Loft Insulation

mccleanp · 22-04-2009 10:43pm #1

I am in the process of building a new house, and I'm currently deciding on loft insulation.

Its a two-storey house with attic trusses (200mm depth). At the moment, I am not considering using the loft space for anything other than storage. I was thinking of installing 200mm fibre-glass between rafters, and another 200mm fibre-glass across the rafters.

I have attached a vapour barrier to underside of the trusses before the plasterboard, in an attempt to avoid any damp problems. I am also installing a MHRV system in the house.

I am intending to floor the central part of the loft, and hence wish to avoid the additional layer of fibre-glass across the rafters. Having spoken with the technical departments in one of the insulation companies, I am considering installing 50-100mm of rigid foil faced PIR insulation on the floored section. I was also advised to install 6mm plywood below the PIR insulation to avoid point loading. Finally I will install 18mm chipboard floor boards above the PIR insulation.

Does anyone have any comments/suggestions?

I am considering installing the fibre-glass loft insulation and 6mm plywood before the house is plastered and the floor screeds are installed, but I am concerned about all the moisture these processes would produce and whether this may harm the insulation. What do you think?

sinnerboy · 23-04-2009 12:35am

400 fibreglass is pretty good . It is important that it should breathe into your vented attic space so placing PIR over is not a good idea

Assuming you only want to floor over the high mid part of your attic simply run 225 x 50 joists at 600 c/c accross your ceiling joists in this mid part with the top 200 layer of fibreglass in between this upper run of joists

Then use floorboarding across these top joist but don't fit them close-to - leave 50mm between each one - a little like hot press shelving if you like

Make sure not to choke your vent paths into the attic See detail 1.09 here http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,18751,en.pdf

mccleanp · 23-04-2009 9:17pm

I can understand what you are saying, but I thought the combination of the vapour barrier below and the MHRV system would negate the need for the loft insulation to 'breathe'?

If I go down the route you suggest, how should I fix the 225x50 joists? That's some really long screws!

Incidentally, I wonder why the recommended loft chipboard flooring comes in tongue & groove, which effectively blocks the path for vapour?

sinnerboy · 23-04-2009 11:14pm

Vapour will be present in the attic atmosphere . During the colder months it will condense close to the top of the quilt . Best not to inhibit its dispersal .

The topmost joists can simply rest on the ceiling joists . When the floorboards are screw fixed over that assembly wont go any where .

I would not recommend t+g loft chipboard

Carlow52 · 23-04-2009 11:22pm

the counter joists can be nailed from the side as per the normal way ceiling rafters are fixed to wall plate.

Alternatively u can use a 12mm or 16mm wood auger bit to drill maybe a 150 mm deep 'counter sink' in the joist and then screw it down: 150mm long screws are easily got

Carlow52 · 24-04-2009 10:18am

Following on the theme in this thread, airborne particles in an glass fibre/similar insulated attic can be an issue for some people. Given the need to ventilate through any flooring as per Sb's suggestion, would putting a layer of breathable membrane, such as Tyveck on top of the final layer of insulation allow the glass fibre to breath and still reduce the airborne particles?
Thanks.

sinnerboy · 24-04-2009 11:30am

Unless the quilt is disturbed exess particles will settle down or be removed over time by the ventilation flow .

But the tyvek could be done alright - but overkill in my opinion

Carlow52 · 24-04-2009 6:46pm

SB: As always tks for input
The situation I was trying to address is a client who wants to upgrade the attic insulation but is asthmatic and keeps the Xmas decos in the attic:)

PirateShampoo · 26-04-2009 12:59pm

You should look into ***** spray foam insulation. Its perfect for people with breathing problems.

Carlow52 · 26-04-2009 5:47pm

PirateShampoo wrote: »

You should look into Icynene spray foam insulation. Its perfect for people with breathing problems.

Thanks for the suggestion: the difficulty I have with that sort of stuff is that there is no 'post-application audit' available to show what you paid for is what you actually got.

To most people including myself it is very hard to know what you got is not some cheap carcinogenic crap.

mccleanp · 26-04-2009 9:58pm

Sinnerboy,

You have completely put me off the PIR (Kingspan) sandwich I idea I started with. I don't want to risk condensation in my roof timbers, especially somewhere I wouldn't see. So I guess its the other option of raising the floor with cross-joists.

I am a little concerned about the additional weight of the cross-joists. My roof is constructed of attic trusses, and the area I am intending to floor is approx 12m x 5m.

I have came across timber I-Joists (or I-Beams), which appear to offer less weight and look easier to fix. Would these be suitable? Anyone any idea (ballpark) what price these are and whether they are stocked by standard builders merchants?

Carlow52 · 27-04-2009 1:24am

mccleanp wrote: »

Sinnerboy,

You have completely put me off the PIR (Kingspan) sandwich I idea I started with. I don't want to risk condensation in my roof timbers, especially somewhere I wouldn't see. So I guess its the other option of raising the floor with cross-joists.

I am a little concerned about the additional weight of the cross-joists. My roof is constructed of attic trusses, and the area I am intending to floor is approx 12m x 5m.

I have came across timber I-Joists (or I-Beams), which appear to offer less weight and look easier to fix. Would these be suitable? Anyone any idea (ballpark) what price these are and whether they are stocked by standard builders merchants?

I-beams as u call them are rather expensive and in your case will only make sense if u use them to span from wall to wall and keep them off the intermediate rafters. not that easy to get, try http://www.mcmahongrp.com/default.aspx
and persevere: else try Timber framed manufacturers.

will access for such long timbers be an issue?

One issue with them is that they are pretty deep to start with as the Irish guys only carry a ltd range
see here for full range http://ilevel.com/floors/f_TJI_joists.aspx

I use them when ever I can as I like them:)

PirateShampoo · 28-04-2009 11:29am

Carlow52 wrote: »

Thanks for the suggestion: the difficulty I have with that sort of stuff is that there is no 'post-application audit' available to show what you paid for is what you actually got.

To most people including myself it is very hard to know what you got is not some cheap carcinogenic crap.

We spray it our selves, but your right we have come across one company so far that have been spraying the fake stuff that you have mentioned and advertizing it has *********. They actually dropped a broucher in the front door. They wont be trading it much longer though.

If you give these guys a ring they will put u in contact with ur local sprayer. They are the only company in Ireland that are aloud to bring the stuff into the country.

******************

sinnerboy · 28-04-2009 12:03pm

mccleanp wrote: »

I am a little concerned about the additional weight of the cross-joists. My roof is constructed of attic trusses, and the area I am intending to floor is approx 12m x 5m.

I would be more concerned with the loading ( goods ) you store up there . Winter clothes and your antique collection of Spiderman comics will be heavy - locate such items over partitions below , with lighter items like the Christmnas tree elsewhere . The joists ( at 600 c/c ) and boarding won't add much weight .

Loft Insulation

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