How windproof should the ceiling/roof cavity of a brand new house be?

Our new home allows so much breeze through the roof cavity (between the roof and ceiling) that it literally sucks the manhole cover entirely off and flings it 'somewhere' in the roof cavity. Our builder is saying that this is perfectly normal but I cant ever remember any other house I have ever lived in with a similar problem. Can anyone advise me if this is normal or not?

No that is not normal.

The "manhole cover." I think that you are referring to the attic access.?. It is usually located on the second floor, in the ceiling inside of a walk-in closet or in a hallway. It is framed, trimmed out with a piece of 3/8" board as the cover which sits on the ledge created by the framing.

As far as your attic and the truss system (the framing that your sheathing and roof shingles ride on). First off if there is no blown insulation that is one issue (not related to this problem but still an issue). Blown insulation looks like cotton candy/fluffy snow and should cover the base of your truss system to a dept of 12-16" I believe. I am not sure if that is per spec as it has been a while since I knew this stuff by heart. Nonetheless, if it is not there all you will see up there is the base of the truss system and the wall board/drywall which is the covering for your second floor ceiling.

Breeze/heat gain/heat loss:

These homes are built to have your HVAC (Heating Ventilation Air Condition/Cooling), which is a 'forced air' system, work in conjunction with your home's framing, insulation, sheathing, windows, and insulation. As far as the attic/truss system the issue that may be at fault is your ridge vent or lack there of. See here: http://www.gklemmroofing.com/images/ridg...

If this component is not installed properly you will experience heat gain/loss and you will hear/experience drafts. Remember that you do want some ventilation from your attic just not total. The properly installed ridge vent is meant to assist in this.

Another potential problem will be the soffit. See here: http://farm2.static.flickr.com/1360/5489...

If this element is installed in-correctly or in an half-assed manner you will experience drafts because this element will provide a path for wind to flow into your attic. If BOTH elements (ridge vent and soffit) are incorrect then wind will blow in through your soffit, up through the attic/truss system, and out through the top of your roof/in properly installed ridge vent or if the ridge vent was not installed and the roof sheathing and associated shingles were made flush at the peak then your attic is not ventilated at all and could result in a sucking action.

Some other items to consider:

When you experience high wind days/nights, do your hear/experience whistling/sucking/breezes near windows?
If the answer is yes then another problem to address is the insulation behind your drywall/around windows.

I would also check your doors leading outside. Do they present drafts etc. If so then you will need to check not only the insulation around the door frame but the door itself. Chances are that it was not 'set' properly. What I mean by that is, when the door was put into your home it was not checked and shimmed properly to insure that it was not only level and plumb (not racked or leaning) but also operating (with the sweep installed) smoothly.

Just some of the items I was able to come up with off the top of my head. Hope this helps.

By the way, if you have purchased this home through a production builder (non-custom builder) then you should check through your warranties etc. and I would definitely add them to your 30-day list of discrepancies. This 30 day list is pretty common and is used by builders to insure quality. It is given to the home owner at closing and allows you to move-in and note problems so the builder can in turn correct them.

Hope this helps. Good luck.

All new construction homes will have the attic space vented. Your's sounds to be a bit excessive. I can't guess what's wrong, but there's definitely more breeze than there should be. (Based on your description.)

you should have your attic vented, both in the summer and winter months, This saves your roof from getting to hot and this will ruin your shingles in the summer months, but you should have plenty of insulation in your attic over your ceiling of your rooms(the floor of your attic) or you will loose your heat through your ceiling in the winter months.