Trussed roof, boarding, and insulation - again

[SimonH2]

I know this has come up a few times, but from the threads I've read, I've not seen my particular idea mentioned.

There's two different properties under consideration. First is a house I own, the other is a bungalow that a colleague at work has just bought. I've been helping the guy at work with some stuff - poor chap, his father wasn't into DIY so he's "lost without map or compass" and the mercy of the good local tradesmen (or in some cases, not so good - one quoted £500 to replace the fuseboard with CU as a pre-requisite before they'd fit a new bathroom ).

So in this bungalow, it's been fitted out with an extra 8" of fibreglass (on top of a few inches of original fibreglass) - I assume under one of those free schemes for pensioners. It's made getting about in the loft a right pain, and that gets me back to the subject. There's f****-all storage in the place other than the loft, so logically it should be part boarded. He'll be limited in what he can put up there by the size of the small hatch !
My house needs more insulation (it's just got 4" fibreglass at the moment), and it is similarly lacking in storage space.

I was thinking about gluing & screwing some extra joists to the truss "uprights" parallel to the existing ceiling joists - with the tops high enough to leave room for the insulation (10 to 12" from the ceiling ?). Doing it this way I was trying to avoid putting any load on the original joists and flexing/cracking the ceilings.
In both cases, I think the trusses are at about 2' centres.

In my house there's just a "W" - ie from the apex the hangers come down and back up once on each side, and the central span is about 8' at ceiling level (about 24' overall span). I reckon I could get longer joists (say 10 to 12') in so I could span this in one go and attach the new joists to all the hangers as shown in red. There are built in wardrobes underneath, so half has a stud wall one side, half has a stud wall the other side - so I could add cross joists (planks on edge) to take support from these as shown in green.

Roof truss & attic boarding proposal

• SimonH2
• 18 Sep 2011
• 1

Proposal for adding additional joists for boarding attic

In my mates bungalow it is a "WW", with the existing joists covering about 30' in 5x6' spans. I was thinking in terms of boarding it to make the central span usable for a "chimpanzee walk", with the span either side forming storage spaces. Without taking a few rows of tiles off the roof, there's no way to get 18' timbers in, so it would have to be 3x6' lengths - or more likley 3x7' or so and overlap them as shown in red on this diagram.
Over part of the area, there is a block wall underneath that could provide support - towards one end of the central span. So there is scope for adding a cross joist (or more like a plank on edge) as shown in green.

Roof truss

• SimonH2
• 18 Sep 2011
• 1

Proposed arrangement for additional joists for boarding

So :
Does this make sense ?
What size timber for the extra joists ?
What should I use for the flooring ?
Anything I've missed ?

 

[Perry525]

It is very easy to make a trap door bigger.
I suggest you visit B&Q or similar and buy a wood ladder complete with frame, this will give a larger opening.

Fibreglass as an insulation is a waste of time and space.
The best solution is spray in foam between the joists, this will be air tight and provide better insulation and enable the loft floor to be boarded. This however is very expensive.

The next best is sheet polystyrene or similar cut to size and fitted between the joists. Three or four inches of polystyrene will keep the home a lot warmer and will quickly pay for itself.

The same applies to your home.

The way roof timbers are designed and fitted, they rely on the weight of the roof to stress the joists, pulling them outwards, holding them like a violin string, as such they are able to carry quite high loads.

 

[geraint]

It is very easy to make a trap door bigger.
I suggest you visit B&Q or similar and buy a wood ladder complete with frame, this will give a larger opening.

Fibreglass as an insulation is a waste of time and space.
The best solution is spray in foam between the joists, this will be air tight and provide better insulation and enable the loft floor to be boarded. This however is very expensive.

The next best is sheet polystyrene or similar cut to size and fitted between the joists. Three or four inches of polystyrene will keep the home a lot warmer and will quickly pay for itself.

The same applies to your home.

The way roof timbers are designed and fitted, they rely on the weight of the roof to stress the joists, pulling them outwards, holding them like a violin string, as such they are able to carry quite high loads.

you seem to be wasted in your day job....

you should be a gas safe plumber.... not an enginneer....

