Benchwayze]

Maybe you could attach small blocks to the noggins, and fix insulated board onto those?

Just guessing really, 'cos my bro'in-law ain't here!

John

 

[MikeG.]

How have I missed this one?

I'll have a look at it this evening........

..........and now I remember saying that about a shed roof the other day, too.........

God, where does the time go?

Mike

 

[garywayne]

Thank you all very much for your replies.

I would like to say that I find it hard to find the right words to explain things. So if in dought, ask.

In my shed there is no ridge board at the apex of the roof, so there will be a through drought in from one side of the shed and out of the other, (both sides are vented). The only obstructions are the noggins, (2" x 4"). One on either side of the roof. 2" of the 4" noggin will be taken up by the 2" thick Kingspan insulation board, which leaves me a 2" depth to play with.

 

[RogerS]

Do you actually still need the noggins? I'm assuming that what looks like OSB sheeting is fixed/nailed to the rafters and so stopping them from moving laterally. So the noggins are superfluous? Could be talking rollocks. Usually do. Need to see what Mike says.

 

[MikeG.]

Roger's asked the question I would ask........what are those bits of 4x2 for? Can you do without them?

Without a continuous air-path from eaves to eaves (impossible because of your ridge), you really should have a 50mm clear continuous cavity to the underside of your OSB.

You can't achieve that with those bits of 4x2 in the way, so......

........can you turn them so that they lay flat? ie stick up 2" instead of 4"?

.........can you do without them altogether?

.......could you fix your insulation across the underside of your rafters, leaving a 2" gap over the "noggins"?

.......could you insulate at joist level?

........could you do some raised ties/ collars at the level of the noggins, and insulate this on the horizontal..........leaving a small attic-type void over to be ventilated at the gables?

If the answer is no all of those, then how about cutting out the 4x2's and replacing them with timbers the size of your rafters.........and those bigger timbers would then be sturdy enough to have some really big holes bored in them.

If none of those are going to work, we'll have to have a bit of a re-think.

Mike

Apologies for the delay in responding.

 

[garywayne]

The OSB is secured to the rafters, so I can remove noggins completely.

Thank you all so much for your help That makes things so much easier.