 

[j3stn]

That seems to be quite a bit of cost and effort in raising the height of joists. As the previous response, there are many other materials to insulate other than fiberglass. This is just used as it covers a large area and is the least expensive. Higher U values are achieved by Celotex/Kingspan etc all basically the same product and is used in cavity walls as the insualtion. However there is a product called Tri-iso which is a very thin foil insulation. You do not need to touch the floor as this can be stapled to the roof joists. They claim it has the same u value as 2 inches of celotex yet its just 20mm thick.

 

[mointainwalker]

They claim it has the same u value as 2 inches of celotex yet its just 20mm thick

Yes and when you add the 25 mm air-gap that the instructions say you must have then it is 70 mm thick. Not so good now is it and that is if the maker's claims are true which are hifghly disputed.

@perry525

Everybody believes that glass-fibre ( note NOT fibreglass this is completely different ) has the same insulation value as polystyrene, except perhaps you and has the additional benefit of being much cheaper.

Fibreglass as an insulation is a waste of time and space

Can you explain your remark above ?

 

[SimonH2]

Hmm, hang on a minute while I get the popcorn

I can sort-of see the point of insulating the roof rather than the bedroom ceilings but for a couple of points :

1) Unless it fits between the rafters, then some of the same problems crop up - principally working around all the structural diagonal bracings etc.

2) Then there's the not so minor detail that the attic is effectively open to the outside via permanent ventilation - there's a vent grill front and back under the eaves that's several inches wide and runs the whole width of the property.

For a new build (or complete gut and rebuild renovation) I could see the attraction of solid sheet insulation - but as a retrofit there's the issue that it's going to be a lot of work (and leave gaps) working round all the existing fixtures - electrics, pipes, and so on.

 

[mointainwalker]

I can sort-of see the point of insulating the roof rather than the bedroom ceilings but for a couple of points :

Simon

Unless you intend doing something in the loft that requires heat, it is a retrograde step to insulate the roof instead of the ceiling. It simply gives you another x % of volume to heat and therefore either increase your fuel bills or lower the average temperature in the house.

 

[SimonH2]

Unless you intend doing something in the loft that requires heat, it is a retrograde step to insulate the roof instead of the ceiling. It simply gives you another x % of volume to heat and therefore either increase your fuel bills or lower the average temperature in the house.

I agree, I wouldn't consider it as a substitute. But I can see some points in it's favour - probably in combination with ceiling insulation.

One benefit is that insulating between the rafters doesn't really affect headroom - the limit being the height of the apex of the middle triangle of the truss. Thus putting some of the insulation in the roof instead of on the ceiling would retain a bit more headroom.

In the same vein, reducing the required thickness on the ceiling would make boarding easier. Eg just cross-joisting with 4" joists would give something like a 7" gap for insulation, while 8" cross joists would be very tall and unstable without further effort (lots of noggins ?) - or add separate joints as per my original thoughts.

Lastly, insulating the roof itself would reduce the extremes - cold freezing pipes and tanks, hot causing electronics to sweat, but again for most purposes that's not a big issue.

The biggest negative I can see is that to be effective, you'd need to reduce the ventilation and I've seen in my parents house what that causes. They had to have some timbers replaced some years ago, and even with the extra ventilation put in, we still get small amounts of "indoor rain" when it's extremely cold outside. I think that will be less of an issue now - the roof has been retiled, and they've used breathable sheeting instead of the old bitumenised felt.

 

[mointainwalker]

But I can see some points in it's favour - probably in combination with ceiling insulation.

Disagree completely but will leave it at that.

 

[SimonH2]

I think the misconception you are labouring under is that somehow a volume of air takes energy to maintain at a constant temperature. It doesn't.

What does require energy is replacing any heat lost through the external boundary of that volume. So if the roof itself were insulated to the same standard then in the steady state the heat lost through the ceilings would be exactly the same as it would be if it were the ceilings that were insulated. There would be an initial extra energy input to heat the air volume and the structures in it, but after that it's a steady state and the heat required to be gained through the ceiling below is only that required to replace what's been lost through the roof.

The key bit there is to the same standard, which is not half as easy and cheap as just throwing a foot depth of glass fibre on the ceilings.

The main reason I'd even contemplate insulating the roof instead of the ceiling would be, as I mentioned, as a means of reducing the physical space required for the ceiling insulation.

If I ever had the cash to do a self build, then I'd either be using a more traditional roof construction (ie not trusses), or talking to the truss manufacturer to have the facility to board out AND insulate well built into the trusses.